Pyrantel Pamoate

Pyrantel pamoate is an oral tetrahydropyrimidine anthelmintic used for roundworm and pinworm infections and, at repeated dosing, hookworm. It is poorly absorbed, acts in the gut, and is safe in young children and pregnancy. It is less effective than albendazole for hookworm and ineffective for whipworm.

Class. Anthelmintic; tetrahydropyrimidine.

Mechanism. Depolarising neuromuscular blocking agent in nematodes, causing spastic paralysis and expulsion of the worm.

Pharmacokinetics. Poorly absorbed from the gut (under 15%); most is excreted unchanged in faeces. Absorbed drug is metabolised and excreted in urine.

Ascariasis (adults and children). 11 mg/kg (as base), maximum 1 g, as a single dose.

Enterobiasis. 11 mg/kg (as base), maximum 1 g, as a single dose, repeated after 2 weeks.

Hookworm. 11 mg/kg (as base) once daily for 3 days (maximum 1 g daily).

Loading dose, maintenance dose and titration. Not applicable. Maximum: 1 g daily.

Main alternatives. Albendazole, mebendazole.

Contraindications. Hypersensitivity; concurrent piperazine (antagonises its effect).

Serious adverse effects. Hepatotoxicity with transient rise in transaminases (uncommon), and hypersensitivity (rare).

Common adverse effects. Nausea, vomiting, abdominal cramps, diarrhoea, headache, dizziness and rash.

Interactions. Piperazine antagonises it; theophylline levels rise; may add to the effect of other neuromuscular blockers.

Common prescribing errors. Using the salt dose and not the base dose, and expecting a single dose to treat whipworm or heavy hookworm.

Special groups. - Pregnancy: considered acceptable when needed. - Breastfeeding: compatible. - Children: from 6 months; dose by weight. - Older adults: usual dose. - Kidney impairment: no adjustment. - Liver impairment: caution; monitor liver tests if severe.

Formulations. Tablets 250 mg (base); suspension 250 mg/5 mL (base).

Administration. With or without food; may be taken with milk or fruit juice; no purgative needed.

Baseline. Diagnosis by stool microscopy where available, pregnancy status, liver tests in liver disease.

Monitoring. Egg clearance on repeat stool test 2 to 4 weeks after in selected patients.

Stopping thresholds. Stop for hypersensitivity or marked liver enzyme rise.

Referral and escalation. Specialist review for persistent infection, heavy worm burden with obstruction, and anaemia from hookworm.