Acute Pain

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.

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).

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.

• 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.

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.

• 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.

• 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.

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.

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.

1. 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).
2. Confirm adequate dosing and correct timing relative to the drug's onset before assuming failure.
3. If on a nonopioid alone and still moderate-severe: add an opioid rather than simply increasing the nonopioid dose.
4. If on an oral opioid and pain remains severe: transition to the parenteral route for faster, more titratable control.
5. 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.
6. 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.

• 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.

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

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.

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.

Local/topical anesthesia

For minor procedures, lacerations, and needle insertion.