That's a very good question and a very good comparison. There are two different kinds of advice I give to employers who ask questions about people with a medical authorization, also known as medical marijuana. The first is that, through a benefit plan, if it's employer-paid then the employer is aware and the employer has a legal obligation, under health and safety law, to not allow them to be in a safety sensitive position, unless they get a medical opinion that the worker is fit for duty.
What we do is that we either write the letter or the employer writes the letter to the doctor authorizing medical marijuana, saying, “Here is the job and here are the risks associated with it. Can we get your medical opinion? Is the person going to be even possibly impaired?” Most times the doctors say, “Yes, don't put them in a safety sensitive position”.
That's easy. The harder part is somebody who might have it and might not want to disclose it because they're afraid of losing their job, but they might be in a safety sensitive position. Then the only way the employer finds out is after an accident, when somebody is injured or killed on the job.
The answer is then that it's not their fault because they have medical authorization. Yes, however, the worker has a duty not to present themselves unsafe. That's why the first recommendation about actually having a formal prohibition against workers coming to work under the influence of cannabis or other drugs makes sense, unless there is a medical authorization as disclosed. It puts some shared responsibility on the worker and their doctor to determine if they could be impaired and could therefore affect the safety of others at work.