Main research Question: The investigators would like to find out if both the Bio-K (lactobacillus acidophilus and lactobacillus casei) drink and capsule are equally effective in the prevention of diarrhea associated with antibiotic use.

Why is this research project important? About 25% of patients who use antibiotics may develop diarrhea as a side effect. Symptoms are mild and consist of watery diarrhea and abdominal pain. Some patients may develop a more severe form of diarrhea, called clostridium difficile-associated diarrhea. Clostridium difficile-associated diarrhea may lead to more serious consequences like inflammation of the large bowel and in some cases, death. There are some studies that suggest Bio-K drink, a probiotic preparation containing lactobacillus acidophilus and lactobacillus casei, decreases the risk of diarrhea from antibiotic use. As a result, St. Joseph's Healthcare has recently to use Bio-K routinely to try to reduce risk of patients developing diarrhea. No other Hamilton hospitals use Bio-K routinely as there is not strong evidence about the effectiveness of Bio-K for the prevention of diarrhea. This study will provide more information about Bio-K's effectiveness in the prevention of diarrhea associated with antibiotic use as both a capsule and a drink.

Inclusion Criteria:

• speak English
• capable of providing consent
• reachable by telephone within the next three months
• willing to comply with study protocols
• age over 65 years
• prescribed clindamycin, fluoroquinolones or cephalosporin for more than one dose

Exclusion Criteria:

• ICU patients
• transplant patients
• patients on immunosuppressant medications (prednisone greater than 50 mg/day for 7 days, azathioprine, cyclosporine, cyclophosphamide, tacrolimus, sirolimus, methotrexate, mycophenolate, anti-TNF agents, interleukin-2 chemotherapy)
• HIV patients with CD4+ count less than 250 cells/mm3
• neutropenic patients with total neutrophil count less than 500
• patients with prosthetic heart valves