Eh...not exactly true. Not true at all actually. Been a medical patient in CA since the early adoption in the 90’s. Bought plenty of guns above board between then and now.
Federal law prohibits gun purchases by an “unlawful user and/or an addict of any controlled substance.” In 2011, the Bureau of Alcohol, Tobacco and Firearms clarified in a letter that the law applies to marijuana users “regardless of whether [their] State has passed legislation authorizing marijuana use for medicinal purposes.” Though a growing number of states are legalizing it for medical or recreational use, marijuana remains illegal for any purpose under federal law, which considers the drug to have a high potential for abuse and no accepted medical use.
The U.S. Circuit Court of Appeals for the 9th Circuit ruled that the federal law passes muster with the Constitution, as “it is beyond dispute that illegal drug users, including marijuana users, are likely as a consequence of that use to experience altered or impaired mental states that affect their judgment and that can lead to irrational or unpredictable behavior.”
The court then concluded that it is reasonable to assume that a medical marijuana cardholder is a marijuana user, and hence reasonable to deny their gun purchase on those grounds.