Taking lactobacillus by mouth or vaginally does not appear to prevent yeast infections in women when they take antibiotics, as frequently happens, according to new research findings.

Australian researchers found that women who took lactobacillus - a “friendly” type of bacteria found in yogurt that many women take to prevent yeast infections - were just as likely to develop yeast infections, or thrush, after a course of antibiotics as women who used inactive “placebo” preparations.

“The trial found no difference in the women who got thrush, whether they were taking lactobacillus, either by mouth or into the vagina, or the dummy medications,” study author Dr. Marie Pirotta told Reuters Health.

She added that she hopes women now think twice before trying lactobacillus to prevent yeast-related vaginitis.

“Armed with this new information, women can either choose to use proven antifungal medications when they develop vaginitis, or discuss treatment with their health care provider,” said Pirotta, who is based at the University of Melbourne.

Pirotta explained that the use of lactobacillus is quite common among women who want to reduce their risk of yeast infections - in one study, up to 40 percent of women who had developed vaginal infections in the past said they had used yogurt or lactobacillus to ward off another infection after taking antibiotics.

To investigate whether this popular treatment works, Pirotta and her colleagues asked 235 female antibiotic-takers to try either a placebo treatment or lactobacillus, either orally or inserted vaginally, and noted who developed a yeast infection.

Nearly one-quarter of women developed yeast infections in the days following their use of antibiotics, and taking lactobacillus did nothing to reduce their risk of infection, the researchers report in the British Medical Journal.

In fact, the researchers chose to end the study early, once they realized they were testing an apparently ineffective therapy.

Pirotta explained that lactobacillus bacteria are the main bacteria present in a healthy vagina, and women who develop yeast infections are just as likely to have a healthy amount of lactobacillus as other women. “So how would adding foreign lactobacillus back into the system help?” she asked.

Pirotta added that some argue lactobacillus may help by making the vagina more acidic, because yeast prefers a less acidic environment. But, again, research shows that women who develop yeast have vaginas that are just as acidic as other women, she noted.

SOURCE: British Medical Journal Online First, 2004.