Generally safe — for healthy people
NCCIH states that probiotics have 'an extensive history of apparently safe use, particularly in healthy people' [1]. For most people, common side effects are mild and digestive (gas, bloating) and settle with time. But 'generally safe' is not 'safe for everyone.'
Who should be cautious
NCCIH is specific that risk is greater in vulnerable groups [1]:
- People who are seriously ill or immunocompromised — including those with weakened immune systems from disease or treatment — face a higher risk of harmful effects, including infections from the organisms themselves.
- Premature infants: NCCIH notes that 'cases of severe or fatal infections have been reported in premature infants who were given probiotics,' and the FDA has warned health care providers about this risk [1].
- People with central lines, short-bowel syndrome, or other serious conditions may also be at higher risk.
Why the risk exists
Probiotics are live microorganisms. In someone whose immune defenses are compromised, organisms that are harmless in a healthy gut can occasionally cause infection — the opposite of the intended benefit.
Quality matters too
Because probiotics are live, strain, dose (CFUs at end of shelf life), and storage affect what you actually get (see what CFU means and probiotics complete guide). Contamination is a concern for any supplement, and more so for vulnerable users (see heavy metals and adulteration).
Practical guidance
- Healthy people can generally use probiotics, starting low to limit gas/bloating [2].
- Seriously ill, immunocompromised people, and parents of premature infants should consult a provider before using probiotics.
- Tell your provider about probiotics, especially around hospitalization, surgery, or chemotherapy (see [supplements during cancer treatment](/learn/supplements-and-chemotherapy-caution)).