Women who are pregnant or breast feeding should steer clear of it. It seems to increase the risk of miscarriage. It may be a mild sedative, so avoid taking it if you are taking tranquilizers. And it may interfere with hypertension medication. Ask whether it might interact with other medications you are taking.
Black cohosh is an interesting herb.
Perhaps good studies will one day be done, so that you could be more certain of its benefits and side effects. Thus far its traditional uses and benefits have no solid support from scientific research.