St John's Wort is a herbal remedy used for low mood, but it is one of the most interaction-prone supplements. It speeds up the breakdown of many medicines, which can make them less effective, including the contraceptive pill (risking unplanned pregnancy), warfarin, some HIV and transplant medicines, and certain heart and epilepsy drugs.
Combined with prescription antidepressants such as SSRIs, it can also raise the risk of serotonin syndrome, a potentially serious reaction. Because it is sold over the counter, people often do not mention it to their doctor, which is exactly why interactions get missed.
Always tell your doctor or pharmacist if you take or want to start St John's Wort. Seek urgent care for symptoms of serotonin syndrome such as agitation, rapid heartbeat, high temperature, muscle twitching, sweating, and confusion.