Other antiprotozoals
Antiprotozoal drugs outside the main groups: pyrimethamine for toxoplasmosis, atovaquone for Pneumocystis, nitazoxanide for Giardia and Cryptosporidium.
What they are
A mixed group of drugs against protozoa (and Pneumocystis, a fungus treated like a protozoan) that do not belong to the antimalarial, nitroimidazole, antitrypanosomal or antileishmanial classes. Each has its own target and its own key infection: toxoplasmosis, Pneumocystis pneumonia, babesiosis, giardiasis, cryptosporidiosis and intestinal amoebiasis.
Mechanism of action
Each member works differently. Pyrimethamine inhibits the parasite's dihydrofolate reductase, so with a sulfonamide it blocks two steps of folate synthesis. Atovaquone imitates ubiquinone and blocks the mitochondrial cytochrome bc1 complex. Nitazoxanide and its metabolite tizoxanide interfere with pyruvate-ferredoxin oxidoreductase (PFOR), an energy enzyme of anaerobic protozoa.
Target: Parasite dihydrofolate reductase (pyrimethamine), mitochondrial cytochrome bc1 (atovaquone) and pyruvate-ferredoxin oxidoreductase (nitazoxanide)
Drugs in this class
- PyrimethamineAntifolate that, with sulfadiazine and folinic acid, is the classic treatment for toxoplasmosis; it can suppress the…Parasite dihydrofolate reductase (bifunctional DHFR-thymidylate synthase of Toxoplasma and Plasmodium)
- AtovaquoneOral suspension for Pneumocystis pneumonia and prophylaxis when trimethoprim-sulfamethoxazole cannot be used; with…Mitochondrial cytochrome bc1 complex (complex III), Qo site, of the parasite or fungus
- NitazoxanideOral broad-spectrum antiparasitic for diarrhoea caused by Giardia and Cryptosporidium; well tolerated, it may tint the…Pyruvate-ferredoxin oxidoreductase (PFOR) of anaerobic protozoa and bacteria
- ParomomycinAminoglycoside that is barely absorbed by mouth: it clears amoebae from the bowel lumen. An injectable form treats…Decoding (A) site of the 16S ribosomal RNA, small ribosomal subunit
Others in the class
- Diloxanide (luminal amoebicide)
- Paromomycin
- Iodoquinol
Test yourself
Think of the answer, then tap the question to see it.
Why is folinic acid (leucovorin) given with pyrimethamine?
At treatment doses pyrimethamine also causes folate deficiency in the patient; folinic acid bypasses the blocked step and restores normal blood cell production.
Which drug of this group is used for cryptosporidiosis in immunocompetent people?
Nitazoxanide.
On which enzyme complex does atovaquone act?
The mitochondrial cytochrome bc1 complex (complex III), at the ubiquinone (Qo) site.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PYRIMETHAMINE tablet (DailyMed (U.S. National Library of Medicine), 2026); Nitazoxanide: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.