A Philadelphia doctor filed a federal suit Thursday challenging a law that prevents him from owning a firearm because he is also a medical marijuana patient.

Matthew Roman, 34, operates a practice in Old City. He uses cannabis to treat his diagnosed Post Traumatic Stress Disorder.

In April, Roman attempted to buy a Smith & Wesson 638 revolver from a federally licensed dealer for self-defense.  He drove to Firing Line in South Philadelphia and, with the advice of a friend, settled on the snub-nosed, five-round handgun.

A federally required question stopped the transaction.

The dealer asked the physician if he used marijuana. Roman said he did.

"The doc answered that question truthfully," said Roman's attorney, John Weston. "And the dealer said, 'I can't sell you a gun.'"

A 1968 law forbids anyone who uses marijuana from owning or using a firearm. In 2016, the U.S. Court of Appeals for the Ninth Circuit ruled that the federal prohibition does not violate the Second Amendment.

The NRA has remained silent on the issue. A spokeswoman did not respond Thursday to requests for comment.

The Bureau of Alcohol, Tobacco, Firearms and Explosives stands firm behind a bulletin sent out to gun shops that allows for no exceptions. A dealer who even suspects a customer may be using cannabis is obliged to cancel a sale.

In all states, firearms dealers must conduct a background check on each customer. There are a handful of situations that might stop a dealer from selling a gun. Along with users of Schedule 1 drugs, convicted felons, fugitives, domestic abusers and "mental defectives" are forbidden from owning a firearm. However, known drunks and people who have spent some time in psychiatric hospitals are permitted to own weapons.

Roman's lawsuit claims that the blanket prohibition against marijuana users violates the constitutional rights of tens of thousands of nonviolent, law-abiding American citizens. The filing, in U.S. District Court for the Eastern District of Pennsylvania, claims the law violates both the Second and Fifth Amendments of the Constitution.

The federal law also creates a legal quandary for thousands of state-legal cannabis businesses across the United States. Because of federal statutes that prevent the businesses from using the banking system, nearly all transactions are completed in cash.

Many medical marijuana dispensaries are owned and operated by people who are patients themselves and who use the medicine to treat myriad ailments. Without the option of protecting themselves, many feel vulnerable to robbery.

Weston said the court will likely attempt to reject the case. But the Philadelphia attorney says this suit stands a good chance of being heard because of Pennsylvania's heavily regulated program and the statutes that it operates under.

"Under the medical marijuana act in Pennsylvania, it's my view that the prohibition doesn't apply at all. You're only prevented from purchasing a firearm if you're an unlawful user of or addicted to a controlled substance.

"Pennsylvania statute defines an unlawful user as someone who does it without a recommendation from a doctor," Weston said. "Under state law, it seems to us that a medical marijuana patient with a state-issued card is not an unlawful user of a controlled substance. That statute should not apply at all."