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Opioid Receptor Agonist (Peripheral — Antidiarrhoeal) Pregnancy: Caution — limited human data; minimal systemic absorption makes risk low; short courses for acute diarrhoea likely acceptable after first trimester

Loperamide

Brand names: Imodium, Arret

Adult dose

Dose: Acute: 4 mg initially, then 2 mg after each loose stool; Chronic: 4–8 mg/day in divided doses
Route: Oral (capsules, tablets, syrup)
Frequency: After each loose stool (acute); twice daily (chronic)
Max: 16 mg/day (prescription); 8 mg/day (OTC for up to 48 hours)
Acute and chronic diarrhoea; NOT for diarrhoea with high fever or bloody stool (may mask serious infection); OTC limited to 48-hour course; MHRA 2017 warning against high-dose misuse

Paediatric dose

Dose: Not recommended <2 years; 2–8 years: 1 mg three times daily initially mg/kg
Route: Oral
Frequency: Three times daily
Max: 3 mg/day (2–8 years); 4 mg/day (8–12 years)
Prescription only in paediatrics; acute gastroenteritis: oral rehydration preferred; BNFc

Dose adjustments

Renal

No dose adjustment required

Hepatic

Use with caution in hepatic impairment — reduced first-pass metabolism may increase systemic levels

Paediatric weight-based calculator

Prescription only in paediatrics; acute gastroenteritis: oral rehydration preferred; BNFc

Clinical pearls

  • MHRA 2017 misuse warning: serious cardiac events (ventricular arrhythmias, cardiac arrest) reported with supratherapeutic doses used for opioid-like CNS effects or self-treatment of opioid withdrawal — emphasise OTC 8 mg/48h limit; do not exceed 16 mg/day even on prescription
  • Peripheral opioid mechanism: loperamide is a mu-opioid agonist that does not cross the blood-brain barrier at therapeutic doses (high P-gp efflux) — acts on myenteric plexus to reduce intestinal motility and secretion; no CNS/analgesic/abuse potential at normal doses
  • C. difficile contraindication: using loperamide to control diarrhoea in C. difficile colitis can trigger toxic megacolon — always check for infective cause before prescribing; if antibiotics recently prescribed, stool culture first
  • IBS-D (diarrhoea-predominant IBS): loperamide reduces stool frequency and urgency in IBS-D — NICE CG61 recommends as first-line antidiarrhoeal; does not reduce global IBS symptoms (pain, bloating)
  • Traveller's diarrhoea: loperamide + antibiotic (azithromycin or ciprofloxacin) combination significantly reduces duration of acute traveller's diarrhoea — loperamide reduces stool frequency while antibiotic addresses underlying infection

Contraindications

  • Acute dysentery (blood/mucus in stool with fever)
  • Antibiotic-associated colitis/C. difficile
  • Active ulcerative colitis (toxic megacolon risk)
  • Ileus or bowel obstruction
  • Children <2 years

Side effects

  • Constipation (dose-dependent)
  • Abdominal cramps
  • Nausea
  • Dizziness
  • Dry mouth
  • QTc prolongation and cardiac arrest (high doses — MHRA 2017 warning re misuse/overdose)

Interactions

  • QTc-prolonging drugs — additive risk at supratherapeutic doses (misuse context)
  • P-glycoprotein inhibitors (quinidine, ritonavir) — increase loperamide CNS penetration; risk of opioid CNS effects at therapeutic doses in combination

Monitoring

  • Stool frequency and consistency
  • Abdominal examination (exclude ileus/megacolon)
  • Resolution within 48h (if not improving, reassess for infective cause)
  • ECG (if high-dose or P-gp inhibitor co-prescribed)

Reference: BNFc; BNF 90; BNFc; MHRA Drug Safety Update 2017 (loperamide cardiac events); NICE CG61 (IBS); NICE CKS Diarrhoea; SPC Imodium. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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