Parasitic infections are treated with antiparasitic medications matched to the specific type of parasite involved. The treatment can be as simple as a single dose of oral medication or as involved as months of repeated dosing, depending on the organism and where it lives in your body. Getting the right diagnosis first is essential, since the drugs that kill intestinal worms do nothing against single-celled parasites, and vice versa.
Diagnosis Comes First
There is no single test that screens for all parasitic infections. Your provider will choose tests based on your symptoms, travel history, and which parasites are suspected. The most common starting point for gut-related symptoms like diarrhea, cramping, and gas is a stool sample, formally called an ova and parasite test. The CDC recommends collecting three or more stool samples on separate days, because parasites and their eggs don’t always show up in every sample.
Blood tests can detect certain infections that stool exams miss. A blood smear, where a technician examines your blood under a microscope, is used for parasites that live in the bloodstream, such as those causing malaria. Serology tests look for antibodies your immune system produces in response to specific parasites. For infections that cause damage to organs, imaging like X-rays, MRI, or CT scans may be needed. If stool tests keep coming back negative but symptoms persist, an endoscopy or colonoscopy lets a doctor visually inspect the intestinal lining for parasites directly.
Treating Intestinal Worms
Intestinal worms, including roundworms, hookworms, pinworms, whipworms, and tapeworms, are treated with a class of drugs called anthelmintics. These medications typically require just one to three days of oral treatment, and symptoms generally begin to improve within a few weeks.
The most widely used drugs in this category work by disrupting a structural protein that worm cells need to function. Without it, the parasite can no longer absorb nutrients from your gut, can’t reproduce, and eventually dies. The dead or paralyzed worms then pass out of your body naturally through your stool. For some infections like pinworm, a second dose is taken about two weeks later to catch any newly hatched larvae that survived the first round.
Tapeworms and flukes (parasites that can infect the liver, lungs, or blood vessels) often require a different medication. For schistosomiasis, a fluke infection that affects hundreds of millions of people worldwide, treatment involves three oral doses spaced four to six hours apart, all completed in a single day. Liver flukes follow a similar one-day protocol at a slightly higher dose per body weight.
Treating Single-Celled Parasites
Protozoan infections, caused by microscopic single-celled organisms, require a completely different set of drugs. These include infections like giardiasis (a common waterborne illness), amoebic dysentery, trichomoniasis (a sexually transmitted infection), and toxoplasmosis.
Giardiasis is one of the most common parasitic infections in the developed world, and it illustrates how treatment approach matters. A five-to-seven-day course of metronidazole cures over 90% of cases. A single dose of a related drug, tinidazole, achieves the same 92% cure rate in one shot, which makes it a popular alternative since you don’t have to remember a week of pills. By contrast, if metronidazole is taken for only one day, success drops to just 36 to 60%, rising to 67 to 80% with two days of treatment. Completing the full prescribed course makes a measurable difference.
Malaria requires its own specialized medications, selected based on which species of parasite is involved and where the infection was acquired, since drug resistance patterns vary by region. Toxoplasmosis, which most healthy people fight off without symptoms, only needs medication in people with weakened immune systems or during pregnancy, when it can harm the developing baby.
Ivermectin for a Range of Parasites
Ivermectin is one of the more versatile antiparasitic drugs in use. Originally developed for veterinary medicine, it has become a cornerstone of treating several human parasitic diseases. It works by paralyzing the nerve and muscle cells of parasites, which kills them or prevents them from holding on inside your body.
For strongyloidiasis (a roundworm infection that can persist for decades in the body), treatment is typically two days of oral dosing. River blindness, caused by a parasitic worm transmitted through blackfly bites, requires a single dose repeated every six to twelve months until the patient is symptom-free. Ivermectin is also used for lymphatic filariasis, skin-migrating larvae, and even ectoparasites like scabies and head lice, where oral doses are taken on day one and again about a week later. In the case of scabies and certain skin conditions caused by mites, a topical cream version is also available.
Treating Parasites on the Skin
Not all parasites live inside you. Scabies mites burrow into the top layer of skin, while lice attach to hair shafts and feed on blood from the scalp or body. These ectoparasites are treated differently from internal infections.
Scabies is most commonly treated with a topical cream applied from the neck down and left on overnight, then washed off. All household members and close contacts often need to be treated at the same time, even if they don’t have symptoms yet, because scabies spreads easily through prolonged skin contact. Bedding and clothing should be washed in hot water or sealed in a bag for several days, since the mites can’t survive long without a human host.
Head lice treatments include over-the-counter topical products as a first step. If those fail, prescription options or oral medications provide a stronger approach. Oral ivermectin, dosed on day one and day eight, is one such option for stubborn cases.
Side Effects During Treatment
Most antiparasitic medications are well tolerated for the short courses typically prescribed. The most common side effects are gastrointestinal: nausea, stomach cramps, diarrhea, or a metallic taste in your mouth. These tend to be mild and resolve once the medication course ends. Metronidazole, one of the most frequently prescribed antiprotozoals, can cause nausea and should not be combined with alcohol, which intensifies the side effects significantly.
When large numbers of parasites die off at once, some people experience a temporary worsening of symptoms. This reaction happens because dying parasites release proteins that trigger an immune response. It’s more common with heavy worm burdens or infections like river blindness. Providers may prescribe anti-inflammatory medication alongside antiparasitics in these situations to manage the reaction.
Why Reinfection Happens
One of the biggest challenges with parasitic infections is that treatment kills the current infection but doesn’t prevent you from getting reinfected. If you’re exposed to the same contaminated water, undercooked food, or soil that caused the original infection, the cycle can start over. This is especially relevant for travelers, people with ongoing occupational exposure, and communities in regions where sanitation infrastructure is limited.
For pinworm, the most common worm infection in the U.S., reinfection is extremely common in households with young children. The eggs can survive on surfaces, clothing, and bedding for two to three weeks, so thorough cleaning and treating all household members simultaneously is part of breaking the cycle. Good hygiene, including handwashing before eating and after using the bathroom, is the single most effective prevention measure across nearly all parasitic infections.
Drug Resistance Is a Growing Concern
Resistance to antiparasitic drugs has been documented across most classes of dewormers, particularly in livestock like cattle, sheep, goats, and horses. In veterinary medicine, this is already a serious problem globally and within the United States. In human medicine, the picture varies by parasite. Malaria parasites have developed resistance to multiple drug classes over the decades, which is why treatment recommendations differ by geographic region. For intestinal worms and protozoa affecting humans, widespread clinical resistance is less common, but the patterns seen in animals serve as a warning that overuse or incomplete treatment courses could accelerate resistance development. Taking the full course of medication as prescribed, rather than stopping when you feel better, helps reduce this risk.