We see a lot of posts in various gun groups about whether you can use cannabis and still legally buy a gun now that pot has been legalized for either recreational or medical use in some form in 47 states. (See Wikipedia here for a comprehensive chart of current permitted and prohibited uses by state.) According to the ATF, the answer is a resounding no.
The ATF has been consistent in their position on this. In an Open Letter on September 21, 2011, the ATF stated specifically that holders of medical marijuana cards, even if legal in the state of residence, could not legally buy a gun and that FFLs could assume that holders of such cards were prohibited from such purchases. That same letter stated that anyone who uses marijuana in any form or, presumably, frequency, should answer yes to question 11(e) on the Form 4473, effectively disqualifying recreational users in states where it is legal from purchasing firearms. The ATF’s position was upheld in Wilson v. Lynch, 835 F.3d 1083 (9th Cir. 2016), and is thus explicitly the law in at least the states covered by that Circuit.
In October, 2016, the ATF issued the revised Form 4473, making their stance known to all prospective purchasers. Question 11(e) on the the form now contains the following language: “Warning: The use or possession of marijuana remains unlawful under Federal law regardless of whether it has been legalized or decriminalized for medicinal or recreational purposes in the state where you reside.”
It is worth noting that all the guidance issued by the ATF thus far goes directly to the purchase or transfer of a firearm through an FFL, not the mere possession of a firearm. However, all gun owners should be aware – it is a felony to lie on the form 4473. Thus, at this time, it is, according to the ATF and at least one federal appellate court, illegal to both use pot in any form and purchase a firearm. You are free, of course, to draw your own conclusions on the rationality of this position.