An employer can lawfully regulate an employee’s use of alcohol and illegal drugs at work. Employment rules about alcohol or drug use are allowed.
An employer can require a drug test to a prospective worker. If an employee fails that test, an employer can refuse to hire that worker or rescind an offer. An employee should notify an employer of any legal prescription drugs that may result in a failed drug test. The employer needs to keep any drug testing information private, as it is considered medical information.
Finally, an employer can probably fire a worker for marijuana use, even if it used to treat a disability consistent with medical marijuana laws. The state and federal laws do not stop employers for firing a worker for marijuana use. It is possible this law could change, or a judge could modify the rule in the future, but for now, an employee could be fired for using marijuana.
The laws are complicated, and there are additional rules and some exceptions to these general rules, so talk to a Bay Area employment law attorney for more details.