3.
Trichomoniasis—Metronidazole is the treatment of choice. A single dose of 2 g is effective. Metronidazole-resistant organisms can lead to treatment failures. Tinidazole may be effective against some of these resistant organisms.
Nausea, headache, dry mouth, and a metallic taste in the mouth occur commonly. Infrequent adverse effects include vomiting, diarrhea, insomnia, weakness, dizziness, thrush, rash, dysuria, dark urine, vertigo, paresthesias, encephalopathy, and neutropenia. Taking the drug with meals lessens gastrointestinal irritation.
FIGURE 52–3 Structural formulas of other antiprotozoal drugs.
Pancreatitis and severe central nervous system toxicity (ataxia, encephalopathy, seizures) are rare. Metronidazole has a disulfiramlike effect, so that nausea and vomiting can occur if alcohol is ingested during therapy. The drug should be used with caution in patients with central nervous system disease. Intravenous infusions have rarely caused seizures or peripheral neuropathy. The dosage should be adjusted for patients with severe liver or renal disease. Tinidazole has a similar adverse-effect profile, although it appears to be somewhat better tolerated than metronidazole.
Metronidazole has been reported to potentiate the anticoagulant effect of coumarin-type anticoagulants. Phenytoin and phenobarbital may accelerate elimination of the drug, whereas cimetidine may decrease plasma clearance. Lithium toxicity may occur when the drug is used with metronidazole. Metronidazole and its metabolites are mutagenic in bacteria and tumorigenic in mice. Data on teratogenicity are inconsistent. Metronidazole is thus best avoided in pregnant or nursing women, although congenital abnormalities have not clearly been associated with use in humans.
Iodoquinol (diiodohydroxyquin), a halogenated hydroxyquinoline, is an effective luminal amebicide. Pharmacokinetic data are incomplete but 90% of the drug is retained in the intestine and excreted in the feces. The remainder enters the circulation, has a half-life of 11–14 hours, and is excreted in the urine as glucuronides. Iodoquinol is effective against organisms in the bowel lumen but not against trophozoites.
Infrequent adverse effects include diarrhea—which usually stops after several days—anorexia, nausea, vomiting, abdominal pain, headache, rash, and pruritus. Some halogenated hydroxyquinolines can produce severe neurotoxicity with prolonged use. Iodoquinol is not known to produce these effects at its recommended dosage, and this dosage (Table 52-5) should never be exceeded. Iodoquinol should be taken with meals to limit gastrointestinal toxicity. It should be used with caution in patients with optic neuropathy, renal or thyroid disease, or nonamebic hepatic disease. The drug should be discontinued if it produces persistent diarrhea or signs of iodine toxicity (dermatitis, urticaria, pruritus, fever). It is contraindicated in patients with intolerance to iodine.