Black cohosh has estrogen-like effects and women who are pregnant or breastfeeding should avoid using this herb. High doses of this herb may cause unusual pain, nausea, headaches, and dizziness. Women taking estrogen should consult their physician before using black cohosh. High doses of black cohosh can cause symptoms of poisoning, most notably nausea and dizziness, and may also lead to miscarriage. Black cohosh should not be used in people with severe rubella or difficulty breathing.