Guide
Probiotics side effects: what the research reports
Randomized trials found no significant increase in side effects with short-term probiotic use, but harms were poorly reported and long-term effects are largely unknown.
In randomized trials, short-term probiotic use did not cause more side effects than control: a review of 622 studies found no significant increase in overall or serious adverse events. That is weaker than it sounds: safety reporting was often poor and long-term effects are largely unknown. Case studies describe rare bloodstream infections.
What the trials reported
The 2011 AHRQ evidence report on probiotic safety reviewed 622 studies. In the randomized trials, probiotics did not significantly raise the relative risk of overall adverse events (RR 1.00, 95% CI 0.93 to 1.07) or serious adverse events (RR 1.06, 95% CI 0.97 to 1.16) compared with control, and the events reported were primarily gastrointestinal. The report's own conclusion was that the literature "is not well equipped to answer questions on the safety of probiotic interventions with confidence," that rare events are hard to assess, and that long-term effects are largely unknown.
How well harms were reported
A 2018 review in the Annals of Internal Medicine examined 384 randomized trials of probiotics, prebiotics and synbiotics. No harms-related data were reported in 28% of them, 37% gave no safety results, and 80% did not give the number of serious adverse events per study group. The authors concluded that safety cannot be broadly assumed without adequate reporting. Read "well tolerated" in a trial summary with that in mind.
Rare serious events
The AHRQ review noted that a number of case studies described fungemia and some bacteremia, infections of the bloodstream, potentially associated with the administered probiotic organisms, and that controlled trials did not routinely monitor for them. These are rare and reported in case studies, so the frequency is unknown. They are the reason our label tells anyone with a weakened immune system or a medical condition to ask a physician first. See who should not take probiotics.
What our label says
Electric Tiger Probiotic 20 Billion CFU's warning reads: do not exceed the recommended dose; ask a physician before use if pregnant or nursing, under 18, have a weakened immune system or a medical condition; keep out of reach of children; do not use if the seal is broken; store cool and dry. Ingredients are 20 billion CFU at the time of manufacture from 13 listed organisms in one 317 mg row, plus cellulose (a vegetable capsule) and brown rice flour. The panel prints no "Contains" allergen line, but if you have a food allergy, read the full ingredient list. The count per capsule is for that one capsule: none of the research above tested our product or its dose.
Questions people ask
What are the side effects of probiotics?
In the trials reviewed, mostly gastrointestinal events, at no higher rate than control. Our research does not give a symptom list or rates for specific symptoms, and we do not invent one. If a symptom is new after you start a supplement, stop and ask a doctor.
What are the side effects of too much probiotics, or of a high CFU count such as 60 billion?
The reviews pooled many products and doses and did not set an upper limit. Do not exceed the label's dose, and ask a doctor if you want to take more.
Are there side effects in women?
The reviews above did not report results by sex. Women's-health products, including vaginal probiotics, are separate products with separate research and are not covered here.
Are there side effects when taking probiotics with antibiotics?
That has its own article: probiotics and medications.
Do probiotics cause diarrhea?
Gastrointestinal events were the type most reported in trials, at a rate not different from control. If diarrhea is persistent or severe, see a doctor rather than adjusting the supplement.
Do side effects differ for dogs, or for prebiotics added to probiotics?
Pet probiotics are not covered. Combined probiotic-and-prebiotic products are a different product; the safety reviews above covered probiotics, prebiotics and synbiotics in different trials.
Related reading: how much probiotics per day and probiotics benefits: what the research does and doesn't show.
Sources
- Safety of probiotics used to reduce risk and prevent or treat disease Short-term trials found no more side effects with probiotics than with control, but safety reporting was poor and long-term effects are largely unknown.
- Harms Reporting in Randomized Controlled Trials of Interventions Aimed at Modifying Microbiota: A Systematic Review A review of 384 trials found harms were often poorly reported, so safety cannot simply be assumed.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
