One of the major problems associated with the use of antibiotics is the occurrence of antibiotic-related diarrhea. Although most such cases of diarrhea are harmless, in individual cases it may lead on to fulminant colitis. In 10-20% of the patients, the causal factor is infection with Clostridium difficile, which multiplies unhindered in patients on antibiotics. In the meantime, controlled, randomized studies have shown that probiotics can exercise a preventive effect in antibiotic-related diarrhea. The administration of Saccharomyces boulardii or Lactobacillus rhamnosus GG reduces the relative risk of an antibiotic-related diarrhea developing by 0.5. The data are more uniform in children than in adults.