Some manufacturers do a better job of delivering viable microbes (identified on the label as CFU, short for colony-forming units) than others. Gary Huffnagle, PhD, a professor at the University of Michigan Medical Center, recommends Align, Culturelle, Florastor, Jarrow, and Theralac.

Freeze-drying, which removes all water from the microbes, makes them more likely to survive in pill form. When you take the supplement, the fluid in your digestive tract reanimates the bacteria.

Although some high-quality supplements don't require refrigeration, most do. Lower temperatures increase the length of time microbes can survive.

Look for a supplement that contains different species of bacteria, such as Lactobacillus and Bifidobacteria. “You want a variety in your probiotics just as you want variety in your diet,” says nutritionist Elizabeth Lipski, PhD, CCN, author of Digestive Wellness (McGraw-Hill, 1995). And look for a different formulation the next time you shop for supplements. “I generally have people take one supplement for a while and then switch so they get exposed to a wide range of benefits.”

Because probiotics need food as soon as they reanimate, a supplement that contains a prebiotic, such as inulin, can improve the flora's chances of surviving in your digestive system.

“For the average, healthy person, a dose between 5-10 billion microorganisms is protective and preventive, even if you take the supplements only a few times a week. If you've been on antibiotics or are recovering from illness, aim for 30 billion,” Huffnagle recommends. Look for supplements that clearly state each dose's CFU. Avoid those that list ingredients only by weight.

Because probiotics contain live microbes, they won't remain viable forever. Make sure to purchase in quantities that will enable you to finish the supplements before the expiration date.