Some adverse effects of St. John's wort include photosensitivity, dry mouth, constipation, fatigue and weight gain, according to Drugs.com. Other people have feelings of fullness in their stomachs and experience restlessness.
St. John's wort can reduce the effectiveness of birth control pills, notes Drugs.com. However, there is also indication that it can cause infertility in some people, reports MedlinePlus.
Some people who take St. John's wort suffer from insomnia or vivid dreams, explains MedlinePlus. They can also experience headaches, rashes, tingling and diarrhea. Medical experts recommend that patients take St. John's wort in the morning if it interferes with sleep. Women who are pregnant or breastfeeding should not take St. John's wort.