TABLE 52–5 Treatment of amebiasis. Not all preparations are available in the USA.1
Clinical Setting Drugs of Choice and Adult Dosage Alternative Drugs and Adult Dosage
Mild to moderate intestinal infection
Hepatic abscess, ameboma, and other extraintestinal disease Luminal agent: Diloxanide furoate,2 500 mg 3 times daily for 10 days
Iodoquinol, 650 mg 3 times daily for 21 days
Paromomycin, 10 mg/kg 3 times daily for 7 days
Metronidazole, 750 mg 3 times daily (or 500 mg IV every 6 hours) for 10 days
Tinidazole, 2 g daily for 3 days
Metronidazole, 750 mg 3 times daily (or 500 mg IV every 6 hours) for 10 days
Tinidazole, 2 g daily for 3 days
Metronidazole, 750 mg 3 times daily (or 500 mg IV every 6 hours) for 10 days
Tinidazole, 2 g daily for 5 days
Tetracycline, 250 mg 3 times daily for 10 days
Erythromycin, 500 mg 4 times daily for 10 days
Tetracycline, 250 mg 3 times daily for 10 days
Dehydroemetine2 or emetine,2 1 mg/kg SC or IM for 3–5 days
Dehydroemetine2 or emetine,2 1 mg/kg SC or IM for 8–10 days, followed by (liver abscess only) chloroquine, 500 mg twice daily for 2 days, then 500 mg daily for 21 days 1Route is oral unless otherwise indicated. See text for additional details and cautions. 2Not available in the USA.
Metronidazole, a nitroimidazole (Figure 52–3), is the drug of choice in the treatment of extraluminal amebiasis. It kills trophozoites but not cysts of E histolytica and effectively eradicates intestinal and extraintestinal tissue infections. Tinidazole, a related nitroimidazole, appears to have similar activity and a better toxicity profile. It offers simpler dosing regimens and can be substituted for the indications listed below.
Pharmacokinetics & Mechanism of Action
Oral metronidazole and tinidazole are readily absorbed and permeate all tissues by simple diffusion. Intracellular concentrations rapidly approach extracellular levels. Peak plasma concentrations are reached in 1–3 hours. Protein binding of both drugs is low (10–20%); the half-life of unchanged drug is 7.5 hours for metronidazole and 12–14 hours for tinidazole. Metronidazole and its metabolites are excreted mainly in the urine. Plasma clearance of metronidazole is decreased in patients with impaired liver function. The nitro group of metronidazole is chemically reduced in anaerobic bacteria and sensitive protozoans. Reactive reduction products appear to be responsible for antiprotozoal and antibacterial  activity. The mechanism of tinidazole is assumed to be the same.
1.
Amebiasis—Metronidazole or tinidazole is the drug of choice in the treatment of all tissue infections with E histolytica. Neither drug is reliably effective against luminal parasites and so must be used with a luminal amebicide to ensure eradication of the infection.
2.
Giardiasis—Metronidazole is the treatment of choice for giardiasis. The dosage for giardiasis is much lower than that for amebiasis, and the drug is thus better tolerated. Efficacy after a single treatment is about 90%. Tinidazole is at least equally effective, and can be used as a single dose.