State laws legalizing marijuana are spreading. Colorado and Washington allow marijuana for recreational use, eighteen states and the District of Columbia have legalized medical marijuana, and several states have pending legislation. However, federal law still classifies marijuana as an illegal class one drug, so under the federal Fair Housing Act landlords aren't required to allow it as a "reasonable accommodation" for people with disabilities.
Regardless of where you live, you can have a no-smoking policy in your rentals as long as you do not discriminate against a protected class in implementing and enforcing the policy. In fact, landlords are increasingly instituting no-smoking policies to deal with odor and smoke issues as well as out of a concern for tenant health.
If you have a tenant who is disabled and smoking marijuana medicinally, and you live in a state where medical marijuana is not legal, you are not required to accommodate medical marijuana usage.
However, things get murky for landlords in states allowing medical marijuana because of possible claims under state fair housing laws that medical marijuana use is a "reasonable accommodation" for say, people undergoing cancer treatment. Because state medical marijuana laws directly conflict with federal law, the enforceability and legal liability of any medical marijuana policy is in question. You have options when it comes to developing a medical marijuana policy, but none should be considered fail safe.
If you live in a state with legal medical marijuana and you have a disabled tenant who is smoking it, a mediator could help resolve the situation by helping you and your tenant agree on mutually acceptable solutions. In addition, consider consulting an attorney. And if you take Section 8 vouchers, check with your local Fair Housing about the rules.