St John's Wort is supposed to have similar effects to SSRI antidepressants and there have been some medical trials of its use for major depression (as opposed to bipolar depression). So far as I can see, the jury is still out on whether SJW could be used in place of SSRIs like Prozac, Zoloft etc, but it does seem to have an effect on serotonin levels in the brain and there are warnings about using SJW at the same time as SSRIs.
Because the effects of SJW are similar to SSRIs in raising serotonin levels, it can have the same effects as SSRIs in triggering mania or hypomania in bipolar patients. If doctors prescribe SSRIs for bipolar patients, it is usually alongside a mood stabiliser which stops the mood becoming too high as serotonin levels increase. If bipolar patients are self-medicating in depressive episodes with SJW, they may find similar effects to using SSRIs alone - i.e. mania or hypomania (although in my case, the effect of SSRIs used without a mood stabiliser was a mixed episode).
However, the effect does depend on the quality and dosage of SJW that is used. A lot of herbal 'remedies' on the market have very little active ingredient.
There's a lot of info online about this on the internet.
Hope this helps