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# Other antiprotozoals

Antiprotozoal drugs outside the main groups: pyrimethamine for toxoplasmosis, atovaquone for Pneumocystis, nitazoxanide for Giardia and Cryptosporidium.

Updated Nov 1, 2026

## 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)

## Key points

- Toxoplasmosis: pyrimethamine + sulfadiazine, always with folinic acid (leucovorin) to protect the bone marrow.
- Folic acid is not a substitute for folinic acid: folinic acid is already reduced and bypasses the blocked enzyme in human cells.
- Atovaquone is an option for Pneumocystis pneumonia (mild to moderate) and prophylaxis when trimethoprim-sulfamethoxazole cannot be used; with azithromycin it treats babesiosis.
- Nitazoxanide treats diarrhoea from Giardia and Cryptosporidium; it is less effective in immunocompromised people and may turn urine greenish.

## Drugs in this class

- [**Pyrimethamine** Antifolate 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)](https://durgical.com/en/pharmacology/pyrimethamine/)
- [**Atovaquone** Oral 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](https://durgical.com/en/pharmacology/atovaquone/)
- [**Nitazoxanide** Oral 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](https://durgical.com/en/pharmacology/nitazoxanide/)
- [**Paromomycin** Aminoglycoside 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](https://durgical.com/en/pharmacology/paromomycin/)

## 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.

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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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=2c57a992-619b-40ce-9395-f12ac47a2c67) (DailyMed (U.S. National Library of Medicine), 2026); [Nitazoxanide: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a603017.html) (MedlinePlus (U.S. National Library of Medicine), 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=P01AX) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/classes/other-antiprotozoals/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/classes/other-antiprotozoals/