Increased safety risk arising from cannabis impairment in the workplace can amount to undue hardship. This will likely continue to be the case until there are reliable technologies that can accurately and effectively measure impairment from cannabis.
The right to accommodation, and the widespread acceptance of medical marijuana, does not mean that employees have a right to use marijuana at work. Safety considerations will be taken into account and although zero tolerance policies will not be automatically enforced, they will be enforced when appropriate.
This blog post provides a primer on the state of undue harship and frustration of contract under Ontario's Human Rights Code.