Although marijuana use violates federal law, many states have passed laws legalizing marijuana. Restrictions vary widely by state; some states only allow medical marijuana, while others have legalized recreational marijuana. As with alcohol and other drugs, you may still prohibit employees from using marijuana at work and reporting to work under the influence of marijuana.
However, you may want to take this opportunity to review your drug policies. Companies in states where marijuana is legal may consider amending existing drug policies to clarify that any use of marijuana is prohibited, regardless of recreational or medical use.