Black cohosh is a tall perennial herb originally found in the northeastern United States. Native Americans used it primarily for women's health problems, but also as a treatment for arthritis, fatigue, and snakebite. European colonists rapidly adopted the herb for similar uses. In the late nineteenth century, black cohosh was the principal ingredient in the wildly popular Lydia E. Pinkham's Vegetable Compound for menstrual cramps.
What Is Black Cohosh Used for Today?
Black cohosh’s main use today is for the treatment of
. Meaningful but far from definitive evidence indicates that black cohosh extract might reduce hot flashes as well as other symptoms of menopause.
In the past, black cohosh was believed to be a phytoestrogen, a plant-based substance that has actions similar to estrogen. However, as we describe below, growing evidence indicates that black cohosh does not have general estrogen-like actions. Rather, it may act like estrogen only in certain places: the brain (reducing hot flashes), bone (potentially fighting osteoporosis), and vagina (reducing vaginal dryness).
Black cohosh has also been tried for reducing hot flashes in women who have undergone surgery for
, but it does not appear to be effective for this purpose.
Finally, black cohosh is sometimes recommended as a kind of general women’s herb, said to be effective for a variety of menstrual issues, such as
premenstrual syndrome (PMS)
and irregular menstruation. However, there is as yet no meaningful evidence at all that the herb is effective for these conditions.
What Is the Scientific Evidence for Black Cohosh?
The body of evidence regarding black cohosh for menopausal symptoms remains incomplete and inconsistent.
The best study was a 12-week,
double-blind, placebo-controlled trial
of 304 women with menopausal symptoms.
This study appeared to find that black cohosh was more effective than placebo. The best evidence was for a reduction in hot flashes. However, the statistical procedures used in the study were somewhat unusual and open to question.
Promising results were also seen in a 3-month, double-blind study of 120 menopausal women.
Participants were given either black cohosh or fluoxetine (Prozac). Over the course of the trial, black cohosh proved more effective than fluoxetine for hot flashes, but fluoxetine was more effective than black cohosh for menopause-related mood changes.
Previous smaller studies have found improvements not only in hot flashes but also in other symptoms of menopause. For example, in a double-blind, placebo-controlled study, 97 menopausal women received black cohosh, estrogen, or placebo for 3 months.
The results indicated that the herb reduced overall menopausal symptoms (such as hot flashes) to the same extent as the drug.
In addition, microscopic analysis showed that black cohosh had an estrogen-like effect on the cells of the vagina. This is a positive result because it suggests that black cohosh might reduce vaginal thinning. However, black cohosh did not affect the cells of the uterus in an estrogen-like manner; this too is a positive result, as estrogen’s effects on the uterus are potentially harmful. Finally, the study found hints that black cohosh might help protect bone. However, a great many of the study participants dropped out, making the results less than reliable.
One study, too small to have reliable results from a statistical point of view, found black cohosh equally effective as 0.6 mg daily of conjugated estrogens.
A study reported in 2006 found that black cohosh has weak estrogen-like effects on vaginal cells and possible positive effects on bone (specifically, stimulating new bone formation).
An earlier study also found multiple benefits with black cohosh, but its results are difficult to trust. This trial followed 80 women for 12 weeks and compared the effects of black cohosh, estrogen, and placebo.
Again, black cohosh improved menopausal symptoms and vaginal cell health. However, in this study estrogen proved less effective than placebo. This result is so difficult to believe that it casts serious doubt on the meaningfulness of the results.
Several other studies are also often cited as evidence that black cohosh is useful for various symptoms of menopause, but in reality they prove nothing at all.
These trials lacked a placebo group. Although women reported improvements in symptoms, there is no way to know whether black cohosh was responsible. Women given placebo reliably report improvements in menopausal symptoms too; a 50% reduction in hot flashes is fairly typical.
Thus, it is possible that the benefits seen in these studies had nothing to do with black cohosh.
A substantial (244-participant) double-blind study published in 2007 compared black cohosh against the synthetic hormone tibolone, and found them equally effective for treating menopausal symptoms.
Though not approved as a drug in the US, tibolone does appear to be effective for menopausal symptoms, therefore these results are somewhat promising.
However, this study lacked a placebo group, and since the placebo effect is powerful for this condition, this omission significantly reduces the meaningfulness of the result.
One interesting double-blind study evaluated a combination therapy containing black cohosh and
St. John's wort
in 301 women with general
as well as depression.
The results showed that use of the combination treatment was significantly more effective than placebo for both problems.
A smaller study using a combination of the same two herbs found improvements in overall menopausal symptoms as well as
In contrast, several other studies failed to find benefit. For example, in a 12-month, double-blind, placebo-controlled study of 350 women, participants were given either black cohosh, a multibotanical containing 10 herbs, the multibotanical plus soy, standard hormone replacement therapy, or placebo.
The results showed that hormone replacement therapy was more effective than placebo, but the other treatments were not. In addition, a double-blind study of 122 women failed to find statistically significant benefits with black cohosh as compared to placebo,
as did another study enrolling 132 women,
as well as one that involved 124 women using a black cohosh/soy isoflavone combination.
These negative outcomes were possibly due to the relatively small sizes of the black cohosh groups. In a condition such as menopausal symptoms, where the placebo effect is strong, and when the treatment is relatively weak, large numbers of participants are necessary to show benefit above and beyond the placebo effect. Nonetheless, this is an impressive number of negative studies, and some question must remain about the efficacy of this herb.
The bottom line: Black cohosh may be modestly effective for reducing hot flashes and other symptoms of menopause, but doubts remain.
Some interesting information has developed regarding how black cohosh may work. In the past, the herb was described as a phytoestrogen, a plant-based chemical with estrogen-like effects. However, subsequent evidence indicates that black cohosh is not a general phytoestrogen, but may act like estrogen in only a few parts of the body: the brain (reducing hot flashes) and bone (potentially helping to prevent or treat osteoporosis), and, perhaps to some extent, in the vagina.
It does not appear to act like estrogen in the breast or the uterus.
If this theory is true, black cohosh is a selective-estrogen receptor modifier (SERM) somewhat like the drug raloxifen (Evista). However, more evidence is needed to establish the facts of the matter.
Because of it estrogenic properties, researchers investigated whether black cohosh might be helpful in women who were having difficulty conceiving. Women with unexplained
who were not responding clomiphene, a commonly used medication to induce ovulation, were randomly divided into two groups. Both groups continued to receive clomiphene, but the women in one of the groups also received 120 mg of black cohosh. Pregnancy rates were significantly higher in the black cohosh plus clomiphene group compared to the clomiphene only group.
Breast Cancer Survivors
Women who have had treatment for breast cancer frequently experience hot flashes, often but not always due to the use of the estrogen-antagonist medications like
. Estrogen treatment is not an option for this problem, as it might increase risk of cancer recurrence. Because black cohosh does not seem to have estrogen-like actions in the breast, researchers felt safe to try it in 85 women who had undergone treatment for breast cancer.
Unfortunately, the results were not encouraging: in this 2-month, double-blind, placebo-controlled trial, black cohosh did not reduce hot-flash symptoms.
Black cohosh seldom produces any side effects other than occasional mild gastrointestinal distress. One rigorous study looked for possible deleterious effects on cholesterol levels, blood sugar, and blood coagulability, and did not find any.
Studies in rats have found no significant toxicity when black cohosh was given at 90 times the therapeutic dosage for a period of 6 months.
Since 6 months in a rat corresponds to decades in a human, this study appears to make a strong statement about the long-term safety of black cohosh.
Unlike estrogen, black cohosh does not stimulate breast-cancer cells growing in a test tube.
However, black cohosh has not yet been subjected to large-scale studies similar to those conducted for estrogen. For this reason, safety for those with previous breast cancer is not known. Also, because of potential hormonal activity, black cohosh is not recommended for adolescents or pregnant or nursing women.
There are a growing number of case reports in which it appeared that use of a black cohosh led to severe liver injury.
However, it is not clear whether the cause was black cohosh itself, or a contaminant present in the product.
One highly preliminary study found that black cohosh might reduce the effectiveness of the chemotherapy drug
Safety in young children or those with severe liver or kidney disease is not known.