St John's wort is not sold as a medicine so the products you can buy are subject to much lower levels of quality control. St Johns Wort varies in quality across different brands.
Plus St Johns Wort interacts with practically everything.
Sources: http://www.nytimes.com/2010/05/26/health/policy/26herbal.htm...
>16 of the 40 supplements tested contained pesticide residues that appeared to exceed legal limits, the investigators found. In some cases, the government has not set allowable levels of these pesticides because of a paucity of scientific research.
http://www.nytimes.com/2013/11/05/science/herbal-supplements...
>DNA tests show that many pills labeled as healing herbs are little more than powdered rice and weeds.
>Two bottles labeled as St. John’s wort, which studies have shown may treat mild depression, contained none of the medicinal herb. Instead, the pills in one bottle were made of nothing but rice, and another bottle contained only Alexandrian senna, an Egyptian yellow shrub that is a powerful laxative.
http://well.blogs.nytimes.com/2014/10/21/dangerous-dietary-s...
>out of more than two dozen supplements that were pulled from shelves after they were found to contain anabolic steroids or powerful prescription drugs, roughly two-thirds were back on the market a year later with the same illicit ingredients
Not trying to be contrary, but do you have a source?
I think it's more complicated than a simple either-or proposition.
http://www.ncbi.nlm.nih.gov/pubmed/16814590 http://www.ncbi.nlm.nih.gov/pubmed/19739671