Probiotics, the beneficial bacteria found in yogurt and other cultured foods, have long been touted for their ability to ease digestive woes. The strongest evidence for probiotics is in treating diarrhea caused by a viral infection or from taking antibiotics. Do probiotics also work for the opposite problem — constipation? A report from King’s College in London showed that taking probiotics can help soften stools, making them easier to pass, and can increase the number of weekly bowel movements. What we don’t know is which probiotic species and strains are most effective, how much to take, and for how long.