Black cohosh (Actaea racemosa)

Clinical studies

Three randomised controlled trials in female breast cancer patients were identified for black cohosh 13 14 22; two of these were double-blind and placebo-controlled 13 22. Results showed that black cohosh was not significantly better than placebo in relieving hot flushes. Black cohosh administered to patients receiving tamoxifen appeared better than usual care with tamoxifen alone; these findings are, however, not conclusive.

In the first double-blind, placebo-controlled trial (n=85), both black cohosh and placebo groups reported a decline in the number and intensity of hot flushes 13. However, the differences between groups were not statistically significant. Significant differences in improvement were observed only for sweating. Black cohosh intake did not alter follicle-stimulating hormone (FSH) or luteinizing hormone (LH) levels. The major limitations of this trial include the small sample size, the high dropout rate (18.8%), the short treatment period (two months), and the low dose of black cohosh (40 mg/day).

The second double-blind, randomized, cross-over clinical trial 22 involving 132 patients reported a mean decrease in hot flash score of 20% (comparing the fourth treatment week to the baseline week) compared with a 27% decrease for patients receiving placebo (P=0.53). Mean hot flash frequency was reduced by 17% with black cohosh and 26% on placebo (P =0 .36). Thirty-four percent of patients preferred the black cohosh treatment, 38% preferred the placebo, and 28% did not prefer either treatment.

The third trial specifically examined the effectiveness of black cohosh for the relief of hot flushes in women with a history of breast cancer 14. A 12-month treatment period of black cohosh combined with tamoxifen showed significantly better effects on the number and severity of hot flushes compared to tamoxifen (usual care) alone. Forty-two out of 90 patients (46.7%) in the herbal treatment group were free of hot flushes while no patient was without hot flushes in the tamoxifen group. Severe hot flushes were reported by 22 patients (24%) in the intervention group, and 34 (74%) in the tamoxifen group. However, this randomised trial has methodological weaknesses, such as being open-label, not placebo-controlled and not blinded 14.

Biological mechanism

Black cohosh has been demonstrated to have organ-specific oestrogenic effects and is characterised as selective oestrogen-receptor modulator 1. However, the most recent investigations do not support an oestrogen receptor-mediated mechanism of action for black cohosh 2. The mechanism of action of black cohosh is still not well understood. Studies found that an ethanol extract of black cohosh did not stimulate growth of oestrogen- and progesterone-positive breast cancer cells in vitro 15 16. Administration of black cohosh has not changed serum FSH or LH levels or vaginal cytological findings 13 17 18 19.