For most healthy adults, the short answer is reassuring: probiotics have a long history of use, and serious problems are uncommon. The longer answer includes some important exceptions, and they're worth knowing before you start.
Here are straight answers to the safety questions people ask most.
Are probiotics safe for healthy adults?
The evidence points to generally safe for most people. In a review for infectious disease specialists, the authors wrote that the long history of safe use, data from clinical trials, and animal and lab studies all support the assumption that probiotics are generally safe for most populations (Doron and Snydman, 2015).
The largest safety review, prepared for the US Agency for Healthcare Research and Quality, included 622 studies. In randomized trials, short-term probiotic use wasn't linked to more adverse events overall (relative risk 1.00) or to more serious adverse events (relative risk 1.06) than in control groups (Hempel et al., 2011).
So why do experts still sound cautious? Because there are real gaps in the data. That same review found that 235 of the 622 studies only described products as "well tolerated" without reporting specific adverse events, that products were often poorly described, and that long-term effects are largely unknown. The authors concluded that the research is "not well equipped" to answer safety questions with confidence, especially about rare events (Hempel et al., 2011).
What side effects are most common?
Controlled trials have mainly reported digestive side effects (Hempel et al., 2011), and gastrointestinal effects are among the possible risks experts list (Doron and Snydman, 2015).
A practical approach: if you notice mild gas or changes in your stool when you start, give it a week or two. If symptoms clearly get worse and stay that way after two to three weeks, stop and reassess.
Who should talk to a doctor before taking a probiotic?
Case reports suggest that people whose health is already compromised are the most likely to have problems with probiotics (Hempel et al., 2011). Check with your doctor first if you:
- Have a weakened immune system, for example from chemotherapy, an organ transplant or immune-suppressing medication
- Are critically ill or in the hospital, especially in intensive care
- Have a central venous catheter or other long-term IV line
- Have acute pancreatitis or have recently had major abdominal surgery
- Are pregnant or nursing
It's also worth asking your pharmacist if you take immune-suppressing drugs, antibiotics or antifungal medication. Bring the product, or a photo of the label, so they can see exactly which strains it contains.
Can probiotics cause infections or other problems?
Rarely, and mostly in people who are already very ill. Systemic infections are one of the recognized theoretical risks (Doron and Snydman, 2015), and the AHRQ review found case reports of fungal and bacterial bloodstream infections potentially linked to probiotic organisms (Hempel et al., 2011).
A study from intensive care units made that link more concrete. Patients given probiotics had a markedly higher risk of Lactobacillus bloodstream infection than those who weren't. Genome sequencing showed the bacteria in patients' blood were essentially indistinguishable from those in the probiotic product, supporting direct transmission from capsule to bloodstream (Yelin et al., 2019).
Experts also list a few theoretical risks, mostly relevant to susceptible people: harmful metabolic activity, excessive stimulation of the immune system, and the transfer of genes between bacteria (Doron and Snydman, 2015). A later expert panel named the possible transfer of antibiotic-resistance genes as one of the research gaps still to be addressed (Merenstein et al., 2023).
Has a probiotic trial ever caused serious harm?
Yes, in a very specific setting. In a Dutch trial, 298 patients with predicted severe acute pancreatitis received a multispecies probiotic or a placebo into the digestive tract twice daily for 28 days. The probiotic didn't reduce infections. And 24 patients (16%) in the probiotic group died, compared with 9 (6%) in the placebo group. Nine people in the probiotic group developed bowel ischemia, eight of them fatally, compared with none on placebo (Besselink et al., 2008).
The researchers concluded that probiotic prophylaxis should not be given to patients in this category (Besselink et al., 2008). These were critically ill hospital patients, not people taking a supplement at home. But the trial is a clear reminder that "natural" doesn't mean harmless in every situation.
Does product quality affect safety?
It can. An expert panel convened by the International Scientific Association for Probiotics and Prebiotics recommended that probiotics intended for patient populations meet stringent quality standards, preferably verified by an independent third party. The panel also emphasized whole genome sequencing, which confirms a strain's identity and checks for genes linked to virulence, toxins and antibiotic resistance (Merenstein et al., 2023).
Two other points from that panel are useful to know. Probiotics made from species without a history of safe use need to be judged case by case. And researchers still need to collect and report side effects more rigorously, which echoes the gaps the AHRQ review found more than a decade earlier (Merenstein et al., 2023).
The same panel concluded that established practices are sensibly addressing the safety of traditional probiotics used by the general population (Merenstein et al., 2023). For everyday shoppers, the practical version is to choose brands that name their strains and can explain how their products are tested.
When should I stop and call a doctor?
If you're healthy and decide to try a probiotic, a few habits make it easier to spot a problem early: start one new product at a time, keep the bottle or a photo of the label, and jot down any new symptoms with the date. Then stop and call your doctor if you notice any of these:
- Fever, chills or feeling seriously unwell after starting a probiotic, especially if you have a central line or a weakened immune system
- Severe or worsening abdominal pain
- Blood in your stool, or black stools
- Symptoms that clearly get worse and stay worse after two to three weeks
- Unexplained weight loss, or new bloating most days with early fullness, pelvic pain or urinary urgency
Sources
- Doron S, Snydman DR. Risk and safety of probiotics. Clin Infect Dis. 2015;60(Suppl 2):S129-34. PMID 25922398
- Hempel S, Newberry S, Ruelaz A, et al. Safety of probiotics used to reduce risk and prevent or treat disease. Evid Rep Technol Assess (Full Rep). 2011;(200):1-645. PMID 23126627
- Yelin I, Flett KB, Merakou C, et al. Genomic and epidemiological evidence of bacterial transmission from probiotic capsule to blood in ICU patients. Nat Med. 2019;25(11):1728-1732. PMID 31700189
- Merenstein D, Pot B, Leyer G, et al. Emerging issues in probiotic safety: 2023 perspectives. Gut Microbes. 2023;15(1):2185034. PMID 36919522
- Besselink MG, van Santvoort HC, Buskens E, et al. Probiotic prophylaxis in predicted severe acute pancreatitis: a randomised, double-blind, placebo-controlled trial. Lancet. 2008;371(9613):651-659. PMID 18279948
This article is for general information and isn't medical advice. Talk to your healthcare provider before starting a probiotic, especially if you're immunocompromised, seriously ill, pregnant, nursing or take prescription medication.
