Sixteen states have legalized cannabis for those who qualify. The Bureau of Alcohol, Tobacco, and Firearms (BATFE) will make sure you lose your Second Amendment rights if you admit to it. From Brian Doherty's piece at ReasondotCom:
Merely having a state medical marijuana card, BATFE insists, means that you fall afoul of Sect. 922(g) of the federal criminal code (from the 1968 federal Gun Control Act), which says that anyone “who is an unlawful user of or addicted to any controlled substance” is basically barred from possessing or receiving guns or ammo (with the bogus assertion that such possession implicates interstate commerce, which courts will pretty much always claim it does). While the BATFE has not yet announced any concerted program to go after people who may have had legally purchased weapons before getting a marijuana card, Morgan Fox of the Marijuana Policy Project says that it’s common practice in medical marijuana-related busts that “if weapons are present, there will be gun charges added on as well.”The federal laws that restrict gun ownership were passed to deny people of color access to firearms just as the federal law that makes cannabis illegal does. Possession of crack cocaine has been recently leveled to mirror racial equality.
President Obama: end the drug war and legalize cannabis now!
Update, 18 December: Barney Frank holds George Will's feet to the fire on cannabis and the rights of consenting adults.