Feedback from around 350 postmenopausal women who were randomized to receive either the herbal, conventional treatment, other supplements or a placebo, failed to support the efficacy of black cohosh reported in other trials.
"Given the serious health risks associated with hormone therapy, we were disappointed to discover that a popular herbal alternative appears to be no more effective than the placebo," said Katherine Newton, lead researcher on the new study published in this month's issue of Elsevier journal Maturitas (16:134-46).
"Regrettably, we found that black cohosh has little potential to play an important role in the relief of vasomotor symptoms."
Since major trials revealed the health risks of hormone replacement therapy (HRT)- until then the most popular and proven therapy for menopause symptoms - the use of supplements containing natural products like black cohosh, soy isoflavones and red clover has surged.
Black cohosh preparations have been approved by the German government as non-prescription medications for treatment of menopausal symptoms and are among the most widely-used natural alternatives to hormone replacement therapy (HRT).
But their use is controversial among some healthcare professionals who say there is little evidence to support their efficacy.
In the new trial, funded by the US National Institute on Aging and the National Center for Complementary and Alternative Medicine, researchers from four institutions in Seattle enrolled followed the women for a year.
They had been randomly assigned to one of five daily regimes: 160mg of black cohosh (supplied by US-based Pure World); a multiple botanical supplement containing 10 different herbs, including black cohosh; the same multibotanical supplement together with counselling to increase dietary soya; hormone therapy (Premarin or Prempro); and placebo.
The participants kept diaries, recording the frequency and intensity of their hot flushes at the start of the trial and at three, six, and 12 months.
At three months the diaries recorded hot flush frequency to be down by 35 per cent with black cohosh, 15 per cent with the multibotanical, 30 per cent with the multibotanical plus dietary counselling, 21 per cent with the placebo, and 88 per cent with hormone therapy. The findings were similar at six and 12 months.
The same pattern was found when the researchers used a menopause symptom score. At three months, the frequency of hot flushes decreased 27 per cent with black cohosh, 14 per cent with the multibotanical, 23 per cent with the multibotanical plus soy, 15 per cent with the placebo, and 76 per cent with hormone therapy. The findings were also similar at six and 12 months.
The authors noted that differences between the herbal regimens and the placebo were not significant but that the difference between the hormone therapy group and the other groups was considered important.