It may take up to four weeks for individuals to feel the full effect of St. John’s wort, according to the University of Maryland Medical Center. Individuals typically take St. John’s wort to relieve mild to moderate depression.
St. John’s wort is not effective for all forms of depression, and it shouldn’t be used as a substitute for medical care. According to the National Center for Complementary and Alternative Medicine, inadequate treatment may cause the depression to become severe. In addition, St. John’s wort can interfere with prescription medications such as antidepressants, birth control pills and warfarin. Taking St. John's wort and antidepressants concurrently may increase the body’s serotonin levels to life-threatening concentrations.