In an open letter to all Federal Firearms Licensees (FFLs) back in September of 2011.

Any person who uses or is addicted to marijuana, regardless of whether his or her State has passed legislation authorizing marijuana use for medicinal purposes, is an unlawful user of or addicted to a controlled substance, and is prohibited by Federal law from possessing firearms or ammunition. Such persons should answer “yes” to question 11.e. on ATF Form 4473 (August 2008), Firearms Transfer Record, and you may not transfer firearms or ammunition to them.