From the New Mexico Independent:
Though medical marijuana is legal in New Mexico, the drug is still regarded as an illegal scheduled substance by the federal government. Given the federal government sets the rules on who can own guns, medicinal marijuana smokers of this state and 15 others are barred from owning guns.
The point was reiterated in a late September letter written (PDF) by the Bureau of Alcohol, Tobacco, Firearms, and Explosives and sent to federal firearms licensees. Owners of gun stores are instructed to withhold the sale of arms or munitions to anyone suspected of having an interaction or addiction to scheduled drugs, including marijuana. The letter specifies individuals known to have a medicinal marijuana card can be reasonably assumed to be an abuser of a controlled substance and gun shop owners must refuse purchase.
Moreover, the letter affirms the illegality of a medicinal marijuana smoker purchasing weapons. Already, those who seek to purchase firearms or ammunition must fill out ATF Form 4473. Question 11.e. specifically asks: “Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance?” Answering ‘yes’ legally bars the individual from purchasing guns or ammunition.
The ATF letter several times referred to marijuana as an addictive drug. According to a summary of the book The Science of Marijuana (2008) in Psychology Today, a person’s risk of developing an addiction to marijuana is roughly 9 percent, compared to 33 percent for tobacco users and 15 percent for alcohol users.