I hate to seem to agree with Scutty's 'alarmist' tendency but he's correct about SJ Wort.
It's surprising if the GP doesn't know about it... it's one of the very few supplements that most are aware of (simply on account of the large number of potentially very serious interactions with many prescription medicines).
On the other hand it's considered effective for depression (or as effective as an SSRi). Yet unlike carefully-measured doses of SSRIs (which indeed can have more side-effects it seems that SJWort) it's very hard to know how much you're getting. 60% of the St. John's wort supplements tested by ConsumerLab.com did not contain expected amounts of one or more key plant chemicals. There was an enormous range, suggesting you could get as much as 280 times more active ingredients from one product than from another. Even respected brands like Now Foods and Swansons failed tests (though in their cases for containing a much much lower dose than stated on the product). The plants can accumulate cadmium or lead, both toxic; and most manufacturers won't test for it. Some examples of prescribed drugs that SJWort can seriously interfere with include omeprazole, warfarin, statins, digoxin, MAO inhibitors, tramadol. They are reported to cause potentially excessive serotonin when taken with SSRIs.
Having said that, if you are not on any meds at all it's probably safe.