What amount st. johns wort is too much?

900mg/d =usual dose: Many studies of St Johns Wort have examined doses of around 300 mg three times daily. (www.nlm.nih.gov/medlineplus/druginfo/natural/329.html) There is a report of potential for overdose (www.drugs.com/npp/st-john-s-wort.html). As a rule of thumb, if a product is intended to effect the brain and body, then some caution is prudent, whether available over-the-counter or by prescription only.

No good studies yet: Different studies have examined different doses, and different companies put different recommendations on their bottles; you've probably noticed. The stuff seems quite safe used judiciously, and some people do wel on higher doses, but it's very dangerous combined with prescription SSRI's, and is a famous sun-sensitizer.