They do the same thing, and since St. Johns Wort is a herbal preparation it can vary in effect depending on person and give unpredictable results, like a food/drug interaction. Since many people are on anti-depressants in order to live at all, you can see why the docs don't want 'unpredictable results'! Now, St. Johns Wort is actually effective for depression on its own, provided it is MILD depression. Medium, strong and strong with melancholia, it has no effect on. (German studies to back all this up - if you read German. It is commonly used there, by prescription.)