A widely used herbal supplement marketed to ease problems associated with an enlarged prostate appears to be useless, providing no more relief than a placebo, scientists will report in a study published today.

The extract, derived from an indigenous Florida palm, is so popular that the number of men who take it in the United States is estimated at 2 million a year. Most have been sold on the widely held belief that saw palmetto provides a reprieve from round-the-clock trips to the rest room. Indeed, dozens of smaller studies have supported the supplement’s efficacy.

But Dr. Stephen Bent of the Osher Center for Integrative Medicine in San Francisco found in the most rigorous study ever conducted on the supplement’s impact on benign prostatic hyperplasia, that it has no more therapeutic value than a placebo, which mimicked saw palmetto capsules in every way. The dummy capsules had an oily substance, they contained a brown-coloring agent, and carried the supplement’s pungent odor.

The yearlong analysis, involving 225 men, was designed to be identical to studies in the past that had found a benefit when two 160-milligram capsules were taken daily. By study’s end, 40 percent of men in the saw palmetto group and 46 percent on placebo thought they had gotten the genuine article.

Curiously, participants in both groups improved during the trial, but there was no dramatic difference among those taking saw palmetto, which suggested the effect could be attributed to mind-over-matter.

“This adds to the doubt in a major way,” Bent said. His findings appear in today’s New England Journal of Medicine.