St John's what?

St. John's Wort Hypericum perforatum

In my dealings with depression, I have been put on various "meds'. I stopped taking the nasty little things months ago, and finally feel like they are out of my system (brain shocks, anyone?)
Anyway, the depression remains.
Apparently St John's Wort is an anti-depressant.

It is yet another one of those odd "co-incidences" that a herbal anti-depressant is named after
St John, the writer who has had the biggest impact on my Christian journey. I have a lot of time for St John. If I could meet one of the Evangelists it would be him. His thought and expression has been a constant source of spiritual nourishment and intellectual stimulus since reading his Gospel for the first time last year.

Why is it called St John's Wort? Why not St Luke's Wort. Luke was supposed to be a doctor.
The following sort of helps:

St. John's Wort Hypericum perforatum
The name derives from eikon (a figure, possibly an unwanted apparition) and hyper (above), which relates to the ancient use of St. John's wort to exorcise evil spirits or influences [10], since the plant may have been placed over religious icons as a symbol of protection. The common name, St. John's wort, is obviously a reference to St. John. Its earliest use may date back to the 6th century AD when, according to Gaelic tradition, the missionary St. Columba always carried a piece of St. John's wort because of his great regard for St. John [12]. Some early Christian authors claimed that red spots, symbolic of the blood of St. John, appeared on leaves of Hypericum spp. on August 29, the anniversary of the saint's beheading, while others considered that the best day to pick the plant was on June 24, the day of St. John's feast [10]. In the Christian tradition, St. John represents light, hence the flowers were taken as a reminder of the sun's bounty [13].
http://www.christopherhobbs.com

Whatever the reason for it's name, it makes sense. Hypericum coming from "unwanted apparition" is logical in the sense that depression used to be regarded as possession by evil spirits. The St John part is slightly more difficult to understand.

John the Evangelist
Hans Memling (1480?)

Anyway, I am going to try St John's Wort. My sister in law had an unwanted bottle of tablets, and I have some teabags. Apparently it takes a week or so to have any effect. Maybe reading St John's works at the same time will make it more efficacious. Or at least reveal while this plant has been given his name.