In traditional Chinese medicine, horny goat weed is usually combined with yin tonic herbs when used long term for support of sexual health.²⁶ If this is not done, it is believed the herb could cause problems such as aggressiveness, irritability, fever, or other “hot” symptoms (such as racing heart beat). In animal studies, prolonged use of excessive amounts of horny goat weed was associated with decreased thyroid activity.²⁷