Medical marijuana should be treated like any other medication.
Recreational marijuana should be treated just like any other legal drug aka alcohol or sugar that employees consume.
If you can't do those things and perform your job safely and effectively, then disallow it. Contextually, some cultures (even by region in the US) respond differently to having a drink at lunch. Your business probably knows which clients will respond negatively and which will not be phased. The same logic applies with weed.