Apparently, it's a thing now. Medical marijuana is legal in 23 states, and it seems to be generating some issues for employers, along with some income for lawyers. At least, that's the claim in a Slate article.
Regardless of how you feel about marijuana, there are certain rules employees and employers need to follow when it comes to drugs in the workplace. If you make a mistake, you could find yourself in court.
"Failure to accommodate" seems to be a likely charge, but it also seems easy to beat; once again, Slate seems to be breathlessly inflating things:
Unlike insulin for a diabetic, Paxil for a person with depression, Propranolol for a person with tremor, and countless other combinations of situations and drugs, those are all prescribed by a doctor.
Not so, in the case of medical marijuana; you're simply interviewed and then handed a card that says you can buy the stuff. Oh, and it's going to cost you $200 to get that card. That's entirely different from a prescription that allows you to buy, say, oxycodone; the prescription's free in the sense that your doctor doesn't charge more for the office visit because she wrote a prescription.
Oxycodone is a controlled substance under federal law, but doctors may prescribe it for pain control despite the fact that for some people, the opioid can be highly addictive. By contrast, marijuana is at present classed as an illegal substance under federal law; doctors cannot prescribe it - the most they can do is recommend that you be issued a card permitting you to purchase it from an approved dispensary in your state. Therefore, although an employer may be liable for "failure to accommodate" if they deny an employee the right to inject himself with insulin at lunch, they are under no such obligation to accommodate a stoner who wishes to toke up at lunch.
The protestations of medical marijuana "patients" notwithstanding, the differences between medical marijuana use and use of prescribed drugs are significant - and clear.