Quick answer

In healthy people, probiotics may cause gas, and side effects are usually minor digestive complaints that go away on their own. A 30 day trial in 100 healthy adults reported no adverse events. Rare serious illness from the probiotic microbes has been reported, mostly in people who were severely ill or had weakened immune systems.^1,2,3

Key facts

In a 30 day trial in 100 healthy adults, three single strains and a mix of two caused no change in digestive complaints, bowel movement frequency or stool consistency.²
In a 3 week trial, stool became looser as the daily dose rose from 100 million to 100 billion CFU, and the highest doses were well tolerated.⁴
At least 60 reports since 1966 have linked probiotics containing Saccharomyces yeast to fungal bloodstream illness, many of them in intensive care.³
FDA warned in 2023 that probiotic products given to preterm infants can cause invasive, potentially fatal illness.⁵
Few studies have looked at probiotic safety in detail, so how often side effects happen is not well known.^6,7

What most healthy people notice

In healthy people, probiotics may cause gas. Side effects are usually minor digestive symptoms that go away on their own.^1,3

In a 30 day trial in 100 healthy adults, three single strains and a mix of two of them caused no change in digestive symptoms, bowel movement frequency or stool consistency compared with placebo, and no adverse events were reported.²

In a 3 week trial in healthy young adults, stool became looser as the daily dose of a two-strain mix rose from 100 million to 100 billion CFU, and the high doses were well tolerated.⁴

Both trials used particular strains at set amounts, so they show that those products were tolerated by healthy adults over weeks, not that every probiotic will be. NIH describes the side effects of probiotics as usually minor and limited to the digestive system.^2,3,4

Reviews call the overall safety record of probiotics excellent for healthy people, but note that trials report safety inconsistently and that how often rare harms occur is not well described.^7,8

Rare but serious problems

Rarely, probiotics have been linked to bacteria or fungi in the blood and to infections causing severe illness, mostly in people who were severely ill or had weakened immune systems.^3,6

At least 60 reports since 1966 have linked probiotics containing the yeast Saccharomyces to fungal bloodstream infections. Many of the patients were in intensive care, were fed by tube or vein, had a central venous catheter or had received broad-spectrum antibiotics.³

A systematic review of probiotic safety found that the most serious harms reported were sepsis, fungal bloodstream spread and ischemia of the gut. They were seen mostly in critically ill people in intensive care, critically sick infants, people after surgery or in hospital and people with weakened immunity.⁹

Some case reports did not confirm that the probiotic strain caused the illness, while in others the strain was confirmed to be the culprit. In an analysis of 22,174 intensive care patients in Boston, six of 522 who received Lactobacillus rhamnosus GG had it found in their blood, compared with two of 21,652 who did not receive it.³

Premature babies and infants

FDA reported that probiotic products used in hospitals for preterm infants contributed to invasive disease, including one infant death in 2023, and were linked to more than two dozen other adverse events in the US since 2018.⁵

Do not give a probiotic supplement to a premature baby. FDA has approved no probiotic as a drug or biological product for infants of any age, so ask a pediatrician before giving any probiotic to a baby or child.⁵

Harms researchers worry about

Possible harms also include harmful substances made by the probiotic microbes and the transfer of antibiotic resistance genes to other gut bacteria. Few studies have looked at probiotic safety in detail, so how often side effects happen is not well known.⁶

A review of probiotic risk lists the theoretical ones as bloodstream spread, harmful metabolic activity, excess immune stimulation in susceptible people, gene transfer and digestive side effects. These are described in case reports and experimental models, and how often they happen is not well described.^7,8

What is in the bottle

Some probiotic products have been reported to contain microorganisms other than those listed on the label.⁶

In laboratory tests of eight commercial products, the freeze-dried ones held fewer live organisms than their labels stated, while the liquid ones matched or beat their labels. That is a small test of a few products, but it is a reason to prefer a label that gives each strain and a count through the expiration date.^1,10

Pregnancy and breastfeeding

In trials in pregnancy, Lactobacillus and Bifidobacterium probiotics did not change rates of cesarean birth, birth weight or length of pregnancy, and a 2021 review found no serious problems for mother or infant. One study reported more vaginal discharge and changes in stool with L. rhamnosus and L. reuteri. No trials have tested Saccharomyces yeast in pregnancy, so its safety there is unknown.^11,12

Ask your doctor or midwife before you start a probiotic while you are pregnant or nursing, and bring the label. The reviews pooled many products and are not designed to find rare problems.^11,12

Medicines and who should ask first

Ask your pharmacist or doctor first

Antibiotics can kill helpful bacteria in the digestive tract, and NIH says antibiotic and antifungal medicines might make some probiotics less effective. Ask your pharmacist how to time a probiotic around your medicine, and keep taking the antibiotic exactly as prescribed.^1,13,14,15

Expected from how the two work, not shown in studies

Ask your doctor first if you have a weakened immune system, for example from chemotherapy, HIV, an organ transplant or long-term steroid treatment^1,6
Ask first if you are seriously ill, in hospital, fed through a tube or vein, or have a central venous catheter³
See a doctor, rather than starting a probiotic, if you have fever, vomiting, bloody diarrhea or severe abdominal pain⁶
Ask first if you take an antibiotic or an antifungal medicine¹³

Questions people ask

Is it safe to take probiotics every day?

NIH says probiotics are unlikely to cause harm in healthy people, and a 30 day trial in healthy adults reported no adverse events. There is no official recommendation for healthy people, so ask a provider about long-term use.^1,2,3

Do probiotics have different side effects for women or men?

The sources we read list the same side effects for everyone and give no separate list by sex. Women who are pregnant or nursing should ask a doctor or midwife first.^3,12

Can probiotics cause skin problems?

The NIH and NCCIH pages we read do not list skin reactions as a side effect of probiotics taken by mouth. If you get a rash, hives or swelling after starting one, stop it and see a clinician.^1,6

Is it safe to take probiotics if you have diabetes?

We found no source that gives advice specific to people with diabetes. NCCIH says anyone with a health problem should consult a provider before taking a probiotic supplement, so ask your doctor.⁶

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.