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You’ve probably heard that probiotics—the active cultures in some yogurts, for example—are good for your health. But not all probiotics are created equal.  Here’s what you need to know to boost immunity, soothe itchy skin, quell an upset stomach, and more.

Michelle Klawiter was nine days into a course of antibiotics for a sinus infection when the gut pain hit. Bloody diarrhea quickly followed. The 42-year-old secretary and mother of three in Chandler, Ariz., had developed a nasty intestinal infection, the kind that sometimes occurs when antibiotics kill your body’s good bacteria along with the bad and lower your defenses to other invaders.

Doctors prescribed a series of increasingly potent antibiotics to try to knock out the new bad bug, Clostridium difficile (C. diff), but nothing worked. Her tummy troubles turned into antibiotic-associated colon inflammation.

And even after three months, “sometimes the pain and cramping was so bad I thought I would pass out,” Klawiter says. Having lost 20 pounds and feeling unable to cope at her job in a small office with two bathrooms, she quit.

The nightmare ended only after Klawiter tried a probiotic that her gastroenterologist had read was effective against C. diff. Klawiter found the over-the-counter product, called Florastor, at her drugstore. “Within a week,” she recalls, “I was eating normally again.”

Scientists have known for decades that probiotics can boost your health. That’s why yogurt’s “active cultures,” or good bugs, are touted so often. And now these bugs can be found in cereal, cheese, energy bars, soup, and a wealth of other products filling up grocery and pharmacy shelves. Their labels promise everything from fewer tummy aches to faster cold and flu recovery.

But probiotics aren’t all created equal, says University of Western Ontario microbiologist Gregor Reid, PhD, who has studied them. The strain that helped Klawiter—Saccharomyces cerevisiae boulardii—probably won’t do a thing for eczema, and vice versa, studies show. In fact, Klawiter had tried eating more yogurt and popping generic probiotic supplements, including acidophilus. Still her condition worsened because she wasn’t using the right kind of probiotics.

So how do you decide which one to take? Read on for tips on which probiotics help your most common complaints—then check out this handy food and supplement guide.

Next: Fight antibiotic side effects; get well “down there”