St. Johns Wort is typically sold in capsule form, containing powdered ariel portions of the plant. Look for brands that have 0.3% standardized hypericin content. Capsule will typically range from 200mg to 500mg, with between 200mg and 1000mg being the recommended daily dose. Start out taking St. Johns Wort on a limited basis to see how your body will respond.
Because St. Johns Wort can interact with certain prescription medicines consult with your doctor before taking this supplement. St. Johns Wort does have several side effects so stop taking it immediately if you have an adverse reaction.