Disclaimer: I am not a psychiatrist.
I do not know how the FAA treats herbal remedies. I doubt that they would need to be listed.
St. John's wort is an over the counter remedy for the treatment of mild depression. I believe that it was more popular a few years ago. I do not know why it appears to be less frequently used now. I suspect it has been replaced by prescription antidepressants which can be as cheap as $4 per month. A review article in the New England Journal of Medicine in 2000 briefly mentioned it.
Herbal Remedies
Hypericum perforatum (St. John's wort), an herbal remedy marketed as a dietary supplement, appears to be more effective than placebo and as effective as low-dose tricyclic antidepressants for the treatment of mild depression.58,59 Side effects are infrequent
http://www.nejm.org/doi/full/10.1056/NEJM200012283432607
I have seen references to other studies but don't have the time to hunt all of them down.
http://www.sciencedaily.com/releases/2008/10/081007192435.htm
St. John's wort has many drug interactions so I would ask a pharmacist if it is a problem with anything you are currently taking.
http://www.rxlist.com/script/main/art.asp?articlekey=96348&page=3#Interactions
Pilots often want to self treat to avoid issues with obtaining a medical certificate. This can be a big problem if treatment for certain medical conditions is delayed or avoided entirely. If suicidal ideation is present a prompt medical evaluation is necessary as this can be a symptom of major depression.