Black cohosh should not be used for more than 6 months at a time at normal doses. Generally, when taking herbs over the long term they should be taken for 3 weeks on and one week off. For menstrual stimulating & abortifacient purposes any herb including black cohosh should not be used any longer than 10 days, since doses taken are often higher than the recommended dosages, and toxic build up or side effects could result, even with herbs generally known as being safe.
Contradictions: Black cohosh can depress heart rate, anyone with any type of heart disease should not use this herb. Other possible side effects include dizziness, diarrhea, nausea, headaches, shakes, low pulse, vomiting. Side effects occurring with prolonged use may include uterine irritation, abdominal pain (see cautions on ectopic pregnancy), abnormal blood clotting (which could cause problems when using herbs to end pregnancy, possibly hemorrhage), liver problems, it could encourage breast tumors. And should not be taken by anyone who has been advised not to take oral contraceptives.7