According to my neurologist, St John's Wort's chemical effect is the same as a SSRI (selective serotonin reuptake inhibitor). It is less potent than something like Zoloft (SSRI prescribed for depression, among other things), but it's over-the-counter herbal, which means it is also less regulated.

So, it can have all the side effects you might see in an SSRI anti-depressant.

For the record, this came up with my neurologist because SSRIs often trigger severe migraines in people prone to them. He was *very* annoyed my therapist suggested St John's Wort after I had trouble with other prescriptions due to migraines.