Head Lice
Clinical Snapshot
Head lice (pediculosis capitis) is caused by Pediculus humanus capitis, a small, wingless, obligate blood-feeding insect that lives exclusively on the human scalp and hair — unlike body lice, which live in clothing seams, head lice never leave the head under normal circumstances.[1] The louse feeds every 3-6 hours by injecting saliva with vasodilatory and anticoagulant properties into the scalp to draw tiny amounts of blood, and the female cements 5-6 eggs (nits) per day directly onto the hair shaft, close to the scalp, using a glue-like secretion strong enough that nits are difficult to remove even after the louse itself is dead.[1]
Head lice is one of the most classic pharmacist-managed minor ailments in Canada, and it is overwhelmingly a condition of school-aged children.[2] A pooled analysis of 55 studies found worldwide prevalence ranging from 0% to 64.1% depending on geographic location, and a United States national survey found 1.6% of children actively infested with live lice and 3.6% with nits at the time of screening; U.S. head lice treatment costs alone are estimated at roughly $500 million annually.[1] Prevalence and cost aside, the single most important message to deliver at every encounter is this: head lice are not a primary health hazard, a sign of poor hygiene, or a vector for disease, and infestation does not discriminate by socioeconomic status, race, or hair type or length.[2][1] Despite this, stigma remains disproportionate to the actual clinical significance of the condition, and children are still sometimes ostracized from school, social events, or friend groups over an infestation that is neither dangerous nor a reflection of anything the family did wrong.[1] A pharmacist who leads with this reassurance, rather than with the product recommendation, measurably improves the encounter.
Recognize the Condition
Life cycle — why timing drives the whole treatment plan
The adult louse is 2-3 mm long (about the size of a sesame seed) and lives 3-4 weeks on the scalp.[1] Once mature, the female can lay up to 10 eggs per day, cemented to the hair shaft roughly 2 mm from the scalp.[1] Eggs are incubated by body heat and typically hatch in 8-9 days (range 7-12 days, faster in warm climates, slower in cold), after which a nymph passes through 3 nymphal stages over the next 9-12 days before reaching the adult stage; a newly matured female can herself begin laying viable eggs about 1.5 days after becoming an adult, so an untreated infestation repeats its full cycle roughly every 3 weeks.[1] This life cycle is exactly why a single pediculicide application is rarely enough — see Choose Initial Treatment.
Symptoms — and why they can take weeks to appear
Itching (pruritus) of the scalp, neck, and behind the ears is the dominant symptom, but it results from a person becoming sensitized to components of louse saliva — with a first-ever infestation, this sensitization typically takes 4 to 6 weeks to develop, so a newly infested child can be genuinely asymptomatic, and unknowingly contagious, for a month or more before any itching begins.[1] A person with previous sensitization from an earlier infestation may itch much sooner after reinfestation. A typical infested scalp has surprisingly few live lice on it — usually fewer than 10 — most easily found at the nape of the neck or behind the ears.[1]
Nits — the finding almost everyone actually sees
Because a typical scalp carries fewer than 10 live lice, most patients and caregivers actually present having found nits, not a live louse. This is the central diagnostic pearl for head lice: a viable, potentially hatching nit is firmly cemented within roughly 0.6-1 cm of the scalp, appears camouflaged to match the hair colour, and under magnification shows an intact, hydrated mass or a visible developing embryo; an old, already-hatched, or non-viable nit is found farther than about 1 cm from the scalp (having been carried outward as the hair grows), appears empty and dull white against darker hair, and does not represent an active or ongoing infestation.[1] Nits are also easy to confuse with dandruff, hair casts (pseudonits), scabs, other hair debris, or even a fungal hair-shaft infection (piedra) — none of which slide along the hair shaft the way a true nit, cemented in place, does not.[1]
Risk Factors
- School- and daycare-age children are by far the most commonly affected group — direct, prolonged head-to-head contact is simply far more frequent in this age group (classroom activities, sleepovers, sports, playing together) than in adults.[2][1]
- Any setting involving close, sustained head-to-head contact: sleepovers, group sports with shared headgear, and other close-contact play or activities.
- A household or close contact with an active infestation — the person most likely to reinfest a successfully treated patient is an untreated family member or close contact (see Non-Pharmacologic Treatment and Monitoring/Follow-Up).
- Head lice infestation is not influenced by hair length, how often hair is brushed, or how often it is shampooed — published reports have not found any of these factors to meaningfully affect infestation risk, despite persistent public belief otherwise.[1]
- Head lice infest all types and textures of hair and do not discriminate by socioeconomic status, race, or ethnicity, despite some public claims to the contrary; there is no consistent supporting evidence for a race- or ethnicity-based difference in risk in the medical literature.[1]
- Head lice are strictly species-specific to humans — a household pet cannot carry or transmit human head lice, and pets do not need to be treated, examined, or excluded from the home.[2]
Red Flags & Refer
Head lice itself is almost always a self-limited, self-care and pharmacist-managed presentation. Escalate beyond routine pediculicide counselling for any of the following:
- Secondary bacterial infection of the excoriated scalp — with heavy infestation and prolonged scratching, impetiginization, folliculitis, or cellulitis of the scalp can develop and needs antimicrobial management rather than a pediculicide alone.[2]
- Persistent live lice after two correctly timed, correctly applied treatments of the same product — this pattern points to true treatment failure (most often resistance, misapplication, or reinfestation — see Treatment Failure/Next Step) and warrants switching to a different mechanistic class or referral for a prescription option, not a third attempt with the same product.[1]
- Suspected hypersensitivity or allergic reaction to a pediculicide — permethrin does not cause true allergic reactions, but pyrethrin-based products are extracted from the chrysanthemum family and can rarely trigger a genuine allergic reaction, particularly in a patient with a known ragweed allergy; modern extraction techniques have made this rare, but it remains a real, reportable possibility.[2][1]
- Significant scalp excoriation, marked regional lymphadenopathy, or fever accompanying the infestation — evaluate for a spreading secondary infection rather than attributing these findings to the lice themselves.
- Persistent itching with no live louse ever actually found, despite a careful wet-combing examination — reconsider the diagnosis (see the differential in Assessment & Diagnosis) rather than continuing to treat empirically for lice.
Assessment & Diagnosis
Diagnosis requires seeing a live louse — not just a nit
Identification of a live nymph or adult louse with the naked eye (magnification helps) establishes the diagnosis; nit presence alone documents that an infestation happened at some point, but does not, by itself, confirm an active, ongoing infestation.[1] This distinction matters clinically and practically: it is the reason nit-only findings should not automatically trigger treatment or school exclusion (see Non-Pharmacologic Treatment).
Wet-combing outperforms visual inspection alone
A study comparing detection methods in parasitologists found that using a fine-toothed louse comb was four times more effective and twice as fast at detecting live lice than visual examination of the scalp alone — the comb's very fine teeth are able to separate hair strands and pull off lice and nits that the eye alone misses, especially since lice actively avoid light and can crawl quickly away from a parted scalp.[2][1] A lubricant (water, hair conditioner, or oil) is sometimes used to "slow down" lice during combing, and the comb teeth should contact the scalp first before being drawn out to the hair ends.[1] For actual treatment purposes, combing appears to work best when performed on wet, conditioned hair (see Non-Pharmacologic Treatment).
Where to look, and what a viable nit looks like
Check first at the nape of the neck and behind the ears, where nits are most easily seen.[1] A viable nit sits within roughly <1 cm of the scalp (most reliably <0.6 cm), is camouflaged to the hair colour, and appears as an intact, hydrated mass or visible embryo under magnification; a nit found more than 1 cm from the scalp is very unlikely to still be viable and reflects a past, not active, infestation.[1] Do not confuse a true nit — firmly affixed and requiring real effort to slide along the hair — with dandruff, hair casts (pseudonits), scabs, other hair debris (including insects blown in and caught in hair), or piedra, a fungal hair-shaft infection.[1]
Nits alone are a poor predictor of active infestation
A prospective study of 1,729 school-aged children screened for head lice found that among children with 5 or more nits within <1 cm of the scalp, only about 32% went on to develop a confirmed active infestation; among children with fewer than 5 close nits, only about 7% did.[1] In other words, most children found to have nits alone — even a meaningful number of nits close to the scalp — do not go on to develop an active infestation. This finding underlies the guidance, discussed further in Non-Pharmacologic Treatment, against reflexive treatment or school exclusion for a nit-only finding.
Differential diagnosis
Dandruff (seborrheic flaking, not adherent to the hair shaft), hair casts/pseudonits (cylindrical, freely slide along the hair, unlike a true cemented nit), piedra (a fungal infection producing firm nodules on the hair shaft), scalp psoriasis or seborrheic dermatitis (diffuse scale, not discrete cemented specks), and simple environmental debris (dandruff, hairspray droplets, or even unrelated insects caught in the hair) can all be mistaken for nits or lice.[1]
Investigations
No laboratory testing is needed for a typical presentation — diagnosis is entirely clinical, made by direct visualization of a live louse.
Treatment Targets
- Confirm an active infestation (a live louse actually seen) before starting treatment, rather than treating a nit-only finding by default.
- Eradicate live lice and viable nits with a pediculicide matched to local resistance patterns, correct technique, and the specific product's directed retreatment interval.
- Recognize that most pediculicides are not fully ovicidal, and plan the second application from the outset rather than treating it as an afterthought.
- Relieve pruritus and identify/treat any secondary bacterial infection of the scalp.
- Prevent transmission to close contacts through appropriately scoped, evidence-based measures — not exhaustive or anxiety-driven household decontamination.
- Actively destigmatize the diagnosis and correct the "no-nit" school-exclusion misconception at every opportunity.
- Recognize true treatment failure (resistance) versus the much more common causes of apparent failure — misdiagnosis, incorrect technique, missed retreatment, or reinfestation — and escalate appropriately.
Choose Initial Treatment
The stepped ladder
1st line permethrin 1% crème rinse or pyrethrins + piperonyl butoxide shampoo remain first-line, guideline-endorsed treatments in Canada.[2] Both are synthetic or natural pyrethroid neurotoxins that keep voltage-gated sodium channels in the louse nervous system open abnormally long, causing spastic paralysis and death.[1] 2nd line a dimethicone-based product or an isopropyl myristate/cyclomethicone product when there is evidence of treatment failure with first-line therapy.[2] Persistent/refractory infestation prescription spinosad or topical ivermectin lotion for infestation confirmed resistant to, or genuinely failing, an adequate trial of first- and second-line options.[1] Resistant to every topical agent oral ivermectin, prescriber-directed, reserved for infestation that has failed correctly applied topical treatment across mechanistic classes.[1]
Permethrin 1% crème rinse — first line
FDA/Health Canada product labelling for permethrin generally covers children 2 months and older.[1] The Canadian Paediatric Society's own application instructions direct: wash the hair with a conditioner-free shampoo, rinse, towel dry, then apply enough permethrin crème rinse to fully saturate the hair and scalp; leave in place for 10 minutes; rinse well with cool water, minimizing exposure elsewhere on the body; towel dry; repeat in 7 days.[2] Note that this Canadian-specific towel-dried-hair instruction differs from some other permethrin product monographs, which direct application to fully dry hair with a longer 9-10 day retreatment window and specifically warn against shampooing for 24-48 hours afterward to preserve the residual pediculicidal film that kills newly hatching nymphs.[1] Always follow the specific product's own package instructions rather than assuming one application technique or retreatment interval applies to every permethrin product. Permethrin does not cause true allergic reactions and, of the pediculicide options, is generally regarded as the preferred agent in pregnancy.[1]
Pyrethrins + piperonyl butoxide shampoo — first line
Acceptable for children 2 years and older.[2] Applied thoroughly to dry hair (a minimum of about 25 mL), left in place 10 minutes, then lathered and rinsed; repeat 7-10 days later.[2] True allergic reactions are rare with modern extraction techniques but remain a real possibility, particularly with a ragweed allergy.[2]
Why a second application is (almost) always needed
Neither permethrin nor pyrethrin is fully ovicidal — roughly 20-30% of nits can remain viable after a first application.[1] The second, correctly timed application is what kills the nymphs that hatch from any surviving eggs, before they mature into egg-laying adults and restart the cycle; skipping or mistiming the second treatment, not "resistance," is the single most common reason a family reports the product "didn't work" (see Treatment Failure/Next Step).
Resistance to permethrin and pyrethrins is real and increasingly documented
Clinical effectiveness of these compounds has declined over the past four decades from near 100% when first introduced in the 1980s to as low as 25% in some studies, and resistance prevalence varies meaningfully by community and country.[1] A Canadian genetic-resistance survey found the knock-down resistance (kdr) allele in 133 of 137 head lice populations tested nationally, and a separate international survey specifically reported a kdr mutation frequency of 0.97 in Canada — among the highest frequencies reported anywhere.[2][1] The precise relationship between carrying this resistance gene and an individual patient's actual treatment failure is not fully established, so a single failed course should not be assumed to reflect resistance without first ruling out the much more common explanations in Treatment Failure/Next Step.[2][1]
Dimethicone — second line, and a genuinely different, non-neurotoxic mechanism
Dimethicone (100 cSt silicone oil, marketed in Canada as NYDA) is a physically acting pediculicide: rather than acting on a nerve or muscle target the way permethrin and pyrethrins do, it works by suffocation — flowing into the louse's breathing (spiracle) system and coating nymphs and egg embryos, causing death by asphyxiation rather than neurotoxicity.[2] Because this mechanism does not depend on the same target as the pyrethroid neurotoxins, dimethicone-based products carry a favourable resistance profile even in a community with high pyrethroid resistance — a genuinely useful counselling point when a patient has already failed first-line therapy. Apply to dry hair and scalp, massage in thoroughly, leave in place at least 30 minutes (some protocols use an overnight application), then comb out; repeat in 8-10 days. Not recommended under age 2.[2]
Isopropyl myristate/cyclomethicone (50%/50%) — second line
A different physical mechanism: this combination dissolves the waxy exoskeleton of the louse, leading to dehydration and death, rather than suffocation or neurotoxicity.[2] Apply to dry hair, leave 10 minutes, then rinse; repeat in 7 days. Not recommended under age 4.[2]
Spinosad 0.9% suspension — prescription, for treatment failure
Age 6 months and older, prescription-only. Spinosad has broad insecticidal activity and is both ovicidal and pediculicidal, disrupting neuronal activity long enough to affect developing larvae before they can form an intact nervous system.[1] It has demonstrated real superiority over permethrin in head-to-head comparison: treatment success of 84-87% with spinosad versus 43-45% with permethrin.[1] Apply to dry hair and scalp (may need a full bottle for longer/thicker hair), leave 10 minutes, then rinse; because it is genuinely ovicidal, retreatment is needed only if live lice are still seen at day 7 — a shorter, conditional retreatment window than the neurotoxic first-line agents.[1]
Topical ivermectin 0.5% lotion — prescription, for treatment failure
Age 6 months and older. Ivermectin increases chloride ion permeability in louse muscle cells, causing hyperpolarization, paralysis, and death; because these glutamate-gated chloride channels do not exist in mammals, the mechanism is not shared with any current human neurologic target.[1] It is applied to dry hair and scalp (saturating the scalp and working outward to the hair ends), left 10 minutes, then rinsed, with no shampooing for 24-48 hours afterward.[1] Unlike permethrin, topical ivermectin is not itself ovicidal, but any nymph that does hatch from a treated egg is unable to feed (its pharynx is paralyzed) and dies before it can mature or lay further eggs — so a single application is usually sufficient, without a routinely scheduled second dose.[1] In two randomized trials, 71-76% of patients were free of live lice 14 days after a single application; adverse effects (skin/eye irritation, burning, dryness) were uncommon.[1][3] Confirm current local availability and prescription status before recommending, since regulatory status differs between countries.
Oral ivermectin — for infestation resistant to every topical option
Prescriber-directed; reserve for topical treatment failure across mechanistic classes. A comparative-effectiveness review by the Canadian Agency for Drugs and Technologies in Health found that oral and topical ivermectin achieved comparable eventual cure rates (100% and 97%, respectively, after a second dose in one small comparative study) but identified no direct trials comparing ivermectin, in either formulation, against a standard pediculicide such as permethrin — a genuine evidence gap worth being transparent about with a patient being offered this option.[4] A typical regimen is two single oral doses of 200-400 mcg/kg spaced 7-10 days apart.[1] Use with particular caution, and generally avoid, in a patient weighing <15 kg — the theoretical concern is greater central-nervous-system drug penetration in a very young child, even though ivermectin's actual mechanism of action targets glutamate-gated chloride channels that are absent in mammals.[1] Oral ivermectin has an FDA-approved indication for head lice in adults, but not specifically in children — pediatric use for this indication is off-label, extrapolated from its approved use for other pediatric parasitic infections, and should be prescriber-directed rather than pharmacist-initiated.[1]
Malathion 0.5% lotion — an additional prescription option in resistant infestation
An organophosphate (cholinesterase inhibitor) pediculicide with high ovicidal activity, historically among the most effective single-application agents in comparative trials.[1] Applied to dry hair, allowed to air dry naturally (never with a hair dryer, curling iron, or flat iron, and never near an open flame or a person smoking — the product's high isopropyl alcohol content is genuinely flammable), left 8-12 hours, then washed off; retreat at 7-9 days only if live lice are still seen. Labelled for ages 6 years and older; safety under age 6 has not been established.[1]
Post-treatment itching does not mean the treatment failed
Itching or mild scalp burning from the inflammatory skin reaction to a topical pediculicide can persist for days after the lice themselves are dead, and is not by itself a reason for retreatment; a short course of a topical corticosteroid or an oral antihistamine can relieve these symptoms while the skin settles.[2]
Non-Pharmacologic Treatment
Wet-combing — a genuine but limited-evidence adjunct, not a reliable stand-alone cure
A large trial of 4,037 Welsh schoolchildren compared wet-combing (every 3-4 days for 2 weeks) against malathion lotion and found cure in only about 38% of the wet-combing group, versus about 78% with malathion.[2] Adding wet-combing on top of a permethrin course did not meaningfully improve outcomes either (about 73% cure with combing added versus about 78% without).[2] Wet-combing (ideally on wet, conditioned hair, working from root to tip in small sections) remains genuinely useful as a detection tool and as an adjunct for removing already-dead lice and loosening nits after a pediculicide course, but should not be recommended as a stand-alone cure when a family wants to avoid a topical product entirely, without a clear conversation about its materially lower success rate.
Environmental measures — appropriately scoped, not exhaustive
Because transmission is overwhelmingly head-to-head rather than fomite-driven (see Recognize the Condition and Risk Factors), environmental cleaning of the wider home is genuinely not warranted — nits are very unlikely to hatch at room temperature away from the scalp's warmth, and exhaustive household decontamination measures have not been shown to add benefit.[2] What is reasonable: wash only the items that had direct head contact in the 1-2 days before treatment — pillowcases, hats, hair accessories, headphones — in hot water (at least 54-60°C/130-140°F) and dry on a hot dryer cycle for about 15 minutes, or seal non-washable items in a plastic bag for 2 weeks if laundering is not practical.[2][1] Combs and brushes used by an infested person should be soaked in hot water. Do not use a pediculicide spray on furniture, carpets, or car seats — it is not necessary and is not recommended.[1] Shaving the head is effective but is not required and can be traumatizing to a child or the caregiver; it should not be presented as a routine option.[1]
Household and close-contact screening
Check all household members and close contacts for live lice or nits within <1 cm of the scalp, and treat those who are actively infested; also treat a family member who shares a bed with the infested person even if no live lice are found on them, since close overnight head contact carries genuine transmission risk.[1] Discourage sharing combs, brushes, hats, and hair accessories during an active infestation, but reassure families that a child does not need to refuse to wear a helmet or other protective headgear out of fear of lice — the actual risk from this kind of brief, shared contact is low.[1]
School, daycare, and the "no-nit" policy controversy
A prospective study of 1,729 school-aged children found that screening for nits alone poorly predicts an active infestation (see Assessment & Diagnosis for the numbers), and routine classroom or school-wide head lice screening has not been shown to meaningfully reduce infestation incidence in a school community over time, nor to be cost-effective.[1] On this evidence base, both the Canadian Paediatric Society and the American Academy of Pediatrics — together with the UK's Public Health Medicine Environmental Group and the U.S. National Association of School Nurses — explicitly advise against "no-nit" policies that exclude a child from school or daycare until every nit is removed: there is no sound medical rationale for this practice, since head lice have low contagion within a classroom, exclusion for nits alone can cause children to miss school unnecessarily, and blanket exclusion has been argued to raise real civil-liberties and stigmatization concerns.[2][1] No healthy child or adolescent should be excluded from school, or sent home early, because of head lice or nits alone — the correctly counselled plan is to have the child finish the school day, start treatment that evening, and return to school as usual; the child should simply be counselled, gently, to avoid close head-to-head contact with others until treatment is complete.[1] Confidentiality matters: notifying a classroom or school community about a specific case should be handled carefully to avoid inadvertently identifying the affected child, since this is a well-documented source of social stigma.[1]
Special Populations
Pregnancy
Permethrin is generally regarded as the drug of choice for head lice treatment during pregnancy, and topical pediculicides in general have very limited systemic absorption, so risk of harm to the fetus from topical head lice treatment is expected to be minimal.[1] Oral ivermectin appears likely safe in pregnancy based on limited available human data, but should remain a prescriber-directed decision made with the full picture of alternatives already tried.[1] Malathion's product-specific pregnancy data are more limited; discuss with the prescriber if it is being considered specifically for a pregnant patient with documented resistant infestation.
Breastfeeding
Topical permethrin, dimethicone, and isopropyl myristate/cyclomethicone are considered compatible with breastfeeding given minimal systemic absorption from topical use; confirm specifics against each drug's own Drug Database entry and current product monograph before recommending.
Pediatrics — confirm the age cutoff for the specific product chosen
Age indications differ meaningfully by product and should be checked before every recommendation: permethrin 1% crème rinse is generally acceptable from 2 months of age; pyrethrins + piperonyl butoxide and dimethicone from 2 years; isopropyl myristate/cyclomethicone from 4 years; spinosad and topical ivermectin lotion from 6 months; and malathion from 6 years, with safety under age 6 not established.[2][1] Oral ivermectin should be avoided, or used with particular caution and only under prescriber direction, in a child weighing <15 kg regardless of age.[1]
Older adults and congregate/institutional settings
Head lice is far less common in adults than in school-aged children, but outbreaks can occur in long-term care or other congregate settings through close caregiving contact; the same diagnostic and treatment approach applies, with attention to screening and treating close contacts within the facility.
Monitoring & Follow-Up
- Confirm the caregiver understands, and has scheduled, the product-specific second application before the encounter ends — this single point is the most common reason a genuinely appropriate first-line treatment appears to "fail."
- Recheck for live lice at the retreatment visit (or by phone/photo follow-up) rather than assuming success; wet-combing is the most sensitive way to confirm the scalp is actually clear.[1][2]
- Screen and treat active infestation in household members and close/bed-sharing contacts alongside the index patient, since an untreated contact is the most common source of apparent reinfestation.[1]
- Reassure explicitly that residual itching or mild scalp irritation after a successful treatment does not indicate reinfestation or a need for retreatment — it reflects the skin's inflammatory response to the pediculicide itself and can be managed symptomatically (topical corticosteroid or oral antihistamine) while it resolves.[2]
- Confirm the family has accurate, destigmatizing information for school or daycare (no exclusion needed for nits or treated lice) and address any misinformation from a school's own no-nit policy directly, since this is a common source of unnecessary missed school days and caregiver anxiety.[1]
Common Errors to Avoid
- Treating, or excluding a child from school for, a nit-only finding without ever confirming a live louse — most nit-only presentations do not go on to become an active infestation.[1]
- Applying or endorsing a "no-nit" school exclusion policy — this is explicitly discouraged by the Canadian Paediatric Society, the American Academy of Pediatrics, and school-nursing bodies as unsupported by evidence and a real source of unnecessary missed school and stigma.[2][1]
- Skipping or mistiming the scheduled second application — this, not "resistance," is the single most common reason first-line treatment appears to fail.[1]
- Assuming shared hats, brushes, bedding, or furniture are the primary transmission route and steering counselling toward exhaustive household decontamination — transmission is overwhelmingly head-to-head, and environmental cleaning of the wider home is not warranted.[2][1]
- Using or recommending a hair dryer, curling iron, flat iron, or open flame while a flammable product such as malathion is on the hair.[1]
- Shampooing too soon after a permethrin or ivermectin application, which strips the residual pediculicidal film that is meant to kill newly hatching nymphs.[1]
- Using or condoning an unproven home remedy (mayonnaise, olive oil, petroleum jelly, essential oils) as a substitute for an evidence-based product, without being clear about the comparatively weak and inconsistent evidence behind these approaches.[1]
- Using a veterinary/animal ivermectin product on a person — this is never appropriate; formulation, dosing, and inactive ingredients differ from human products and are not designed for human tolerability.[1]
- Recommending a pediculicide spray for furniture, carpets, or car seats — not necessary and not recommended.[1]
- Using stigmatizing or hygiene-blaming language when discussing the diagnosis, rather than leading with the fact that head lice is not a health hazard and does not reflect poor hygiene.[2][1]
Clinical Decision Algorithm
Drug Tables
How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.
First-Line Pediculicides
Neither agent is fully ovicidal — a correctly timed 2nd application is expected for most patients, not a sign of treatment failure. Always follow the specific product's own application and retreatment instructions.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Permethrin 1% crème rinse | Standard use: Saturate hair/scalp, leave 10 min, rinse | Mild scalp burning; does not cause true allergic reactions | None significant at standard topical doses | Age ≥2 months. Apply to towel-dried (or, per some labels, fully dry) hair/scalp; repeat per the specific product's label (commonly 7 days in Canada, 9-10 days on some other labels). Generally the preferred pediculicide in pregnancy. |
| Pyrethrins + piperonyl butoxide | Standard use: Apply to dry hair, minimum ~25 mL, leave 10 min, lather and rinse | Rare true allergic reaction, particularly with ragweed allergy | None significant at standard topical doses | Age ≥2 years. Apply to dry hair; repeat 7-10 days later. No monograph exists in this database for this product; presented as plain text. |
Second-Line Pediculicides — Physical (Non-Neurotoxic) Mechanisms
Reserve for documented or suspected treatment failure with a first-line agent. Because these act physically (suffocation or exoskeleton dissolution) rather than on a nerve/muscle target, they retain efficacy against pyrethroid-resistant lice.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Dimethicone | Standard use: Apply to dry hair/scalp, massage in, leave ≥30 min (or overnight), comb out; repeat in 8-10 days | Local irritation (uncommon) | None significant at standard topical doses | Age ≥2 years. Suffocation mechanism (blocks the louse's breathing/spiracle system) — unaffected by pyrethroid (kdr) resistance. |
| Isopropyl myristate/cyclomethicone (50%/50%) | Standard use: Apply to dry hair, leave 10 min, rinse; repeat in 7 days | Local irritation (uncommon) | None significant at standard topical doses | Age ≥4 years. Dissolves the louse's waxy exoskeleton, causing dehydration and death. No monograph exists in this database for this product; presented as plain text. |
Prescription Options for Treatment Failure
Reserve for infestation genuinely failing an adequate, correctly applied trial of first- and second-line agents — confirm adherence, technique, retreatment timing, and reinfestation have been ruled out first (see Treatment Failure/Next Step).
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Spinosad 0.9% suspension | Standard use: Apply to dry hair/scalp, leave 10 min, rinse | Local skin/eye irritation | None significant at standard topical doses | Age ≥6 months, prescription. Genuinely ovicidal and pediculicidal; demonstrated superiority over permethrin (84-87% vs 43-45% cure). Retreat at day 7 only if live lice are still seen. Not present in this database; presented as plain text. |
| Ivermectin 0.5% lotion (topical) | Standard use: Apply to dry hair/scalp, leave 10 min, rinse | Local skin/eye irritation, burning, dryness | None significant at standard topical doses | Age ≥6 months, prescription. Not itself ovicidal, but paralyzes the pharynx of newly hatched nymphs so they cannot feed — a single application is usually sufficient. The platform's only ivermectin monograph (ROSIVER) is a topical rosacea cream, not this antiparasitic formulation, so this row is deliberately presented as plain text rather than linked. |
| Malathion 0.5% lotion | Standard use: Apply to dry hair, air-dry only, leave 8-12 h, wash off; retreat 7-9 days if live lice seen | Local irritation; flammable product | None significant at standard topical doses | Age ≥6 years; safety under age 6 not established. High historical ovicidal activity. Never use a hair dryer, curling iron, flat iron, or open flame during the application/dry period. Not present in this database; presented as plain text. |
| Ivermectin (oral) | Standard use: 200-400 mcg/kg PO, 2 doses spaced 7-10 days apart | GI upset, dizziness; theoretical CNS concern in very young children | None significant at standard doses; prescriber to confirm against other current medications | Prescriber-directed only; reserve for infestation resistant to every topical class. Avoid, or use with particular caution, if weight <15 kg. FDA-approved for head lice in adults; pediatric use for this indication is off-label. The platform's only ivermectin monograph (ROSIVER) is a topical rosacea cream, not this oral antiparasitic formulation, so this row is deliberately presented as plain text rather than linked. |
Symptomatic Relief
For pruritus or mild scalp irritation after treatment; itching alone, with no live louse found, is not a reason for retreatment.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Cetirizine | Adults: 10 mg PO once daily | Mild sedation, dry mouth | Additive sedation with other CNS depressants | Non-sedating option preferred for daytime pruritus. |
| Diphenhydramine | Adults: 25-50 mg PO at bedtime PRN | Sedation, anticholinergic effects | Additive sedation with other CNS depressants | Reserve for itching disrupting sleep. |
| Hydrocortisone (topical) | Adults/children: Apply a thin layer to the scalp 1-2 times daily, short course | Local skin thinning with prolonged use | None significant with short-term topical use | Short course only, for irritation from the pediculicide itself. |
Secondary Bacterial Infection of the Scalp
Reserve for impetiginization, folliculitis, or cellulitis arising from excoriation of the scalp; refer promptly if infection is spreading or systemic signs appear.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Mupirocin (topical) | Adults/children: Apply 3 times daily for 5-10 days | Local irritation, burning | None significant with topical use | For limited, localized impetiginization. |
| Cephalexin | Adults: 500 mg PO 4 times daily for 7 days | GI upset, hypersensitivity reactions | May increase INR with warfarin (uncommon) | First-line oral choice for infection needing systemic therapy. |
| Clindamycin | Adults: 300-450 mg PO 4 times daily for 7 days | GI upset, Clostridioides difficile-associated diarrhea | None significant at standard doses | Alternative for penicillin allergy or suspected MRSA. |
| Sulfamethoxazole-trimethoprim | Adults: 1 double-strength tablet PO twice daily for 7 days | GI upset, photosensitivity, hypersensitivity reactions | May increase INR with warfarin | Alternative when MRSA is specifically suspected or confirmed. |
References
- Head Lice — Pediatrics, American Academy of Pediatrics Committee on Infectious Diseases, Committee on Practice and Ambulatory Medicine, and Section on Dermatology (Nolt D, Moore S, Yan AC, Melnick L). Pediatrics 2022;150(4):e2022059282
- Head lice infestations: A clinical update — Canadian Paediatric Society, Infectious Diseases and Immunization Committee (Cummings C, Finlay JC, MacDonald NE). Paediatr Child Health 2004;9(9):647-651 (CPS Practice Point, most recently reaffirmed/updated September 2025)
- Ivermectin Lotion (Sklice) for Head Lice — American Family Physician (Early J, MacNaughton H). Am Fam Physician 2014;89(12):984-986
- Ivermectin for Parasitic Skin Infections of Lice: A Review of Comparative Clinical Effectiveness, Cost-Effectiveness, and Guidelines — Canadian Agency for Drugs and Technologies in Health (CADTH) (Young C, Argáez C). CADTH Rapid Response Report, May 14, 2019
Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.