16.3 Pain & Fever Self-Care

Paracetamol (Acetaminophen)

Paracetamol is first-line for mild-to-moderate pain and fever. It is hepatically metabolised (CYP1A2/2E1 to the toxic intermediate NAPQI, detoxified by glutathione) and has a narrow therapeutic window.

Adult dosing

• 500–1000 mg every 4–6 hours as needed
• Maximum 4 g per 24 hours (eight 500 mg tablets)
• Reduce the maximum to 2–3 g/day in: elderly (≥65), hepatic impairment, alcohol misuse, low body weight (<50 kg), malnutrition or chronic dehydration (reduced glutathione stores)

Paediatric dosing

• 10–15 mg/kg/dose every 4–6 hours (oral or rectal)
• Maximum 60 mg/kg/day (not exceeding the adult ceiling)
• Practical dosing tables use age-banded volumes of 120 mg/5 mL or 250 mg/5 mL suspensions — always confirm strength before dispensing

Overdose and antidote

Paracetamol overdose causes delayed hepatotoxicity (peak injury at 72–96 h). Patients may be asymptomatic early — never reassure on clinical grounds alone. Management includes:

• N-acetylcysteine (NAC) — the antidote; replenishes glutathione, most effective when given within 8 hours of ingestion
• Treat per body weight nomogram (acetylcysteine 150 mg/kg IV over 1 h, then 50 mg/kg over 4 h, then 100 mg/kg over 16 h, in the standard UK regimen)
• Activated charcoal within 1 h if presenting early and a single large ingestion

Ibuprofen and NSAIDs

Ibuprofen is effective for pain with an inflammatory component and for fever unresponsive to paracetamol.

• Adult: 200–400 mg every 6–8 hours with food, maximum 1.2 g/day OTC (up to 2.4 g/day on prescription)
• Paediatric: 5–10 mg/kg/dose every 6–8 hours (≥3 months, ≥5 kg)

NSAID cautions — counsel before sale

Aspirin in Children — Reye Syndrome

Aspirin is avoided in children under 16 years for fever or viral illness because of the association with Reye syndrome (hepatic encephalopathy with high mortality). Use paracetamol or ibuprofen instead. Aspirin remains appropriate in children only for specific rheumatological or Kawasaki-indication prescribing under specialist supervision.

Combining analgesics safely

• Paracetamol + ibuprofen can be alternated for paediatric fever difficult to control; document timing to avoid duplicate dosing.
• Check all OTC combination products (cold/flu remedies) for hidden paracetamol to avoid exceeding 4 g/day — a common overdose source.

Refer for:

• Pain or fever persisting beyond 3 days (adults) or 24–48 h (children, elderly)
• Severe or sudden pain (possible surgical abdomen — appendicitis, ectopic pregnancy, AAA)
• High fever with rash, neck stiffness (meningitis), or breathing difficulty
• Signs of dehydration in infants/elderly
• Chest pain, neurological deficit, jaundice, unexplained weight loss

Worked example — hidden paracetamol

A 45-year-old with a cold asks for paracetamol for headache. He is already taking a combination cold/flu remedy (paracetamol 500 mg + phenylephrine + guaifenesin) two tablets four times a day — that is 4 g/day of paracetamol already. Adding regular paracetamol would exceed the 4 g/day ceiling and risk hepatotoxicity. Counsel to take either the cold remedy or plain paracetamol, not both; if switching, dose paracetamol 1 g every 6 h to stay within 4 g/day. Always read the label of combination products.

Worked example — pregnancy analgesia

A 28-year-old at 28 weeks' gestation has back pain. Paracetamol 500–1000 mg every 6 h (max 4 g/day) remains first-line in pregnancy for short-term use. Ibuprofen is avoided from 20 weeks' gestation because of fetal renal impairment and oligohydramnios; it is also avoided in the third trimester (premature ductus arteriosus closure). If paracetamol is inadequate, refer for medical review rather than supplying an NSAID.

Worked example — paediatric dosing

A 5-year-old (weight 18 kg) has fever 39°C. Ibuprofen 100 mg (5 mg/kg) every 8 hours with food or paracetamol 270 mg (15 mg/kg) every 4–6 hours (e.g. 11 mL of 250 mg/5 mL suspension). Confirm suspension strength before dispensing; do not exceed 60 mg/kg/day paracetamol or 30 mg/kg/day ibuprofen. Avoid aspirin (Reye syndrome). Review at 24–48 hours if fever persists.

Key exam traps

• Aspirin is avoided in children under 16 for fever/viral illness (Reye syndrome).
• The adult 4 g/day paracetamol ceiling is reduced in elderly, hepatic impairment, low body weight, and alcohol misuse.
• NSAID-sensitive asthma is a contraindication to ibuprofen; cross-reactivity with aspirin and other NSAIDs is typical.
• Paracetamol overdose is treated with N-acetylcysteine; most effective within 8 hours of ingestion.
• NSAIDs from 20 weeks' gestation are avoided because of fetal renal effects.