What you should take to prevent diarrhea depends on the cause. Probiotics, fiber supplements, digestive enzymes, and bismuth subsalicylate (the active ingredient in Pepto-Bismol) all work, but each targets a different trigger. Picking the right option means knowing whether you’re dealing with antibiotics, travel, food intolerances, or recurring loose stools.
Probiotics for Antibiotic-Related Diarrhea
Antibiotics kill off beneficial gut bacteria along with the harmful ones, and that imbalance causes diarrhea in roughly one out of five people taking them. Probiotics are the most effective preventive measure here, but the strain matters. Two options have the strongest evidence behind them.
Saccharomyces boulardii is a beneficial yeast, not a bacterium, which means antibiotics don’t destroy it while you’re taking them. In clinical trials, only about 5.7% of people taking this probiotic alongside antibiotics developed diarrhea, compared to 19% of those on a placebo. The typical adult dose is 500 mg once daily for prevention, started at the same time as the antibiotic course and continued for a few days after finishing it.
Lactobacillus rhamnosus GG (often labeled LGG) is the other well-studied option. A meta-analysis of 11 randomized trials found it cut the risk of antibiotic-associated diarrhea nearly in half, from 22.4% down to 12.3%. The evidence is strongest in children. For adults, the benefit was clearest in people taking antibiotics for stomach ulcer treatment. You’ll find LGG in products like Culturelle, typically in capsule form.
Start either probiotic on the first day of your antibiotic course. Taking it a few hours apart from your antibiotic dose helps ensure the probiotic organisms survive long enough to colonize your gut.
Bismuth Subsalicylate for Travel
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, is the most accessible over-the-counter option for preventing traveler’s diarrhea. A JAMA study found that taking two tablets four times daily (totaling about 2.1 grams per day) significantly reduced diarrhea in travelers to high-risk regions. This regimen can be used safely for up to three weeks.
The compound works by coating the stomach lining, reducing inflammation, and having a mild antibacterial effect against the pathogens you’re most likely to encounter in contaminated food and water. It does turn your tongue and stool black temporarily, which is harmless.
One important limitation: bismuth subsalicylate contains a salicylate, the same class of compound found in aspirin. It should not be given to children or teenagers because of the risk of Reye’s syndrome, a rare but serious condition affecting the liver and brain. Adults who take blood thinners or are allergic to aspirin should also avoid it.
Fiber Supplements for Recurring Loose Stools
If you deal with chronic or recurring loose stools rather than a single episode, psyllium husk (sold as Metamucil and generic equivalents) can help normalize stool consistency. Psyllium is a soluble fiber that absorbs excess liquid in the intestines, forming a bulkier, more solid stool. While most people think of it as a constipation remedy, doctors also prescribe it for diarrhea-predominant conditions like irritable bowel syndrome.
Start with a small dose (one teaspoon or one capsule) mixed into a full glass of water and increase gradually. Taking too much too quickly can cause bloating and gas. Consistency matters more than dose: daily use over several weeks produces better results than occasional use.
Lactase for Dairy-Related Diarrhea
If dairy products reliably give you loose stools, cramping, or gas, you’re likely not producing enough of the enzyme that breaks down lactose. Undigested lactose pulls water into the intestines, causing osmotic diarrhea. Lactase supplements (sold as Lactaid and store brands) replace the missing enzyme and prevent symptoms entirely when used correctly.
The key is timing: take the supplement with your very first bite of dairy food. Taking it after you’ve already eaten or waiting until symptoms start won’t help, because the enzyme needs to be present in your gut at the same time as the lactose.
Zinc for Children in High-Risk Settings
The World Health Organization recommends zinc supplementation for children who experience frequent diarrheal episodes, particularly in developing regions where diarrheal disease is a leading cause of childhood illness. The recommended dose is 20 mg per day for children older than six months and 10 mg per day for younger infants, given for 10 to 14 days. Zinc supplementation reduces both the severity of current episodes and the likelihood of new episodes for two to three months afterward. This recommendation is most relevant for families in areas with limited access to clean water and sanitation.
Rotavirus Vaccine for Infants
Rotavirus is the most common cause of severe diarrhea in young children worldwide. Two oral vaccines are available in the United States. RotaTeq is given in three doses at 2, 4, and 6 months of age. Rotarix is given in two doses at 2 and 4 months. The first dose of either vaccine needs to be given before 15 weeks of age, and all doses should be completed before 8 months. These vaccines have dramatically reduced hospitalizations for childhood diarrhea since their introduction.
Hygiene Habits That Actually Work
No supplement replaces basic hygiene, especially for preventing viral diarrhea caused by norovirus, the bug behind most stomach flu outbreaks. One finding that surprises many people: alcohol-based hand sanitizers are not effective against norovirus. A survey of 161 long-term care facilities found that those relying primarily on hand sanitizer for routine hygiene were six times more likely to experience norovirus outbreaks than facilities emphasizing soap and water. Among facilities that preferentially used sanitizers, 53% had confirmed norovirus outbreaks, compared to just 18% of those that favored hand washing. The CDC now recommends against using hand sanitizer as a substitute for soap and water when norovirus is a concern.
When traveling to areas with questionable water safety, the standard precautions still apply: drink bottled or boiled water, avoid ice in drinks, skip raw fruits and vegetables you can’t peel yourself, and eat food that’s been cooked and served hot. These simple steps prevent more cases of traveler’s diarrhea than any pill.
Matching the Right Option to Your Situation
- Starting antibiotics: Saccharomyces boulardii or LGG probiotics, taken from day one through a few days after the course ends.
- Traveling to a developing region: Bismuth subsalicylate tablets four times daily, plus strict food and water hygiene.
- Chronic loose stools or IBS: Daily psyllium husk fiber, gradually increased to a comfortable dose.
- Dairy triggers diarrhea: Lactase enzyme taken with the first bite of any meal containing dairy.
- Young children with recurrent episodes: Zinc supplementation for 10 to 14 days, following WHO guidelines.
- Infants: Rotavirus vaccination starting at 2 months of age.
Prevention works best when it’s specific to the cause. A probiotic won’t help lactose intolerance, and a fiber supplement won’t protect you from contaminated street food in Southeast Asia. Identifying your trigger is the first step toward choosing something that actually works.