A herb extract for men with a prostate condition has no more effect on it than a dummy version, a study has found.
Many men with prostate problems take the extract
Saw palmetto is taken to improve urinary symptoms in men with an enlarged prostate gland.
US researchers carried out a year-long study of 225 men, none of whom knew if they were taking the real herb or not.
Prostate experts in the UK said the New England Journal of Medicine study contradicted anecdotal reports from men of benefits from saw palmetto.
Having an enlarged prostate - also known as benign prostate hyperplasia (BPH) - is very common, affecting about one third of men over 50.
The US research was carried by doctors at the San Francisco VA Medical Center and the Northern California Kaiser Permanente Division of Research.
Patients, who all had enlarged prostates - were randomly assigned to take either the herbal extract or the dummy pill twice a day.
They were assessed regularly to see if there had been any change in their condition, or in urinary function.
Dr Stephen Bent, who led the research, said: "If you look at the change in symptoms over time between the two groups, it was almost identical.
"There was no statistically significant difference at any time point during the study."
The researchers also looked at particular groups of patients, comparing those with more and less severe symptoms and those with larger and smaller prostates - but found no difference in any of the subgroups between the herbal extract and the dummy version.
Dr Bent added: "The results of this study clearly do not support a strong clinical benefit of saw palmetto."
But he said it was possible other groups of patients might respond differently to the herbal extract.
He said the findings of this study did contradict earlier research.
However, he said: "Prior studies were generally small in size and short in duration.
"Plus, the vast majority of them did not use the standard symptom score that we used for assessing the severity of BPH."
He said it had also previously been difficult to mimic the distinctive and strong smell and taste of saw palmetto - making it easier for patients to know during the study whether or not they are taking the real thing.
Debbie Clayton, head of information at the UK's Prostate Cancer Charity, said: "This is an interesting study because it contradicts the findings of previous studies.
"We await the results of future trials to see if these results can be duplicated.
"We also need to see the effect of higher doses studied - the doses available from high street shops in the UK, are higher than those used in this study."
She added: "Men report anecdotally that they find saw palmetto helps with their symptoms of BPH.
"We would advise men who are considering taking saw palmetto to discuss this with their doctor first, especially as it can mask a man's true PSA l[prostate specific antigen] level.
"We would also advise men to be aware that saw palmetto does not prevent prostate cancer."