Fatigue is not a reported side effect of black cohosh. According to the website, Drugs.com common side effects include:
"Large doses of black cohosh have been reported to cause nausea, dizziness, seizures, visual disturbances, reduces pulse rate and increased sweating. Stomach upset has been reported as a common side effect with the use of therapeutic doses of black cohosh.
Is the fatigue preventing from doing your normal activity? Something else may be the underlying cause. If the tiredness persists, consult your physician, You may need to have your thyroid levels checked.
Thank you for your question.