How long does it take St. John's wort to take effect?

It takes approximately two months for the effects of St. John’s wort to be noticeable in the treatment of depression, according to WebMD. The typical dosage is 300 milligrams by capsule three times daily.

St. John’s wort is often used to treat mild to moderate depression, but it is also used to treat insomnia, anxiety and seasonal affective disorder. The herb is thought to act as a selective serotonin reuptake inhibitor, or SSRI. It is important to talk to a doctor before taking this supplement because of potential drug interactions. Medications that potentially interact with St John’s wort include antidepressants, birth control, anti-arrhythmics and blood thinners.