It's not really new. On the federal
level, marijuana is still illegal, despite the fact that some states allow its prescription. Therefore, even with a prescription that's legal under state law, a purchaser must answer "yes" on question 12 (e) on the 4473, and is therefore ineligible to buy a firearm from a licensed dealer:
Are you an unlawful user of, or addicted to, marijuana, or any depressant, stimulant, or narcotic drug, or any other controlled
The conflict between federal and state law on the matter is ripe for a lawsuit, but there hasn't been much headway on this yet.