"A lot of studies have shown an association between antibiotic use and resistance," said Dr. Herman Goossens, lead author of the report in the Feb. 10 issue of The Lancet. "But all those studies are based on indirect evidence. There have been no randomized, placebo-controlled, double-blind studies. Ours is the first study to show directly that antibiotic use leads to resistance," he said.

The study was performed at University Hospital Antwerp in Belgium, where Goossens is professor of medical microbiology.

The study showed that "antibiotics will have a tremendous effect on normal flora [microbes] which will exist for at least six months," Goossens said. It also showed that different antibiotics in the same family can have strikingly different effects on these bugs.

In the study, healthy volunteers were given either azithromycin or clarithromycin, members of the macrolide family of antibiotics, while a third group received a placebo.

The researchers periodically tested samples of the Streptococcus family of bacteria, obtained from individuals in each group, to see if they had developed any resistance to the macrolide drugs.

As expected, resistance levels rose in the two groups given the antibiotics -- by 50 percent after eight days in the clarithromycin group, and by more than 53 percent.