I'm not sure about much clarification, but here goes.
The laws all over (state to state to country) are different. In Canada, if it is legal its supposed to be legal equally everywhere. Supposedly you can't get fired (or will not be able to be fired as of October 17th) for off duty Cannabis use providing that you are not still intoxicated when coming in to work *OR* if you are in a safety sensitive position.
I worked in the nutrition lab, Dept. of Animal Science at the University of Manitoba. At the time that I went on indefinite medical leave in May, 2017 my department did not have an MJ policy. But they did have an 'intoxication' policy in place. Summer, 2014 a guy was fired because he was caught swilling whiskey on the job. Immediate termination. I assume this is what will be put in place for MJ.
However with MJ, work would be forced to accommodate a person's medical use (the catch is within reason) on the job.
On any given day, I would work with animals, run gas chromatographs and use nitric, sulfuric, perchloric, acetic and other acids - as well as working with mycotoxins. That was classified as a safety sensitive position. There really isn't anything one can do other than not using on the job and ensuring you're not high when you come in to work. A good human resources dept. is also invaluable.
With my job, there was never a question of using while at work.
Tom
Biochemistry, Microbiology, Biology, Zoology, Physics, etc - what you could call the physical sciences are not likely going to ever be compatible with on job use. Reclassification of your position is a possibility, but unlikely.