A dewormer is a medication that kills or expels parasitic worms living inside the body. These drugs, also called anthelmintics, are used in both humans and animals to treat infections caused by roundworms, hookworms, tapeworms, whipworms, and other parasitic worms that typically settle in the intestines. Dewormers are among the most widely used medications in the world, essential in veterinary care and still commonly prescribed for people in regions where parasitic infections are prevalent.
How Dewormers Work
Parasitic worms depend on specific biological processes to move, feed, and stay anchored inside a host. Dewormers interrupt those processes in different ways depending on the drug class. Some cause the worm’s muscles to seize up in a permanent contraction, a spastic paralysis that leaves the worm unable to hold on to the intestinal wall. Others do the opposite, relaxing the worm’s muscles so completely that it goes limp and gets swept out with normal digestion.
Another group of dewormers works more slowly by blocking the worm’s ability to absorb nutrients or build the internal structures its cells need to survive. These drugs bind to proteins inside the worm’s cells and prevent them from forming the scaffolding that keeps cells functioning. Without that scaffolding, the worm starves and dies over a period of days. Still other dewormers target the worm’s outer covering, making it permeable to calcium, which disrupts the parasite’s ability to regulate its own body chemistry.
The key to all these mechanisms is selectivity. Dewormers target biological features that are different enough between parasites and their hosts that the drug harms the worm without significantly affecting the person or animal taking it.
Major Drug Classes
Three main families of dewormers dominate modern use. Benzimidazoles, introduced in the 1960s, include albendazole and mebendazole. They work by dismantling the worm’s cellular infrastructure and are effective against a wide range of parasites including roundworms, whipworms, and hookworms. These are typically taken by mouth.
Imidazothiazoles, which arrived in the 1970s, include levamisole. These drugs activate receptors on the worm’s muscle cells, forcing them into paralysis. The third major class, macrocyclic lactones, includes ivermectin and was introduced in the 1980s. These paralyze the worm’s feeding apparatus by forcing open specific channels in its nerve cells. In livestock, macrocyclic lactones come in injectable and pour-on forms, while benzimidazoles are usually given orally.
A few important drugs fall outside these three families. Praziquantel is widely used against tapeworms and the parasites that cause schistosomiasis. Pyrantel pamoate, which paralyzes roundworms and hookworms through a different mechanism, is one of the most accessible dewormers available.
Dewormers for Humans
Parasitic worm infections in humans are far more common than many people realize. Soil-transmitted helminths, including roundworms (ascariasis), whipworms, and hookworms, infect hundreds of millions of people worldwide. In the United States, the CDC lists four oral medications available for treating these infections: albendazole, mebendazole, ivermectin, and pyrantel pamoate.
For pinworms, the most common worm infection in the U.S., pyrantel pamoate is available over the counter without a prescription. It comes as an oral suspension, capsules, or chewable tablets and is given as a single dose based on body weight, then repeated two weeks later to catch any worms that were eggs during the first dose.
Treatment for other worm types is straightforward but varies. A single dose of albendazole can clear a roundworm or hookworm infection, while whipworm typically requires three days of treatment. Mebendazole follows a similar pattern, with a short course of twice-daily doses for most infections. These prescription medications are generally well tolerated. The most common side effects are digestive: bloating, gas, and diarrhea. Serious reactions like signs of liver problems (yellowing of the skin or eyes) or severe allergic reactions are rare.
Dewormers for Pets
Puppies and kittens are frequently born with worm infections or pick them up from their mother’s milk, which is why deworming starts early. The Companion Animal Parasite Council recommends beginning treatment at just 2 weeks of age, repeating every 2 weeks until regular broad-spectrum parasite prevention begins. If year-round prevention isn’t maintained, the recommended schedule is every 2 weeks until 2 months old, then monthly until 6 months, and quarterly after that. Adult dogs and cats on a maintenance plan should receive broad-spectrum treatment four times a year.
Pet dewormers come in chewable tablets, liquids, granules mixed with food, and topical formulations. Many monthly heartworm preventatives for dogs also contain a deworming ingredient that controls intestinal parasites simultaneously. Your veterinarian can run a fecal test to check for eggs in your pet’s stool and recommend the right product based on what parasites are present in your area.
Dewormers in Livestock
For cattle, sheep, goats, and horses, deworming is a core part of herd health management. Livestock pick up worm larvae from contaminated pasture, and heavy infections cause weight loss, anemia, diarrhea, and in severe cases, death. Delivery methods include oral drenches (liquid squirted into the mouth), injectable formulations, pour-on solutions applied to the skin, and medicated feed or mineral blocks.
Unlike pets and humans, where treatment is given when an infection is confirmed, livestock deworming has traditionally followed set schedules. That approach is now shifting because of drug resistance, one of the biggest challenges in animal agriculture.
Why Drug Resistance Matters
Every population of parasitic worms contains a small number of individuals that carry genes making them naturally resistant to a given dewormer. When a whole herd is treated at once, the susceptible worms die and the resistant ones survive, passing those genes to the next generation. After enough rounds of treatment with the same drug class, the resistant worms dominate and the dewormer stops working.
Resistance is already widespread in sheep and goat parasites and growing in cattle. The mechanisms are varied: some resistant worms pump the drug out of their cells before it can work, while others have altered the receptor the drug targets so it no longer binds effectively. A dewormer is considered to have a resistance problem when it fails to reduce egg counts by at least 95% in a fecal egg count reduction test. A reduction below 70% signals a serious concern.
The primary strategy for slowing resistance is called “refugia,” which means intentionally leaving a portion of the worm population unexposed to the drug. In practice, this means not treating every animal. Instead, farmers use targeted selective treatment, dosing only the animals showing clinical signs of infection, poor body condition, or high parasite burdens. In sheep flocks where a blood-feeding stomach worm is the main threat, a simple color chart that scores the animal’s level of anemia helps identify which individuals actually need treatment. The untreated animals maintain a population of drug-susceptible worms on the pasture, diluting out the resistant genes.
How Infections Are Diagnosed
In humans, diagnosis usually involves a stool sample examined under a microscope for eggs, or in the case of pinworms, a tape test pressed against the skin around the anus first thing in the morning. A doctor may also order blood tests if a tissue-migrating parasite is suspected.
In animals, fecal egg counts are the standard diagnostic tool. A small stool sample is mixed with a salt or sugar solution that floats parasite eggs to the surface, where they can be counted under a microscope. These counts are useful for monitoring pasture contamination, testing whether a dewormer is still effective, and identifying animals that are naturally more resistant to infection. However, egg counts alone don’t always reflect the actual number of worms inside the animal, since different species produce eggs at very different rates. That’s why veterinarians combine egg counts with clinical signs like weight loss, rough coat, diarrhea, or anemia when deciding whether treatment is needed.