pg, it does exactly the same as SSRI drugs, it's just not very strong. Stuff like Prozac doesn't often work if you are feeling bad in response to an event because different things are happening in your brain than the cyclic stuff (However, the 'danger' is that extreme stress can become cyclic so it's important to control it with behavioural changes ).
Part of long term depression is knowing what is the illness and what is a sane reaction to a horrible situation. It's never a good idea to medicate the second because you'll fuck yourself up, long term. Medicating loss firstly doesn't really work, and secondly leads to less ability to cope next time.
St John's Wort affects hormonal birth control, so it's not always the best plan for women.
(disclosure time: I wasn't depressed upthread, I was just sad because a guy I really liked lied and treated me badly. )