Fever

Fever is a regulated, protective physiologic response in which the hypothalamus raises the body's thermoregulatory set point, most often in response to infection — it reflects the body's beneficial host defence working as intended, not a disease in itself.^[1] A commonly used definition is a rectal or rectal-equivalent temperature consistently over 38°C (100.4°F), with slightly lower thresholds recognizing normal diurnal variation of about 0.5°C — over 37.2°C in the early morning or over 37.8°C later in the day.^[2] The febrile response rarely exceeds 41-42°C, and fever itself is not inherently harmful in most patients. Because fever is a symptom and not a diagnosis, the clinical task is always to identify and manage the underlying cause, while treating the fever itself only for comfort — a distinction that runs through every part of this chapter, especially the pediatric-specific content below.

General presentation

Fever can be accompanied by malaise, fatigue, headache, backache, myalgia, arthralgia, drowsiness, and chills or rigors. These accompanying symptoms track with the degree of temperature elevation or with the underlying illness itself, and their presence or absence does not reliably indicate how serious the underlying cause is.

Atypical presentation by age

Infants younger than 3 months and older adults (65 years and older) may not mount a measurable fever even in the presence of serious infection — they can instead remain normothermic or become hypothermic. A normal or low temperature in these 2 groups does not rule out serious illness and should not be reassuring on its own if the rest of the clinical picture suggests infection.^[1]

Confirming fever: measurement method and route

Every measurement route only approximates core temperature, so the same route should be used for serial comparisons in one patient rather than switching routes and comparing readings directly. Axillary and infrared tympanic measurement are reasonable screening methods through childhood; oral measurement is used once a patient is old enough to cooperate reliably and in adults; rectal measurement gives the most precise reading in young infants and is used specifically when that precision changes management, such as deciding whether a neonate meets the threshold for an emergency sepsis workup.^[3]

• Age under 3 months — the single most important risk factor for occult serious bacterial infection presenting as fever.
• Immunocompromised state, including neutropenia, active chemotherapy, chronic high-dose corticosteroid use, HIV infection, primary immunodeficiency, and functional or anatomic asplenia (including sickle cell disease).
• Incomplete or unknown immunization status.
• Indwelling hardware — central venous catheter, ventriculoperitoneal shunt, urinary catheter, prosthetic joint, or prosthetic heart valve.
• Chronic underlying illness — congenital heart disease, chronic lung disease, chronic renal disease, or a metabolic/genetic disease.
• Recent antimicrobial use, which can partially treat and mask an evolving infection.
• Recent travel to a malaria-endemic or other infectious-disease-endemic region, or a known sick contact.
• A new medication started in the preceding weeks — a prompt to consider drug-induced fever (see Assessment and Diagnosis).

Fever vs. hyperthermia — a critical distinction

These 2 states are commonly conflated but are physiologically different and need different treatment; antipyretic drugs work by lowering the hypothalamic set point, so they are pharmacologically useless for true hyperthermia.

History

• Onset, duration, and pattern of the fever, plus associated localizing symptoms (respiratory, urinary, gastrointestinal, skin, neurologic).
• Immunization status, recent travel, sick contacts, and daycare or school attendance.
• Underlying chronic illness, immunosuppression, or recent surgery/instrumentation.
• Every current medication and supplement, prescription and nonprescription — drug-induced fever is uncommon but genuinely underrecognized; reported causes include allopurinol, amphotericin B, antihistamines, antipsychotics, atropine and other anticholinergics, azathioprine, barbiturates, carbamazepine, corticosteroids, cyclosporine, digoxin, diltiazem, doxepin, epinephrine, folic acid, furosemide, heparin, hydralazine, hydrochlorothiazide, H2-receptor antagonists, insulin, interferon, iodides, iron dextran, isoniazid, MAOIs, methyldopa, metoclopramide, mycophenolate, nifedipine, NSAIDs, oral contraceptives, penicillins/cephalosporins/sulfamethoxazole-trimethoprim and other antibacterials, phenytoin, procainamide, propylthiouracil, quinidine, quinine, rifampin, salicylates, sulfasalazine, tacrolimus, triamterene, and vitamins. Suspect drug-induced fever when fever appears after a new drug is started, no other source is found despite a reasonable workup, and the fever resolves after the drug is stopped.^[6]

Physical examination

A complete examination looking for a focal source, with particular attention to overall appearance (toxic versus well), work of breathing, perfusion, hydration status, rash, and neck stiffness. The degree of illness on examination — not the absolute temperature — should drive the urgency of workup and referral.

When investigations are — and are not — indicated

Aggressiveness of laboratory and imaging workup depends on age, clinical severity, symptoms pointing to a specific focus (urinary, respiratory), and comorbidity or immune status. A well-appearing child older than 3 months with a clear benign source (e.g., a typical viral upper respiratory illness) and reassuring follow-up generally needs no bloodwork or imaging. Nonspecific inflammatory markers — procalcitonin, C-reactive protein, or white blood cell count with or without differential — are not sensitive or specific enough on their own to replace clinical assessment of severity, though procalcitonin has a defined, validated role inside specific low-risk infant prediction rules (see below).^[7] Bacteremia is more likely when the temperature exceeds 41.1°C, a reasonable trigger to lower the threshold for blood cultures in that specific situation.^[8]

• Relieve discomfort and distress caused by the fever itself — the goal is comfort, not achieving a "normal" temperature reading.
• Identify and appropriately manage (treat, or safely observe) the underlying cause of the fever.
• Balance the benefit of symptomatic treatment against the cost, adverse effects, and dosing-error risk of antipyretic medication.
• Relieve caregiver or parental anxiety about fever ("fever phobia") through accurate counselling, without over-treating a well-appearing patient.
• In a pregnant patient, particularly during the first trimester, reduce the risk of adverse fetal outcomes associated with maternal heat exposure.^[10]

• Dress the patient in light, breathable clothing and keep ambient room temperature around 20-21°C — avoid both overbundling, which traps heat, and excessive undressing.
• Encourage increased fluid intake to replace insensible losses from fever and reduce the risk of dehydration.
• Avoid strenuous physical exertion while febrile.
• Tepid sponging is not recommended as a routine measure. It does lower temperature transiently, but the effect is short-lived, and the shivering and vasoconstriction the body mounts to defend its still-elevated set point make it uncomfortable without added benefit once an antipyretic is on board; if physical cooling is used at all, give the antipyretic first, since sponging before the set point is lowered simply drives the body to generate more heat to fight the cooling.^[14]
• Never use alcohol sponging — isopropyl or rubbing alcohol can be absorbed through the skin or inhaled, and has caused hypoglycemia, intoxication, and coma.
• Reserve physical cooling methods (tepid sponging, cooling blankets, fanning) for the rare situation of a core temperature over 41-42°C, and only after the patient has been appropriately investigated — physical methods are for very high fever in a monitored setting, not routine outpatient management.

Pregnancy

Breastfeeding

Acetaminophen, ibuprofen, and naproxen are all considered compatible with breastfeeding at standard doses. ASA should be used with caution, if at all, given case reports of infant platelet dysfunction and the availability of safer alternatives.

Pediatrics — the highest-stakes population for dosing accuracy

Always dose acetaminophen and ibuprofen by the patient's actual weight in kilograms, never by age alone. Confirm the exact product concentration being used every time — infant drops and children's suspensions of the same drug are not interchangeable volume-for-volume, and this exact mismatch is a well-documented cause of real overdoses.^[16] See Assessment and Diagnosis for the age-stratified risk approach to the febrile infant, and Common Errors to Avoid for the specific dosing-error patterns to counsel against.

Immunocompromised and neutropenic patients

Fever in a neutropenic patient — defined as an absolute neutrophil count under 500 cells/mm³, or under 1000 cells/mm³ with an expected decline, together with a single temperature at least 38.3°C or a temperature at least 38.0°C sustained over 1 hour — is a true medical emergency requiring immediate referral for empiric broad-spectrum antibiotics. It is never managed with an antipyretic and reassurance in the community pharmacy setting.^[4]

Older adults

A blunted febrile response means serious infection can present with a normal or only mildly elevated temperature — maintain a lower threshold of concern for this age group than the absolute temperature would otherwise suggest (see Assessment and Diagnosis).

Renal and hepatic impairment

Avoid ibuprofen, and NSAIDs generally, in significant renal impairment or when the patient is dehydrated — prostaglandin inhibition can worsen renal perfusion. Use acetaminophen with caution, and consider a reduced maximum daily dose, in significant hepatic impairment or active liver disease.

Take a temperature only if the patient feels warm or looks unwell, to establish a baseline — routine repeat measurement more than 2-4 times daily is not needed and tends to feed unnecessary anxiety rather than change management, a pattern sometimes described as "fever phobia."^[2] The trajectory of illness and how the patient looks, not the exact temperature trend, should guide ongoing concern. Seek medical attention if fever persists more than 24 hours with no apparent source, persists more than 3 days regardless of cause, or recurs after resolving.

• Monitor every patient started on an antipyretic for rash or signs of an allergic reaction.
• Monitor for signs of dehydration; if using an NSAID, stop it if dehydration develops.
• Monitor for common adverse effects — GI upset with NSAIDs — and watch for signs of hepatotoxicity with high-dose or prolonged acetaminophen use.
• At every fever-related encounter, confirm the correct product, concentration, and measuring device are being used, and that the patient is not exceeding the maximum recommended daily dose — including contributions from other coadministered acetaminophen- or ibuprofen-containing products such as combination cough-and-cold preparations.
• Confirm the patient is not on an interacting medication (see Drug Tables) and counsel to avoid alcohol while taking NSAIDs or acetaminophen.

1. Reassess for red flags before assuming this is simple treatment failure — a response, or lack of response, to an antipyretic does not confirm or exclude a serious underlying cause.
2. Confirm the correct weight-based dose, product concentration, and measuring device are actually being used — a dosing error is a far more common explanation for "the medication isn't working" than true treatment failure.
3. Confirm the maximum daily dose has not already been reached, including contributions from other coadministered products.
4. If a single agent genuinely is not achieving comfort at a correct, maximized dose and interval, consider a pharmacist- or prescriber-directed short course of alternating or combined acetaminophen and ibuprofen with an explicit written schedule, rather than leaving the caregiver to improvise the timing.^[11]
5. If fever persists beyond the expected duration for the presumed cause (more than 24 hours with no apparent source, or more than 3 days regardless of cause), or recurs, refer for medical reassessment rather than continuing to escalate antipyretic therapy at home.

• Treating fever to "break" it or normalize the number on the thermometer, rather than treating for comfort.
• Dosing by age instead of the patient's actual weight — the single most common and most consequential dosing error.
• Confusing infant drops with children's suspension concentrations of the same drug, or switching between formulations without recalculating the volume.
• Using a kitchen spoon instead of the calibrated device supplied with the product.
• Not asking about other coadministered acetaminophen- or ibuprofen-containing products (combination cough/cold/analgesic products), risking an inadvertent cumulative overdose.
• Using ASA for fever in a child or adolescent younger than 18-19 years — Reye syndrome risk.
• Using naproxen for fever in a child younger than 12 years.
• Waking a sleeping patient specifically to give a scheduled antipyretic dose — not necessary.
• Alternating or combining acetaminophen and ibuprofen as a routine default rather than a deliberate, explicitly scheduled exception — the leading cause of dosing confusion between the 2 agents.
• Giving prophylactic antipyretics before or immediately after routine immunization — this has not been shown to meaningfully reduce fever and may blunt the vaccine antibody response; reserve antipyretic use for a genuinely uncomfortable patient after vaccination.^[2]
• Recommending antipyretics to prevent febrile seizures — a persistent myth. Antipyretic therapy does not prevent febrile seizures, and prophylactic antipyretic or anticonvulsant use is not recommended for this purpose.^[5]
• Using alcohol sponging to cool a febrile patient.
• Assuming a fever that responds to antipyretic therapy cannot be a serious infection — response to treatment does not rule out serious illness.
• Missing a drug-induced cause of fever in a patient recently started on a new medication with no other identifiable source.

How to use this table: Doses are self-care doses and require patient-specific confirmation, including weight-based recalculation and renal/hepatic adjustment. Select any drug name to open its complete Pharmasist Drug Database page.

Para-aminophenol derivatives

NSAIDs

Avoid in dehydration or significant renal impairment; give with food if GI upset occurs.

Salicylates

Reserve for adults; not a routine first antipyretic choice even in adults given equally effective, better-tolerated alternatives.

Fixed-dose combination products

Evidence note: Drug inventory, dosing, adverse effects, and comments reflect standard Canadian self-care and prescribing practice, cross-checked against the current AAP clinical report on antipyretic use and current product labelling.^[2]