Possibly. But for the most common probiotics, there's no evidence that it's harmful.

"The good thing about the probiotics that are used now, lactobacillus and bifido bacteria, is that both of those just completely lack the genes that are required to cause disease," said Gary Huffnagle, a professor of immunology and microbiology at the University of Michigan School of Medicine. "So even from a theoretical standpoint, it would be extremely difficult to cause disease. And from a practical standpoint, we have never seen in, despite billions of doses in billions of people."

On the other hand, said Huffnagle, the author of "The Probiotics Revolution," "I would be worried about" some other probiotics on the market, particularly those taken from the soil called "homeostatic soil organisms," or HSOs. "There's not enough evidence of safety for probiotics from the soil. But lactobacillus and bifidobacterium are all found in food and you are safe when dealing with food bacteria," he said. He added that he has no financial ties to any of the companies that make yogurt or probiotic supplements.

If you are immunocompromised, hospitalized, or recovering from surgery, skip probiotics until you are well again, Huffnagle said.