First of all, let’s talk a little bit about terminology. By definition, an antibiotic consists of a molecule able to treat bacterial infection, while an anti-parasitic is used to treat parasitic infections. The latter class includes antiprotozoal agents treating protozoan parasites infections and anti-helminthics treating infections caused by helminths. Furthermore, antimicrobial agent is a more global term including all the drugs used to treat the different types of microorganisms. However, this is only a matter of terminology and boundaries are not so clear in real life as exemplified below.
It is also relevant to know that antimicrobial agents, such as antibiotics and anti-parasitic agents, do not necessarily kill microorganisms. In fact, there are two main classes of antimicrobial agents: the first one actually kill microorganisms while the second one inhibit its growth without killing it allowing the immune system to get rid of it on its own. In practice, some antimicrobial agents can play both roles depending of the concentration used, meaning that growth will be inhibited at low concentration and microorganisms will be killed at higher concentration.
In practice, existing molecules are tested in laboratory to find any effects against a particular type of microorganism. If an antibiotic already used in clinic (or any other type of molecule) is effective against a parasite, it could be used to fight it after clinical trials showing efficiency and potential side effects. For example, it is the case of the anticancer drug miltefosine, which has been proven to be active against the protozoan parasite Leishmania and the human trematode Schistosoma mansoni. Furthermore, efficiency of a particular molecule can also be seen by accident when doctors note that treating one condition with this molecule has also an effect on another condition present in the same patient.
Since the different parasites represent a heterogenic group with different characteristics, it is normal that molecules treating them can vary greatly. For example, the protozoan parasite Leishmania has a lipidic membrane that is more like the ones of fungi. As such, some antifungal molecules, such as Amphotericin B, are usually effective against this parasite.
One example of antibiotic used against parasite is metronidazole, commonly branded under the name Flagyl. This drug is part of the nitroimidazole antibiotics group and is indicated for anaerobic bacteria, which represent bacteria that do not need oxygen to live, and protozoa. It is used to fight infections caused by Entamoeba histolytica, Giardia intestinalis and Trichomonas vaginalis. Tinidazole, an antibiotic belonging to the same class as metronidazole, can also be used to fight Entamoeba histolytica infections.
Paromomycin is another example of antibiotic used against parasites. It belongs to the aminoglycoside antibiotic group. It is used to fight infections caused by Cryptosporidium and Entamoeba histolytica. It is also effective against the cutaneous form of leishmaniasis.
Another example of antibiotic used to treat parasitic infections is represented by the combination of trimethoprim/sulfamethoxazole. The former is part of the dihydrofolate reductase inhibitors class of antibiotics, while the latter is a sulfonamide bacteriostatic antibiotic. This combination of antibiotics is known under several brand names. It is the drug of choice to treat symptomatic infections with the protozoan parasite Toxoplasma gondii.
Finally, as shown in the different examples above, even though it is usually not their first therapeutic indication, some antibiotics can also be effective against parasites. However, it is only a small proportion of them that are used to treat parasitic infections usually treatable using anti-parasitic drugs.
Source: Centers for Disease Control and Prevention