PHARMACOTHERAPEUTICS: Less than 10% of orally administered mebendazole is absorbed. In fact to be precise, only if you take it with a heavy fatty meal, the bioavailability will exceed 3%. The absorbed drug is protein-bound (> 90%), is rapidly converted to inactive metabolites, primarily during its first pass in the liver, and has a half-life of 2–6 hours. It is excreted mostly in the urine, principally as decarboxylated derivatives. In addition, a portion of absorbed drug and its derivatives are excreted in the bile. Absorption is increased if the drug is ingested with a fatty meal.
PHARMACODYNAMICS: 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.
CLINICAL USES: 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 of age. Cure rates are good for pinworm infections and ascariasis, but have been disappointing in recent studies of trichuriasis. For intestinal capillariasis, mebendazole is used at a dosage of 200 mg twice daily for 21 or more days.
ADVERSE REACTIONS, CONTRAINDICATIONS, AND 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 which includes rash and 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 of age because of limited experience and rare reports of convulsions in this age group. Plasma levels may be decreased by concomitant use of carbamazepine or phenytoin and increased by cimetidine.
Mebendazole should be used with caution in patients with cirrhosis.
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