I read an interesting article this morning in the January/February 2015 Realtor magazine. The article was focused on landlords, and whether they have the right to tell their tenants not to use marijuana, which could be a concern in states where medicinal or recreational use is allowed.
The first major takeaway from the article was that a landlord who does not want the use, growth or cultivation of marijuana in their rental unit, can certainly takes steps to prohibit that use. The landlord would need to ensure their new leases had specific wording to that effect, or they would have existing tenants sign an appropriate lease addendum. The wording has to be clear that it is the use of marijuana that is prohibited (not just a "no smoking" statement).
But an issue may arise if a tenant does not abide by those rules. As the article posited, let's say a tenant who has a medical marijuana prescription asks her landlord for permission to smoke at home, and he/she says no but the tenant smokes anyway. The landlord then takes the tenant to court to evict her. The judge could decide that the fact that medical marijuana is legal in that state takes precedence over the landlord's desires. Rules about this issue are still being defined.
Marijuana is still an illegal drug on a national/federal level, but the government has agreed not to prosecute medical marijuana users in states allowing such use. The federal government's stance might decrease a landlord's ability to decide whether pot use is/isn't allowed in their rental unit. Another interesting development is that some states will start requiring disclosure about pot-friendly properties. If that happens, real estate agents might be required to let their tenants know when homes they are interested in are near or in buildings where pot smoking is allowed; this could turn some potential renters off from a perfectly good rental property.
Right now landlords are not allowed to discriminate based on race, color, national origin, religion, sex, familial status, handicap, sexual orientation, source of income, and other state-specific protected classes. Will this list someday also include pot-smoker status?
Can you tell your tenants not to smoke marijuana? Yes but it might not matter. See here for the complete article.