Hemp is marijuana. It is also found in the buds that you can smoke to get high. I believe they are somewhat competing components and their concentration depends on when you harvest a plant. It has either more THC/CBD depending on the time chosen.

Didn't know doctors prescribe it. Hemp is relatively closely related to nettles, which are also used for some therapies for arthritis. Maybe there is another component that helps? Could be a coincidence of course.