Probiotic supplements abound in health food stores these days. The science doesn’t back up many of the bold health claims, though, and there’s minimal federal oversight or quality control of the products, which contain live bacteria meant to supplement or alter the microbial community already present in your gut.

Read more: Probiotics make bold health claims, but the science is shaky

Doctors say there’s no proven medical reason for most people to take probiotic supplements. But they’re unlikely to cause any harm. If you do want to give them a try, here are some tips to make sure you’re getting what you expect:

  • The bacteria should be present in adequate amounts, which means billions of CFUs, or colony-forming units. The human gut contains trillions of resident bacteria, so scientists estimate at least one billion probiotic organisms are needed to make an impact via the intestine.
  • The bacteria should be living. If they are dead or inactivated, they’re not probiotics.
  • Be wary of products sitting on a shelf or in high temperatures for long periods of time. If bacteria experience any moisture or heat, they’ll begin to grow, use up the resources around them, and die. “If you pick a product in a shop window in Texas in the middle of summer, good luck,” said Gregor Reid, director of the Canadian Research and Development Centre for Probiotics at the Lawson Health Research Institute.
  • The bacteria should confer a health benefit, backed up by human trials that showed the probiotics had a positive effect on a specific condition. A group of experts recently compiled the clinical evidence for 40 brand-name probiotic products in Canada, many of which are also sold in the United States. Check out their ratings for each product, Levels I-III, based on the strength of evidence for specific health conditions.

Keep in mind that certain individuals should not take probiotics. Those with bowel damage due to injury, surgery, chemotherapy, or severe infection should steer clear due to the risk that the bacteria could migrate out of the intestines and into the bloodstream, said Dr. Shira Doron, a physician and epidemiologist at Tufts Medical Center. Individuals with compromised immune systems should also be wary, she added, since the immune system is needed to keep gut bacteria in check.

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