Parasites are organisms that live on or inside another organism. Parasitic infections, then, are when an unwelcome parasite invades a host organism and damages that host. Antibiotics are medications that kill or retard the growth of parasites, usually without causing significant harm to the person taking them. There are a few antibiotics that are effective against parasites.
Antibiotics versus Antimicrobials
Antibiotics are members of a larger class of medications called antimicrobials. An antimicrobial is any medication that can be used to treat infections caused by microorganisms, including single-celled eukaryotes called protozoa, fungi and worms. In general, it's much easier to develop antibiotics than other types of antimicrobials because the complex cells of organisms like parasites are much more similar to human cells than are the simple, non-nucleus containing cells of bacteria. This also means, unfortunately, that substances that harm parasites often damage human cells as well. For example, most antifungal medications are significantly more toxic than most antibiotics, with the most effective antifungals being the most toxic. However, there are a few anitbiotics that are safe to use in humans and also are effective against certain types of parasitic infection.
Metronidazole, sold under the trade name Flagyl, is effective against infections caused by certain kinds of parasite. Giardiasis is an infection of the intestinal tract by the protozoan parasite Giardia lamblia that causes, in its acute phase, diarrhea, vomiting and cramps and, in its chronic phase, loss of appetite, malaise and fatigue. Giardiasis is extremely common worldwide, and Flagyl is the treatment of choice for it. Trichomonas vaginalis is a protozoan parasite that causes trichomoniasis, a sexually transmitted infection of the vagina in women and urethra in men. Trichomoniasis symptoms are similar to those of vaginal infections caused by yeast and bacteria, so only a doctor can make the diagnosis. In men, there are often no symptoms, but some men experience discharge from the urethra and burning with urination. Flagyl is used to treat trichomoniasis and can also be used to treat the most common type of vaginal bacterial infection, making it a good choice when the diagnosis is uncertain.
The combination of trimethoprim and sulphamethoxazole, sold under the trade name Bactrim, is an antibiotic used to treat Pneumocystis pneumonia, or PCP, a type of lung infection caused by the protozoan parasite Pneumocystis carinii. PCP is rare in people with a normal immune system, but fairly common in those whose immunity is depressed, such as those with untreated HIV infection. Bactrim is also effective at preventing PCP infection in vulnerable individuals, and is sometimes given for that purpose. Bactrim also has some activity against malaria, which is also caused by a parasite.