The State of California, never content with the idea that employers might know better than the government what's required to actually earn tax dollars, is now prohibiting "employment discrimination" against medical marijuana users:
it is unlawful for an employer to discriminate against a person in hiring, termination, or any term or condition of employment or otherwise penalize a person, if the discrimination is based upon either of the following:
(1) The person's status as a qualified patient. [meaning a medical marijuana user]
(2) The person's positive drug test for marijuana, provided the person is a qualified patient and the medical use of marijuana, as defined in Section 11362.7, does not occur on the property or premises of the place of employment or during the hours of employment, as required by Section 11362.785.
The law contains exceptions for use on the job, as well as for "safety" occupations, and (naturally) for the government itself, but otherwise medical pot smokers are protected from mellow harshing by employers who just don't understand that employees need this stuff to cope with their asthma and anger management issues.
But "safety" and government employment are pretty narrow exceptions. As I read the law, a junior attorney with a medical marijuana prescription could load up in the courthouse parking lot just before a morning session, then begin "hours of employment" nice and baked. His employing firm's clients might not feel too safe knowing their financial welfare or freedom were in the hands of a stoner, but firing the attorney for appearing in court while stoned would be out of the question.
The same risks hold true for accountants, doctors, and just about any other job that isn't necessarily related to safety (whatever that means), but where getting it done right might matter to the customer.
"Dude, these spots on this mammogram are like … totally Jackson Pollock or something."
Here's a solution that might make sense even under current California law. Give employees the right to use marijuana if they have a genuine medical need for it, and give them the freedom to face the consequences if it impacts their job performance.