St John's Wort is Hypericum perforatum, and it is the one herb in this collection whose pharmacological activity is both well established and clinically significant enough to be a practical problem.
Extracts have been studied extensively for mild to moderate depression, and a substantial body of trial evidence and several meta-analyses support an effect, with results generally comparable to standard antidepressants in mild to moderate cases and considerably weaker evidence in severe ones. It is a licensed medicine in Germany and sold as a supplement elsewhere, which produces a large disparity in preparation quality.
The consequential fact is the interactions, and these are not disputed. Hyperforin, one of the principal constituents, is a potent inducer of cytochrome P450 3A4 and of P-glycoprotein. The practical result is that St John's Wort accelerates the clearance of a long list of drugs and can reduce them below effective concentration. Documented examples include oral contraceptives, warfarin, ciclosporin in transplant patients, some antiretrovirals, and several anticancer agents. There is also a serotonin syndrome risk in combination with SSRIs.
That is stated here as fact, not as advice: this is a plant with a real mechanism, and one of the clearest demonstrations available that herbal does not mean inert or safe. A herb potent enough to work is potent enough to interfere.
The plant also causes photosensitivity in livestock that graze it heavily, a recognised agricultural problem, and it is an invasive weed of rangeland in Australia and western North America.