The latest study doesn’t look good for black cohosh. Here’s the clickativity .
Our take: The study was performed by the National Institutes of Health  (your friendly government agency), and looked at just over 300 women – so, not exactly a conclusive study. However, size does not always matter – a good study is a good study. The researchers found that women taking black cohosh for hot flashes had a half-episode less per day than women on the placebo treatment. This marginal difference might be enough for some women to take the herbal supplement, but it’s not significant enough to pass scientific muster (and that’s really a good thing – that’s why science rocks).
Black cohosh has been used for hundreds of years and was a traditional Native American medicine. If you suffer from hot flashes and find that black cohosh has helped you, there’s probably nothing to worry about as it is a fairly harmless herb (though it can cause headaches and stomach discomfort in some).
The study designers did state that the jury’s not out and larger studies need to be conducted.
About a year and a half ago, a big study  was released on echinacea. The report was that the herb did nothing to prevent or alleviate cold and flu viruses. The study was certainly well-designed: participants were locked up dorm-style in a completely controlled environment for the duration of the study. But despite the strict parameters, the scientists forgot something: the echinacea plant has different parts that can be utilized for medicinal purposes. Because herbal supplements are not regulated the same way as drugs are regulated, the type of echinacea in the assortment at your local GNC can literally vary from bottle to bottle. Other studies testing different sources of echinacea have proven a benefit. It just goes to show that “the latest study” is almost never the last word.