Mebendazole probably acts by inhibiting microtubule synthesis; the parent drug appears to be the active form. Efficacy of the drug varies with gastrointestinal transit time, with intensity of infection, and perhaps with the strain of parasite. The drug kills hookworm, Ascaris, and Trichuris eggs.
Mebendazole is indicated for use in ascariasis, trichuriasis, hookworm and pinworm infections, and certain other helminthic infections. It can be taken before or after meals; the tablets should be chewed before swallowing. For pinworm infection, the dose is 100 mg once, repeated at 2 weeks. For ascariasis, trichuriasis, hookworm, and Trichostrongylus infections, a dosage of 100 mg twice daily for 3 days is used for adults and for children older than 2 years. Cure rates are good for pinworm infections and ascariasis but have been disappointing in recent studies of trichuriasis, although efficacy for trichuriasis is better than that of albendazole. Cure rates are also low for hookworm infections, but a marked reduction in the worm burden occurs in those not cured. For intestinal capillariasis, mebendazole is used at a dosage of 200 mg twice daily for 21 or more days. In trichinosis, limited reports suggest efficacy against adult worms in the intestinal tract and tissue larvae. Treatment is three times daily, with fatty foods, at 200–400 mg per dose for 3 days and then 400–500 mg per dose for 10 days; corticosteroids should be co-administered for severe infections.
Adverse Reactions, Contraindications, & Cautions
Short-term mebendazole therapy for intestinal nematodes is nearly free of adverse effects. Mild nausea, vomiting, diarrhea, and abdominal pain have been reported infrequently. Rare side effects, usually with high-dose therapy, are hypersensitivity reactions (rash, urticaria), agranulocytosis, alopecia, and elevation of liver enzymes.
Mebendazole is teratogenic in animals and therefore contraindicated in pregnancy. It should be used with caution in children younger than 2 years because of limited experience and rare reports of convulsions in this age group. Plasma levels may be decreased by concomitant use of carbamazepine, phenytoin, or ritonavir, and increased by cimetidine. Mebendazole should be used with caution in patients with cirrhosis.
Metrifonate is a safe, low-cost alternative drug for the treatment of Schistosoma haematobium infections. It is not active against Schistosoma mansoni or Schistosoma japonicum. It is not available in the USA.
Metrifonate, an organophosphate, is rapidly absorbed after oral administration. After the standard oral dose, peak blood levels are reached in 1–2 hours; the half-life is about 1.5 hours. Clearance appears to be through nonenzymatic transformation to dichlorvos, its active metabolite. Metrifonate and dichlorvos are well distributed to the tissues and are completely eliminated in 24–48 hours.
The mode of action is thought to be cholinesterase inhibition, temporarily paralyzing adult worms, resulting in their transit from bladder vasculature to small arterioles in the lungs, where they are killed. The drug is not effective against S haematobium eggs; live eggs continue to pass in the urine for several months after all adult worms have been killed.