The new findings should help women look for other methods to control the menopause symptoms, the Mayo Clinic scientists told the American Society of Clinical Oncology (ASCO) conference.
The herbal black cohosh (Cimicifuga racemosa) is used increasingly in Europe to help control hot flushes and is currently one of the leading supplements taken by menopausal women.
But although previous studies have indicated some benefit, results from the new randomized, double-blind, placebo-controlled study appear to counter the earlier evidence.
"The findings demonstrated absolutely no improvement of symptoms when women took black cohosh compared to placebo," said surgeon Barbara Pockaj, the lead physician in the study.
"This finding is extremely important, because we can now say to our patients that black cohosh does not work and we have to try other methods to control their symptoms."
The study involved 132 women divided into two groups. One group took black cohosh pills for four weeks and then a placebo for four weeks. The other group took a placebo during the first four weeks, followed by the black cohosh. The supplements were provided by Hi Health.
Participants in the study kept a diary during a baseline week and through the rest of the treatment period to record the daily number of hot flushes and hot flush scores (measured by assigning points to each hot flush based on severity and then adding the points for a given time period).
Mayo researchers measured patient treatment preferences after completion of both treatment periods by ascertaining which treatment period, if any, the patients preferred: 34 per cent of patients preferred the black cohosh treatment, 38 per cent preferred placebo and 28 per cent did not prefer either treatment.
As many as three out of four women in the United States are said to experience hot flushes during menopause. They occur with varying frequency and severity, causing frequent sleep interruptions that can affect a woman's mood or overall health.
Until recently many women used hormone replacement therapy (HRT) to control the side effects of menopause but new evidence that HRT can raise the risk of cancers and heart attack encouraged women to search for natural alternatives.
However in 2003, UK researchers published a review of herbal remedies in the journal Menopause that found little evidence to support their efficacy. They said that black cohosh showed promise but that this was limited by the 'poor methodology of the trials' carried out to date.
The new trial was sponsored by a grant from the National Cancer Institute and conducted by a network of researchers led by the North Central Cancer Treatment Group (NCCTG), a national clinical research group funded by the National Cancer Institute.
Certain surgical or medical treatments for cancer can cause menopause to begin earlier and more suddenly than it would normally occur; so, many women undergoing treatment for cancer must also cope with hot flashes.