With regard to the second part of your question, SJW is known
to have powerful interactions with other drugs.  It accelerates
the efflux and metabolization of other drugs, so diminishing
their effect.  For example, there have been recipients of organ
transplants taking cyclosporin to suppress organ rejection, who
have had rejection episodes caused by SJW reducing the
effectiveness of cyclosporin.  There have also been women
taking birth control drugs who got pregnant as a result of the
effect of SJW on such drugs.  If you are taking any prescription
pharmaceutical, you should not be taking SJW.

There is at least one report in the medical literature of an
alleged case of addiction to SJW.  That's not to say it is
proven that addiction will occur -- merely that there is one
report.  Considering how widely SJW is used, addiction
must be somewhat uncommon, if it occurs at all.

Other than those risk factors, SJW seems to be well tolerated
in most people.  There's a recent paper which has a very
thorough review of the possible adverse effects of SJW:
"St. John's Wort:  A Systematic Review of Adverse Effects
and Drug Interactions for the Consultation Psychiatrist",
_Psychosomatics_, volume 44, number 4, pages 271-282.

I've read most of the literature on the possible bad effects
of SJW, and I would say this is by far the best review.

http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s555147.htm
(Assuming you want it for depression)
Anth

The other folks who have responded gave you some good info.  You should
also know that St. John's wort is only good for mild depression, and
someone with more severe depression will not get any benefit from it.

Kimmer
email:  Lady_Kimmer at altcastlenet dot com

Blood type O people should not take St John's Wort. It causes blood
agglutination.

It takes about 3-4 months, the same as most SSRI drugs.

One important element here is the potency of SJW being bought.  There is a
huge variation....that will certainly be a factor....