This is a randomized, double-blind, placebo-controlled trial. Patients will be randomly assigned to receive Lactobacillus GG (n = 47) or a placebo (n = 47) within 3 days of initiation of antibiotic therapy. Randomization will occur by using a computerized randomization list. A pharmacist, who is not a member of the study staff, will assign each of the 2 supplement groups a number and label the supplement bottles accordingly.