The lactobacillus acidophilus bacterium, a probiotic that is an active ingredient in many yogurts, does not prevent the thrush infections that commonly occur following a course of antibiotics, says Dr. Marie Pirotta of the University of Melbourne's Department of General Practice. Pirotta headed a clinical trial involving 235 Melbourne women.

The study was published Friday in the British Medical Journal.

The results are a reminder that all medicines, including natural ones, need to be scientifically tested for effectiveness, says Pirotta. The findings were a surprise even to her, she acknowledged, but were so conclusive that the study was cut short.

Participants in the trial took probiotic or placebo medicine orally and vaginally until four days after completion of an antibiotic course, recorded symptoms and provided vaginal swabs. Results showed no difference in the incidence of thrush that occurred among the women from the probiotic or placebo groups.