Can probiotics cause diarrhea or loose stools?

Key facts

• Starting a new probiotic could trigger short-term diarrhea, constipation or stomach pain, more so at a large dose or with a sensitive gut. [Cleveland Clinic]
• "Side effects of probiotics are usually minor and consist of self-limited gastrointestinal symptoms, such as gas." [NIH Office of Dietary Supplements]
• Few studies have looked at probiotic safety in detail, so solid information on how often side effects happen, and how severe they are, is lacking. [NCCIH]
• Diarrhea means loose, watery stools three or more times a day, or more often than is normal for you. [NIDDK]
• Adults should talk with a doctor right away about diarrhea lasting more than 2 days, a high fever, 6 or more loose stools a day, or stools that are black and tarry or contain blood. [NIDDK]
• Alcohol, caffeine, high-fat foods and sugar alcohols can make acute diarrhea worse. [NIDDK]

Can probiotics cause diarrhea? What Cleveland Clinic and NIH say

They can, for a short time. Cleveland Clinic takes the question head on: in the short term, a new probiotic could bring on diarrhea, constipation or stomach pain, especially if you take a large dose or your gut tends to be sensitive.

It also gives a reason. Many probiotics produce short-chain fatty acids in the gut. Those byproducts are useful, but a sudden rise in them can loosen stools for a while:

"a sudden influx of them could cause temporary diarrhea."

Cleveland Clinic, Probiotics

Physician's Choice summarizes the federal view this way: According to the NIH Office of Dietary Supplements, probiotics are generally well tolerated by healthy people, though they may cause mild digestive symptoms, such as gas. Probiotics may not be appropriate for everyone, particularly people who are seriously ill or have weakened immune systems. Talk with your healthcare provider before use.

NCCIH, another part of NIH, adds a limit: studies suggest probiotics usually have few side effects, but few have examined their safety in detail, and the data on long-term safety are limited.

How long do probiotic side effects last?

For gas and bloating, a few days, in Cleveland Clinic's words. For loose stools it says only that they are temporary, and no source we checked gives a number of days, so the number to hold on to is NIDDK's limit for diarrhea in adults: 2 days.

The two fit together. Cleveland Clinic expects the extra gas and bloating from a new probiotic to settle within a few days and calls the diarrhea temporary. NIDDK says diarrhea of any cause that runs past 2 days is a reason to talk with a doctor right away. So mild changes can be given a little time; diarrhea past 2 days, or anything worse, does not get waited out.

MedlinePlus sets a longer limit for adults, 5 days, for diarrhea that does not get better and comes with no other warning signs; this page uses NIDDK's shorter one.

Our guide to gas in the first weeks of a probiotic covers gas on its own, and smelly gas from probiotics covers a change in its smell. All the side effects sources list, with what is known about how often they happen, are gathered in probiotic side effects. Stomach pain, nausea and heartburn from a new probiotic have their own guide: can probiotics cause stomach pain or nausea?

What should you do if a probiotic gives you loose stools?

Keep up your fluids, look at what else changed, and set yourself a limit. Cleveland Clinic calls short, mild spells temporary; the warning signs in the table do not wait.

1. Drink plenty of fluids. MedlinePlus puts this first in home care for diarrhea, so that you do not get dehydrated.
2. Ease off what makes it worse. NIDDK lists alcohol, drinks with caffeine, high-fat foods, foods heavy in fructose or lactose, and sugar alcohols such as those in sugarless gum and candies.
3. Read the other ingredients. If what you started is a gummy, look for sugar alcohols on the label, since NIDDK lists them among the things that can make diarrhea worse.
4. Take it the way the label says. Cleveland Clinic notes that some probiotics work better with food and others on an empty stomach.
5. Do not let it delay care. NCCIH is direct about this: "Don't use probiotics as a reason to postpone seeing your health care provider about any health problem."

Is it the probiotic, or something else?

Often something else. NIDDK says the most common causes of acute diarrhea are viral gastroenteritis, food poisoning and medicine side effects.

MedlinePlus calls the stomach flu the most common cause, a mild viral infection that usually clears up within a few days, and names laxatives that contain magnesium among the medicines that can cause diarrhea. What NIH says about magnesium supplements and loose stools is in supplements for bloating. Loose stools that start the day after a meal out, or while others at home are sick, may point somewhere other than the capsule you started last week.

If you changed several things in the same week, NIDDK's advice for longer-running symptoms helps: keep a food journal of what you eat and drink, what symptoms you have and when they occur, and take it to your doctor.

Can probiotics cause constipation?

Sometimes, early on. Cleveland Clinic lists constipation next to diarrhea and stomach pain as possible in the short term when you start a new probiotic. None of the sources we checked says how often it happens.

NIDDK describes constipation as fewer than three bowel movements a week, or stools that are hard, dry, lumpy or difficult to pass, and lists not eating enough fiber and not drinking enough liquids among its causes. If it does not go away with self-care, NIDDK says to see a doctor. What to do about it, step by step, is in our answer to can probiotics cause constipation?