Saw palmetto, a plant extract sold as a natural boost to urinary health, doesn't improve the symptoms of enlarged prostate in middle-aged men, a U.S. study said.

Even at high dosages, men taking saw palmetto daily didn't report any more improvement in symptoms, such as needing to urinate frequently or urgently, than those taking a drug-free placebo pill, according to findings published in the Journal of the American Medical Association.

"Even pushing up to the highest dose ... we couldn't see a benefit greater than placebo," said Michael Barry at Massachusetts General Hospital in Boston, who led the study.

The plant extracts are sold by many supplement and vitamin companies and are available over-the-counter for a few dollars for a month's worth of capsules. They're marketed to support prostate and urinary health.

For the study, Barry and his colleagues randomly assigned 369 men with urinary symptoms to take either saw palmetto capsules or a placebo daily for almost a year and a half. Every few months, researchers stepped up the dose until men taking saw palmetto were getting 960 milligrams each day.

Urinary symptoms were measured on a scale from 0 to 35 based on men's own reports. A two-point decrease in score is considered a "slight" improvement, while at three points men can notice a difference in their everyday lives.

From the onset to the close of the study, men in the saw palmetto group had an average decrease in symptom scores from 14.2 to 12.2 -- a 2.2-point improvement. Men getting the placebo started out with an average score of 14.7, which decreased three points to 11.7.

There was no difference in symptoms between men taking any dose of saw palmetto versus those in the placebo group.

"We've always known that there's a pretty big placebo effect with bothersome urinary symptoms," Barry told Reuters Health.

Barry said the findings don't mean that nobody should take the plant extract, even if its only benefit may come from men believing it will help.

"It appears there were essentially no side effects, and that some men improved. I wouldn't object to men giving it a try to see how it worked for them," he added.

GNC, which manufactures and sells multiple types of saw palmetto capsules to support "healthy prostate function," declined to comment on the study.

But Stephen Bent, who studies alternative therapies at the University of California, San Francisco, and didn't take part in the study, said that the research showed "very definitively" that saw palmetto doesn't ease the urinary trouble caused by an enlarged prostate.

"The science shows that this really is not likely to be a useful product," he said.

Medications known as alpha blockers and 5-alpha-reductase inhibitors are also used to treat urinary symptoms related to an enlarged prostate, and there's much more evidence that they're effective, Barry said. For severe symptoms, prostate surgery can also be considered.

For mild or moderate cases, Bent said, another option is "watchful waiting," because symptoms may get better or worse on their own over time.