Patient Basics: Giardiasis (Giardia Infection)

Giardiasis is a gut infection caused by Giardia, a tiny parasite that spreads through water, food, surfaces or hands contaminated with stool. In humans, giardia causes watery or greasy diarrhea, gas, bloating and belly cramps, usually 1 to 2 weeks after exposure. Prescription medicine cures most cases within days.

Key points

• People catch giardia by swallowing the parasite, most often from untreated water in lakes, streams or wells, or from close contact with someone who has it.
• Symptoms include diarrhea, foul-smelling greasy stools that float, gas, cramps, nausea and tiredness. Some people have no symptoms at all.
• A stool test confirms it.
• Tinidazole, metronidazole or nitazoxanide clear the infection. There is no over-the-counter cure.
• Wash hands with soap and water, and boil or filter water from natural sources before you drink it.

How people get giardia

Giardia lives in the intestines of infected people and animals and leaves the body in stool as hardy cysts. The cysts can survive for weeks in cool water and resist the usual amounts of chlorine in pools. Swallowing even a few can cause infection.

Common ways it spreads:

• Drinking untreated water from lakes, rivers, streams or shallow wells, often while camping or hiking
• Swallowing water in pools, hot tubs, lakes or splash pads
• Changing diapers or caring for someone with diarrhea, then touching your mouth
• Eating raw food washed in contaminated water
• Traveling to places without safe drinking water
• Sexual contact that involves the anus

Children in child care, their families, hikers, travelers and people with a weak immune system are at higher risk. Dogs, cats and wild animals such as beavers can carry giardia, which is why it is sometimes called “beaver fever.”

Giardia symptoms in humans

• Watery diarrhea, which can come on suddenly and be explosive
• Greasy, frothy or foamy stools that float and smell very bad
• Gas, bloating and stomach cramps
• Nausea, burping that tastes like rotten eggs, and loss of appetite
• Tiredness
• Weight loss

Giardia usually doesn’t cause fever or blood in the stool. Without treatment, symptoms often last 2 to 6 weeks, and sometimes longer. Some people have symptoms that come and go for months, with weight loss and trouble absorbing food. In children, a long infection can slow growth. After the infection clears, some people can’t digest milk sugar (lactose) well for a few weeks.

How it’s diagnosed

A stool test is the main way to find giardia. Many labs use a test that detects parts of the parasite or its genes, often as part of a panel that checks for several germs at once. Some labs look for the parasite under a microscope. Because the parasite isn’t always shed, you may be asked for more than one stool sample on different days.

How giardia is treated

• Prescription medicine. Tinidazole is often given as a single dose. Metronidazole is taken for several days. Nitazoxanide comes as a liquid that works well for children. Don’t drink alcohol while taking metronidazole or tinidazole, or for a few days after, because it can cause vomiting and flushing. These medicines may leave a metal taste in your mouth.
• Fluids. Drink plenty of water, broth or an oral rehydration drink to replace what diarrhea takes out. This matters most for babies, young children and older adults.
• Food. Eat small, plain meals. Cut back on milk and other dairy for a few weeks if they make gas or diarrhea worse.

If you are pregnant, tell your doctor, because the choice of medicine may change. People without symptoms usually don’t need treatment, though a doctor may treat them to protect others in the household. If symptoms return after treatment, see your doctor again. The infection may not have cleared, you may have caught it again, or you may have trouble digesting lactose.

Going back to work or child care

Children can usually go back to child care once diarrhea has stopped. Food handlers and health care workers may need to wait longer, and some need a clear stool test. Avoid swimming while you have diarrhea and for about 2 weeks after it stops, because you can still shed the parasite.

Lowering your risk

• Wash your hands with soap and water after using the toilet, changing diapers or handling animals, and before preparing food. Alcohol hand gel doesn’t work well against giardia cysts.
• Treat water from natural sources. Bring it to a rolling boil for 1 minute, or 3 minutes above 6,500 feet. You can also use a filter rated to remove cysts, with a pore size of 1 micron or smaller.
• Don’t swallow water when you swim in pools, lakes or rivers.
• When traveling, drink sealed bottled water and skip ice and raw foods that may have been washed in tap water.
• Test your well if it is shallow or near septic systems, livestock or recent flooding.
• Pick up after pets and wash your hands afterward.
• During sex, avoid contact with stool, and use barriers such as dental dams.

When to see a doctor

• Diarrhea lasts more than 3 days in an adult, or more than a day in a baby or young child
• You have diarrhea after camping, travel or drinking untreated water
• There is blood in the stool, a high fever or bad belly pain
• You are losing weight or symptoms keep coming back
• You are pregnant, older or have a weak immune system

Get emergency care for signs of serious dehydration, such as very little or no urine, dizziness when standing, confusion, or a baby with no wet diapers for 6 hours or more.

Common questions about giardia

How long does giardia last without treatment?

Untreated giardia often lasts 2 to 6 weeks, and in some people it drags on for months. The infection does sometimes go away on its own, but treatment shortens the illness to a few days and lowers the chance of passing it to others. See a doctor rather than waiting it out.

Can you treat giardia at home or over the counter?

No over-the-counter medicine cures giardia, and home remedies don’t kill the parasite. You need a prescription from a doctor. At home you can drink plenty of fluids, eat small, plain meals and wash your hands often to keep from spreading it while the medicine works.

What does giardia poop look like?

Giardia often causes loose, pale, greasy stools that float, look frothy or foamy and smell very bad. The diarrhea can be watery and come on suddenly. Blood in the stool is not typical of giardia, so tell a doctor if you see it, because another infection may be the cause.

Can I get giardia from my dog?

It’s possible but less common than people think. Dogs and cats often carry types of giardia that rarely infect people. Still, wash your hands after picking up pet waste, and have your vet check a pet with diarrhea. Most human cases come from water or from other people.

Questions to ask your doctor

• How did I most likely get this infection?
• Should my family or partner be tested?
• What should I avoid while taking this medicine?
• When can my child go back to child care?
• What should I do if the diarrhea comes back?

Learn more

• About giardia infection (Centers for Disease Control and Prevention)
• Giardia infection (MedlinePlus, U.S. National Library of Medicine)

This guide is general information and doesn’t replace advice from your own doctor. Last updated October 2026.