Antiprotozoal Drugs: Beyond Malaria
Malaria gets the headlines, but a whole family of single-celled parasites causes diarrhoea, vaginal and gut infections, and serious tropical diseases. The good news: a handful of familiar drugs — led by one you already know from anaerobic bacteria — covers most of what you'll meet in practice.
Protozoa are single-celled animal-like parasites — bigger and more complex than bacteria, but far smaller than worms. Some live in the gut and cause dysentery; one causes a common vaginal and urinary infection; others, spread by insects, cause devastating diseases like sleeping sickness and leishmaniasis. You don't need to memorise every exotic organism. What pays off is recognising a few common infections and the small set of drugs that treat them — and noticing that the star player is a drug you've already met.
Metronidazole — the workhorse
The same drug that kills anaerobic bacteria kills key protozoa. You already met metronidazole as the go-to drug for anaerobic bacteria — and it's exactly the same chemistry that makes it a front-line antiprotozoal. It's activated inside organisms that live without oxygen, and several important protozoa fit that bill. Metronidazole (or its cousin tinidazole) is the treatment for three very common infections: trichomoniasis (a sexually transmitted infection causing vaginitis and urethritis — treat the partner too), giardiasis (a watery diarrhoea from contaminated water), and amoebiasis (Entamoeba dysentery and liver abscess). The same practical warnings carry over from the antibacterial chapter: a strong disulfiram-like reaction with alcohol (flushing, vomiting — so no alcohol during treatment), and a metallic taste. For amoebiasis there's an extra wrinkle: after metronidazole clears the invading form, a second 'luminal' drug (such as paromomycin) is given to wipe out cysts left in the gut and prevent relapse or spread.
The rest of the toolkit
Beyond metronidazole, a few drugs cover the other protozoa. Leishmaniasis — spread by sandflies, causing skin ulcers or a lethal disease of the internal organs — is treated with liposomal amphotericin B (the same antifungal you met, here repurposed) or older pentavalent antimony compounds. The trypanosomes cause African sleeping sickness and, in the Americas, Chagas disease; these are treated with specialised agents (such as nifurtimox and benznidazole for Chagas) that are largely the domain of tropical-medicine specialists. And a couple of drugs deserve a mention because they turn up elsewhere: pentamidine treats some of these parasitic infections and also Pneumocystis pneumonia in immunocompromised patients, and nitazoxanide is a broad antiprotozoal used for cryptosporidium and giardia. The practical takeaway is not to memorise every regimen, but to know that most everyday protozoal infections in general practice come down to metronidazole.
- Metronidazole/tinidazole treats trichomoniasis, giardiasis, and amoebiasis.
- It's the same drug used for anaerobic bacteria — activated in oxygen-poor organisms.
- Warnings carry over: disulfiram-like reaction with alcohol; metallic taste.
- Amoebiasis needs a second luminal drug (paromomycin) to clear gut cysts.
- Leishmaniasis → liposomal amphotericin B; trichomoniasis → treat the partner too.
The single most useful fact here is a link back to the antibacterial chapter: metronidazole is a two-in-one drug. Its mechanism — being activated only inside organisms that live without oxygen — makes it lethal to both anaerobic bacteria AND anaerobic protozoa. That's why one drug you learned for a dental abscess or C. difficile is the very same one you reach for in trichomonas, giardia, and amoebic dysentery. Learn metronidazole once, and you've covered a huge slice of both bacterial and protozoal infection — alcohol warning and all.
- Drinking alcohol with metronidazole — disulfiram-like reaction (flushing, vomiting).
- Treating trichomoniasis in one partner only — reinfection follows; treat both.
- Stopping after metronidazole in amoebiasis — add a luminal agent for cysts.
- Assuming all 'parasites' need the same drug — protozoa and worms differ entirely.
Which single drug treats trichomoniasis, giardiasis, amoebiasis, AND anaerobic bacterial infections?
- Metronidazole/tinidazole is first-line for trichomoniasis, giardiasis, and amoebiasis.
- Same drug and mechanism as for anaerobic bacteria; no alcohol during treatment.
- Amoebiasis: add a luminal agent (paromomycin) to clear gut cysts.
- Leishmaniasis → liposomal amphotericin B; specialist drugs for trypanosomes.
- Katzung BG. Basic & Clinical Pharmacology — Antiprotozoal Drugs.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Chemotherapy of protozoal infections.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antiprotozoal drugs.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antiprotozoal drugs.
