Antibiotic-Associated Diarrhea (AAD) is a common complication of antibiotic use. The frequency of AAD can be high (26 - 60%) or moderate (13-29%) during hospital outbreaks and is relatively infrequent in outpatients. The risk factors for AAD include broad-spectrum antibiotics, host factors (age, health status, and gender), hospitalization period and exposure to nosocomial pathogens. AAD occurs 2-8 weeks after exposure to antibiotics as a result of disrupting normal intestinal microflora. One of the roles of normal intestinal microflora is to act as a protective barrier that resists the colonization of intestinal pathogens. These patients are susceptible to infection by opportunistic pathogens without this protective barrier. Probiotic therapy is suited to AAD and Clostridium difficile disease. Probiotics assist in reestablishing the disrupted intestinal microflora, enhancing immune responses and clearing pathogens and their toxins from the host. Studies using probiotics have been reported for the past twenty-eight years (1977~2005), but the studies have been variable in trial designs and types of probiotics, had differing doses and durations of treatment, and thus have yielded controversial results. The investigators will conduct a multi-center, randomized, placebo-controlled, double-blind trial to assess the efficacy of the probiotic Lactobacilli (Lacidofil cap®) for the prevention of AAD in adults.