I have never read or heard of any of this stuff interfering with B C, wise for you to ask. I really do not know about other medicines, but I never had a doctor tell me to stop taking the GLA oils.
My uneducated recommendation is to start small and build up. The pills for hemp oil tend to be 1 gram, the other oils may come in smaller amounts per pill. Hemp oil also comes straight (no pill), not exactly the most delightful thing in the world to take by mouth, but it has a sort of nutty flavor. The hemp powder I just mix with hot cereal when I take it (one scoop, but of course you can start with less.)
Of course if you are worried, ask your doctor. Just hope your doctor has some training in alternative therapies (though most of them don't know much about hemp powder and the efficacy of GLA for inflammations.) I did notice that MSM seemed to block the effect of the hemp powder for around three weeks (with the horse), then it seemed to kick in again.
The hemp seed powder is also a protein supplement, with many, many amino acids including all the essential ones our bodies can't make by themselves. Mia's hooves have gotten stronger and have grown faster since I started giving it to her, and her coat is nice and shiny.
Hope this helps!

STUNNING HORSE! :)