Additionally, sub-para b. specifies that QR&O 31.13(2) still applies, which means you still have the obligation to report the outside medical care even if you don't think it is affecting your job performance. This is no different that someone who goes downtown and pays for their own LASIK, cosmetic surgery etc.
Sub-para c is a pretty good indication that a member who does this is also going to end up with MELs, which also happens in the examples I gave. What those MELs are and how they are going to affect a member's career is always going to be case by case but I haven't heard of medicinal cannabis being prescribed for something short term so it could very well have career implications.