I have used Vogel's 'The Nature Doctor' as a reference for at almost 20 years, he recommends St John's Wrt oil quite a bit, generally for external use (as a topical), and rarely for internal use (the whole herb/flower like you would get in a store bought capsule) but only as needed for short duration.

I also use a handbookof Vitamin and herbal supplements by H Winter Griffith MD, may be out of print have had quite some time also. In that book it lists St John's Wort as being on the FDA' list of unsafe herbs (for long-term internal use). Reason's given are:

-causes photsensitization (bad if you go out in the sun a lot like me)
-interferes with absorbtion of iron and other minerals when taken internally
-slightly depresses central nervous system

chemicals it contains are:
Hypericin, Resins, tannins, and volatile oils.

It is meant for short term use only as far as i can tell. I have used the capsules myself in the past, I can only tolerate small amounts for a few days now and then.