Acute Pain

Clinical Snapshot

Acute pain is temporary, lasting minutes to several weeks, and must be recognized, assessed for cause, and treated as soon as possible — untreated pain causes real physiological and psychological harm, so the absence of a diagnosis (including for undifferentiated abdominal pain) should never be used as a reason to withhold analgesia.^[1] Treatment is stepped by measured severity, not assumed from the diagnosis: a nonopioid analgesic (acetaminophen and/or an NSAID) for mild-to-moderate pain (numeric rating scale <7), escalating to an opioid — oral for moderate pain, parenteral for severe pain (NRS ≥7) given its faster, more predictable onset. Opioids are an Institute for Safe Medication Practices Canada "high-alert" medication class — real risks with inappropriate use — but the correct response to that risk is careful, structured use, not withholding effective treatment from a patient in severe pain.

Recognize the Condition

Recognizing pain, including when the patient cannot self-report

Observe for behavioural signs — agitation, anxiety, crying, gritting of teeth, withdrawal from activities — and solicit self-reports whenever possible. A validated pain scale should be used and recorded like a vital sign, since routine use of a pain scale reduces oligoanalgesia (undertreatment of pain).

Choosing a pain scale by age/cognition

Numeric rating scale (NRS, 0-10)

Validated in adults and children 6 years and older — the standard tool for most patients.

Colour Analogue Scale

Usable from age 4-5 years.

Wong-Baker Faces Pain Rating Scale

Usable from age 3-6 years.

PAINAD (Pain Assessment in Advanced Dementia)

For adults with cognitive impairment who cannot reliably self-report.

Full assessment, in parallel with treatment

Medical history and associated symptoms, physical exam to determine cause and severity.
Medication self-treatment history, and possible allergy or adverse reaction to analgesics — validate against a centralized health record when available, especially before opioids.
Reassess frequently using the same scale to judge both severity and treatment efficacy.

Risk Factors

Any acute injury, procedure, or illness causing tissue damage or nociceptive/inflammatory input.
Cognitive impairment or preverbal age, which raise the risk of oligoanalgesia because pain is harder to recognize and self-report — actively look for behavioural signs in these patients rather than assuming absence of report means absence of pain.
Substance use history, personal or family history of addiction, untreated psychiatric conditions, or trauma exposure — screen for these before initiating an opioid, since they change the risk/benefit calculation and monitoring intensity, though they should not be used to withhold adequate analgesia for genuine acute pain.

Red Flags & Refer

Do not delay analgesia while pursuing a diagnosis — opioid analgesics can be given safely before a full assessment in acute abdominal pain without increasing the risk of diagnostic or treatment error; withholding pain relief to "preserve the exam" is an outdated and unsupported practice.

Pain out of proportion to exam findings — consider compartment syndrome, ischemia, or a surgical abdomen.
Pain with hemodynamic instability, altered mental status, or signs of a serious underlying cause (e.g., chest pain suggesting ACS, severe headache suggesting a vascular cause).
Inadequate analgesic response despite appropriate titration — reassess the diagnosis, not just the dose.
Any sign of opioid toxicity (sedation progressing beyond expected, respiratory depression) — have naloxone available whenever an opioid is given parenterally.

Assessment & Diagnosis

There is no differential-diagnosis table for acute pain itself — the "differential" here is the underlying cause of the pain, evaluated through the standard history, physical exam, and investigations relevant to the specific presenting complaint (see the condition-specific chapter for the suspected cause, e.g., a fracture, an abdominal process, a headache syndrome). What is universal across all causes of acute pain is the assessment framework in Recognize the Condition — a validated pain scale, used consistently and recorded as a vital sign, to both diagnose the severity of pain and to measure response to treatment over time.

Treatment Targets

Recognize that the patient is experiencing pain.
Relieve the pain promptly, in parallel with (not after) identifying and treating its cause.
Identify and treat the underlying cause.

Choose Initial Treatment

Direct treatment rule, by measured severity

NRS <7 (mild-moderate) start with an oral nonopioid analgesic — acetaminophen and/or an NSAID.

If pain persists at a moderate level or worsens, add an oral opioid to the nonopioid rather than switching to opioid monotherapy.

NRS ≥7 (severe) start a parenteral analgesic — usually an opioid — given the faster, more predictable onset versus the oral route.

IV is preferred over IM/SC given faster and more predictable onset with less injection pain.

Do not wait for a full diagnostic workup before starting treatment — give analgesia in parallel with, not after, diagnostic evaluation, including in undifferentiated abdominal pain.

Begin immediate nonpharmacologic measures (immobilize a fracture, dress a burn, apply cold/heat) while pharmacologic treatment is arranged.

Non-Pharmacologic Treatment

Assess quickly, calmly, and with empathy; provide reassurance and actively encourage the patient to verbalize pain throughout treatment, not just at intake.
Immediate physical measures while pharmacologic treatment is arranged: immobilize a fracture, dress a burn, apply cold or heat.
Adjunctive techniques: relaxation, guided imagery, distraction — particularly useful in children and for procedural pain, but should supplement, not replace, adequate analgesia.
Consider adding a sedative alongside analgesia for painful procedures specifically — but a sedative is not a substitute for an analgesic; treat the pain itself, don't just sedate through it.

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Acetaminophen Preferred	Mild-moderate pain, first step	Adults: 325-650 mg PO/PR Q4H, max 4 g/day. Children: 10-15 mg/kg PO Q4H (15-20 mg/kg PR), max 5 doses/day	Hepatotoxicity in overdose/supratherapeutic dosing	Fewer adverse effects/interactions than full-dose NSAIDs but less effective and no anti-inflammatory action. Can combine with an NSAID or opioid for additive effect.
Ibuprofen Preferred	Mild-moderate pain, first step	Adults: 200-400 mg PO Q6-8H, max 1.2 g/day. Children: 10 mg/kg PO Q6-8H, not to exceed adult dose, max 40 mg/kg/day	GI upset; avoid in renal failure, PUD, heart failure, ASA-sensitive asthma	400 mg dose is superior to 200 mg and comparable to acetaminophen/codeine. In children, 10 mg/kg is as safe as acetaminophen 15 mg/kg.
Naproxen Alternative	Mild-moderate pain, longer duration wanted	Adults: 500 mg initial, then 250 mg PO/PR Q6-8H, max 1250 mg/day. Children: 5-7 mg/kg PO Q8-12H, max 1000 mg/day	GI upset; avoid in renal failure, PUD, heart failure, ASA-sensitive asthma	Longer duration of action than ASA/ibuprofen. Naproxen 500 mg is superior to ASA 650 mg.
Ketorolac (parenteral) Special situation	Moderate-severe pain, when an opioid is contraindicated or as an adjunct	Adults: 10-30 mg IM/IV Q4-6H, max 120 mg/day. Children: 0.2-1 mg/kg/dose IM/IV Q4-6H, max 30 mg/dose	Same as oral NSAIDs; cannot be dose-titrated	10 mg IV has equivalent efficacy to 15 and 30 mg. 30 mg ≈ morphine 12 mg. Effective for renal colic. Limit to 5 days.
Morphine Preferred	Moderate-severe pain, gold standard opioid	Oral: adults 10-30 mg Q4-6H; children 0.2-0.5 mg/kg Q4-6H. IV: adults 2.5-10 mg Q2-4H; children 0.1-0.2 mg/kg Q2-4H	Sedation, constipation, nausea, respiratory depression	Predictable adverse-effect profile allows dose titration to a pain-free endpoint. Metabolism not affected by liver disease.
Hydromorphone Alternative	Moderate-severe pain, higher potency needed	Adults: 1-2 mg PO Q4H, or 0.25-0.5 mg IV Q1H	Sedation, constipation, nausea, respiratory depression	More potent than morphine — use lower doses. Immediate-release forms preferred; avoid controlled-release for acute pain.
Fentanyl Alternative	Brief procedures, or when rapid onset/offset is needed	Adults: 50-100 mcg IV Q1-2H, titrate. Children: 0.5-3 mcg/kg IV Q1-2H	Sedation, chest wall rigidity with rapid IV push	30-60 minute duration ideal for short procedures. Minimal hemodynamic effect, no histamine release. Intranasal option when no IV access.
Oxycodone Alternative	Moderate-severe pain, oral route	Adults: 2.5-5 mg PO Q4H	Sedation, constipation, nausea; significant abuse potential	Immediate-release, alone or with acetaminophen, is an effective oral option — use with caution given abuse potential.
Codeine Special situation	Not recommended as first choice — unpredictable response	Not recommended under 12 years; not for post-tonsillectomy/adenoidectomy pain under 18 years	Sedation, constipation, nausea; unpredictable in CYP2D6 ultra-rapid or poor metabolizers	Discouraged as a single agent for adults given unpredictable CYP2D6-dependent response — morphine is a better default choice.
Tramadol Special situation	Not recommended as first-line acute pain analgesic	Per product labelling — not detailed here given its second-line status	More nausea than other opioids; CYP2D6 ultra-rapid metabolizers at risk of respiratory depression	Should not be used as a first-line analgesic for acute pain — less reliable unless combined with acetaminophen.

Special Populations

Pregnancy

Acetaminophen is the drug of choice for mild pain. NSAID use should be restricted to the 1st or 2nd trimester — even short-term use after 32 weeks is associated with a significant increase in premature ductus arteriosus closure. NSAIDs are contraindicated from approximately 28 weeks of gestation onward, per Health Canada's finalized June 2021 safety review of NSAID-associated fetal kidney injury and oligohydramnios.^[2] Between roughly 20 and 28 weeks, use only if a healthcare professional judges it necessary, at the lowest effective dose for the shortest duration, with ultrasound monitoring of amniotic fluid if use extends beyond 48 hours; discontinue if oligohydramnios is detected. For more severe pain, opioids can be used at any stage of pregnancy — monitor the newborn for respiratory depression if opioids are used in late labour, and for withdrawal signs if used at high doses or more than sporadically in late pregnancy, since neonatal withdrawal can occur even at therapeutic maternal doses.

Breastfeeding

Acetaminophen is the drug of choice for mild pain; NSAIDs can be used safely. Most immediate-release opioids transfer into breast milk in small amounts and are reasonably safe if limited to therapeutic doses and short-term use (2-3 days) — counsel the breastfeeding parent on signs of opioid toxicity in themselves (constipation, oversedation) and in the infant (drowsiness, difficulty feeding, breathing difficulty, decreased muscle tone), and to stop breastfeeding if toxicity signs appear. Health Canada and the FDA advise against codeine during breastfeeding unless benefit clearly outweighs risk, given case reports of infant toxicity via variable maternal CYP2D6 metabolism — the interpretation of the underlying case reports remains debated, but the cautious approach is to limit codeine use while breastfeeding when an alternative is reasonable.

Children

Codeine and hydrocodone are not recommended under 12 years of age, and not for pain after tonsillectomy/adenoidectomy under 18 years, given rare but serious reports of death from unpredictable CYP2D6-driven morphine conversion. Use age-appropriate pain scales (see Recognize the Condition) rather than assuming pain severity from behaviour alone.

Opioid-naive, frail, or older adults

Use the lowest recommended starting dose. When switching between opioids, use equipotent-dose conversion tables and account for incomplete cross-tolerance — it may be necessary to reduce the calculated equipotent dose by half and titrate carefully, particularly when switching to a highly potent agent such as hydromorphone.

Monitoring & Follow-Up

Reassess pain frequently using the same validated scale used at baseline, and adjust treatment based on the response, not a fixed schedule. Monitor level of consciousness and adverse effects after every opioid dose, particularly with parenteral administration or in opioid-naive patients. Choose the analgesic and route based on hepatic/renal function and concurrent medications. Allow adequate time for onset before judging efficacy or performing a painful procedure. Consult a specialized acute pain service for complex or refractory cases.

Treatment Failure / Next Step

Reassess the pain score with the same validated scale — confirm treatment failure is real, not a communication or scale-mismatch issue (especially in children or cognitively impaired patients).
Confirm adequate dosing and correct timing relative to the drug's onset before assuming failure.
If on a nonopioid alone and still moderate-severe: add an opioid rather than simply increasing the nonopioid dose.
If on an oral opioid and pain remains severe: transition to the parenteral route for faster, more titratable control.
If on a parenteral opioid without adequate relief despite reasonable titration: reassess the underlying diagnosis (a missed or evolving process, e.g., compartment syndrome or a surgical abdomen, can present as "analgesic failure") and involve a specialized acute pain service.
For opioid-induced adverse effects limiting titration (e.g., significant nausea/sedation before adequate analgesia): consider an opioid rotation, or add an NSAID/acetaminophen for an opioid-sparing effect rather than escalating the same opioid further.

Common Errors to Avoid

Withholding analgesia until a diagnosis is confirmed, including in acute abdominal pain — opioids given before full assessment do not increase diagnostic or treatment error.
Treating pain based on the presumed severity of the diagnosis rather than a measured pain score — under- or over-treating because "this shouldn't hurt that much."
Using meperidine for pain expected to last more than 3 hours, or for more than 24-48 hours — accumulation of its neurotoxic metabolite normeperidine can cause seizures; many hospitals have removed it from formulary for this reason. Avoid in renal failure, liver disease, and within 14 days of an MAOI.
Using codeine as a reliable single-agent analgesic in adults — unpredictable CYP2D6-dependent response makes morphine a better default.
Prescribing an opioid course longer than necessary "just in case" — per current acute-pain opioid stewardship guidance, 3 days is often sufficient and more than 7 days is rarely needed; initiate with immediate-release formulations and avoid long-acting/extended-release opioids for acute pain.^[3]^[4]
Interpreting opioid-induced hives/itching as a true allergy — this is usually histamine release, not IgE-mediated allergy, and often resolves with an antihistamine or a switch to a different opioid rather than requiring the opioid class to be avoided entirely.
Switching opioids at a full equipotent dose without accounting for incomplete cross-tolerance — start lower and titrate, especially when switching to hydromorphone.
Prescribing an NSAID beyond 28 weeks of pregnancy, or without amniotic fluid monitoring for extended use between 20 and 28 weeks.

Clinical Decision Algorithm

Management of acute pain

Patient with acute pain — assess with a validated pain scale immediately, in parallel with diagnostic workup

Begin immediate nonpharmacologic measures (immobilize, dress, cold/heat) while pharmacologic treatment is arranged — do not wait for a full diagnosis

NRS under 7 (mild-moderate)

Start oral nonopioid: acetaminophen and/or an NSAID

NRS ≥7 (severe)

Start a parenteral analgesic (usually an opioid, IV preferred) — faster, more predictable onset

Reassess pain score at an interval matched to the drug's onset of action

Adequate relief

Continue current regimen; reassess regularly; treat underlying cause

Still moderate/worsening on a nonopioid alone

Add an oral opioid to the nonopioid (not opioid monotherapy)

Inadequate relief despite reasonable opioid titration

Reassess the diagnosis — an evolving or missed process can present as "analgesic failure"; involve a specialized acute pain service

Before prescribing any opioid course to take home: use immediate-release only, lowest effective dose, shortest duration — 3 days often sufficient, rarely more than 7

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.

Nonopioid analgesics

Drug	Dosage	Onset	Duration	Adverse effects	Drug interactions
Acetaminophen	Adults: 325-650 mg PO/PR Q4H, maximum 4 g/day Children: 10-15 mg/kg PO Q4H (or 15-20 mg/kg PR), maximum 5 doses/day	under 1 h	4-6 h	Hepatotoxicity in overdose or supratherapeutic dosing	Chronic alcohol use increases hepatotoxicity risk. May increase INR with warfarin if ≥2g/day used for ≥3 consecutive days — check INR and adjust warfarin dose.
ASA	Adults: 325-650 mg PO Q4H, maximum 4 g/day Children: 10-15 mg/kg PO Q4H, maximum 5 doses/day — avoid under 18 years with fever or viral illness (Reye syndrome risk)	1 h	4-6 h	GI upset. Avoid in renal failure, PUD, heart failure, ASA-sensitive asthma	Increases warfarin's anticoagulant effect. May reduce antihypertensive effect of diuretics/beta-blockers/ACE inhibitors/alpha-blockers. Monitor lithium levels. Increased GI bleeding risk with SSRIs.
Ibuprofen (oral)	Adults: 200-400 mg PO Q6-8H, maximum 1.2 g/day Children: 10 mg/kg PO Q6-8H, not to exceed adult dose, maximum 40 mg/kg/day	0.5-1 h	4-6 h	GI upset. Avoid in renal failure, PUD, heart failure, ASA-sensitive asthma	Same interaction profile as ASA (warfarin, antihypertensives, lithium, SSRIs)
Naproxen	Adults: 500 mg initial, then 250 mg PO/PR Q6-8H, maximum 1250 mg/day Children: 5-7 mg/kg PO Q8-12H, maximum 1000 mg/day	1 h	≤7 h	GI upset. Avoid in renal failure, PUD, heart failure, ASA-sensitive asthma	Same interaction profile as ASA (warfarin, antihypertensives, lithium, SSRIs)
Ketorolac (parenteral)	Adults: 10-30 mg IM/IV Q4-6H, maximum 120 mg/day Children: 0.2-1 mg/kg/dose IM/IV Q4-6H, maximum 30 mg/dose	0.5 h	4-6 h	Same as oral NSAIDs; cannot be dose-titrated once given	Same interaction profile as ASA (warfarin, antihypertensives, lithium, SSRIs)

Opioid analgesics

Give parenteral opioids with naloxone on hand. Respiratory depression is rare when the drug is titrated to the level of pain, not given as a fixed dose regardless of response.

Drug	Dosage	Onset	Duration	Adverse effects	Drug interactions
Morphine	Adults, oral: 10-30 mg PO Q4-6H, titrate to effect Children, oral: 0.2-0.5 mg/kg PO Q4-6H Adults, IV intermittent: 2.5-10 mg IV Q2-4H Adults, IV infusion: 1-10 mg/h continuous, 2.5-5 mg breakthrough dose Children, IV intermittent: 0.1-0.2 mg/kg IV Q2-4H Children, IV infusion: 0.01-0.05 mg/kg/h continuous, 0.01-0.05 mg/kg breakthrough dose	Oral: 30 min. IV: 5-10 min	4-5 h	Sedation, constipation; all opioids share this adverse-effect class	Additive sedation with other CNS depressants (alcohol); potential enhancement of opioid effect with lidocaine
Hydromorphone	Adults, oral: 1-2 mg PO Q4H Adults, IV: 0.25-0.5 mg IV Q1H	Oral: 15-30 min. IV: 5 min	4-5 h (oral and IV)	Sedation, constipation	Additive sedation with other CNS depressants; potential enhancement of opioid effect with lidocaine
Fentanyl	Adults: 50-100 mcg IV Q1-2H, titrate to effect Children: 0.5-3 mcg/kg/dose IV Q1-2H	Almost immediate (IV)	0.5-1 h	Sedation, constipation; chest wall rigidity with rapid IV push	Additive sedation with other CNS depressants. CYP3A4 inhibitors (cimetidine, efavirenz, erythromycin, itraconazole, ketoconazole, ritonavir) may potentiate fentanyl's opioid effect.
Oxycodone	Adults: 2.5-5 mg PO Q4H	10-15 min	3-6 h	Sedation, constipation; significant abuse potential	Additive sedation with other CNS depressants
Meperidine (pethidine)	Adults: 50-100 mg IV Q3-4H, maximum 100 mg/dose — not first-line, not for ongoing use Children: 1-1.5 mg/kg/dose IV Q3-4H, maximum 100 mg/dose — not for ongoing use	5 min	2-4 h	Sedation, constipation; seizures in renal failure; tremors, hyperreflexia, hallucinations from normeperidine accumulation	Additive sedation with other CNS depressants. Potentially life-threatening serotonin syndrome with nonselective MAOIs.
Codeine	Not recommended: As a single agent for acute pain in adults — unpredictable response given CYP2D6-dependent conversion to morphine; not for use under 12 years, or under 18 years for post-tonsillectomy/adenoidectomy pain	Variable (CYP2D6-dependent)	Variable	Sedation, constipation, nausea; unpredictable in CYP2D6 ultra-rapid/poor metabolizers	Additive sedation with other CNS depressants

Ketamine (adjunctive)

Subdissociative IV dosing as an opioid-sparing adjunct — provides good analgesia with minimal cardiorespiratory impact; can also be used alone but with more limited supporting evidence.

Drug	Dosage	Onset	Duration	Adverse effects	Drug interactions
Ketamine	Subdissociative adjunct: 0.1-0.5 mg/kg IV	Rapid (IV)	Short, dose-dependent	Dissociative effects at higher doses; nausea	Additive CNS effects with other sedating agents

Local/topical anesthesia

For minor procedures, lacerations, and needle insertion.

Drug	Dosage	Onset	Duration	Adverse effects	Drug interactions
Lidocaine (infiltrative)	Direct infiltration/nerve block: 3-5 mg/kg (maximum 300 mg); up to 5-7 mg/kg with epinephrine co-administration, unless epinephrine is contraindicated	Rapid (local)	Short, dose-dependent	Local: none typical at correct dose. Systemic toxicity (CNS/cardiac) with inadvertent IV injection or overdose.	None significant at local infiltration doses

Topical anesthetics for needle-insertion pain (EMLA, amethocaine/Ametop, liposomal lidocaine/Maxilene) and inhaled nitrous oxide for brief procedural sedation are additional options for minor procedures — choose based on required application time, vasoactive properties, and procedure setting.

References

Pain assessment for pediatric patients in the emergency department — Pediatrics (Drendel AL, Brousseau DC, Gorelick MH). Pediatrics 2006;117(5):1511-1518
Labelling update regarding the use of non-steroidal anti-inflammatory drugs (NSAIDs) beyond 20 weeks of pregnancy and risk of kidney damage to unborn babies — Health Canada. Safety alert, finalized June 2021
CDC Clinical Practice Guideline for Prescribing Opioids for Pain — United States, 2022 — MMWR Recommendations and Reports, Centers for Disease Control and Prevention (Dowell D, Ragan KR, Jones CM, et al.). MMWR Recomm Rep 2022;71(3):1-95
Initiation of Opioid Therapy for Acute Pain — Professional Standard — College of Physicians and Surgeons of Nova Scotia. Approved June 4, 2020
Nonopioid drugs for pain — The Medical Letter on Drugs and Therapeutics. Med Lett Drugs Ther 2018;60(1540):25-32
Comparison of intravenous ketorolac at three single-dose regimens for treating acute pain in the emergency department: a randomized controlled trial — Annals of Emergency Medicine (Motov S, Yasavolian M, Likourezos A, et al.). Ann Emerg Med 2017;70(2):177-184

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.