Many herbal supplements are completely untested, and some are known to cause problems when taken in conjunction with other drugs.
St Johns Wort is a very effective anti-depressant, however it also blocks the affect of several drugs including contraceptives, immuno-suppressants, andtiretroviral drugs and warfain.
They're not 'harmless', and should always be checked with a medical professional.
Definitely check with someone whose profession it is to know such things like your GP or the pharmacist. It can be too dangerous to listen to someone in this forum that does not know exactly what risks this vit supplementer could bring upon you.