The best way for landlords to protect themselves is to be proactive. When entering into new tenancy agreements or leases, make sure to include terms which indicate that all types of smoking (tobacco, marijuana, vaping, etc.) is not allowed in the rental unit. You should also clearly communicate that the cultivation of marijuana plants is prohibited. Sections 10 and 15 of the new Ontario Standard Form of Lease are where you can document these restrictions.  There are exceptions with respect to medical marijuana.