Twenty-one lawmakers have written Veterans Affairs Secretary Robert McDonald urging him to let VA doctors discuss and recommend marijuana as a potential medical treatment in states where it is legal.

Under a VA policy that expires on Jan. 31, VA doctors are not allowed to discuss medical marijuana with their patients or recommend it as a treatment.

Senators and representatives — 19 Democrats and two Republicans, including Rep. Joe Heck of Nevada, a physician and Army Reserve brigadier general who chairs the House Armed Services personnel panel — want a new policy that "removes barriers that would interfere with the doctor-patient relationship" in states where medical marijuana is legal.

The policy, lawmaker say, "disincentivizes doctors and patients from being honest with each other."

"You are in a position to make this change when the current directive expires at the end of this month," Sen. Kirsten Gillibrand, D-N.Y., Sen. Steve Daines, D-Mont., and others wrote Wednesday to McDonald. "We ask that you act to ensure that our veterans’ access to care is not compromised and that doctors and patients are allowed to have honest discussions about treatment options."

Twenty-three states and the District of Columbia have legalized medical marijuana, and 17 states have laws regulating oils derived from marijuana plants.

However, marijuana possession and use continues to be a crime under federal law.

Last year, the Senate Appropriations Committee approved an amendment to the Veterans Affairs spending bill that would have required VA to change its policy on physicians discussing medical marijuana with patients. House lawmakers had introduced similar legislation, but the provision was stripped from the final version of the comprehensive spending bill.

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

Also in 2015, Gillibrand and Sens. Cory Booker, D-N.J., and Rand Paul, R-Ky., sponsored a bill seeking reclassification of marijuana as a Schedule II drug, which would allow researchers to study the plant's effectiveness as a medicine without having to go through the long bureaucratic process currently required.

In November, Democratic presidential candidate Sen. Bernie Sanders, I-Vt., introduced a bill that would repeal all federal penalties for possessing and growing marijuana.

Sanders' bill has been referred to the Senate Judiciary Committee but has no co-sponsors.

The VA recommends that its physicians use "evidence-based" practices — therapies proved by scientific research to be effective — to treat mental and physical health conditions such as post-traumatic stress disorder, depression and pain.

There has been no research in the U.S. on the effectiveness of medical marijuana for relieving symptoms of PTSD or other conditions, although some veterans groups and marijuana legalization advocates say it does help relieve symptoms of combat-related PTS and anxiety.

Last January, the state of Colorado gave a $2 million grant to the Multidisciplinary Association for Psychedelic Studies to conduct research on using marijuana to treat post-traumatic stress.

Dr. Sue Sisley, who is spearheading that research, is expected to start her work this year.

In July, Colorado health officials voted against adding PTSD to the state's list of eligible ailments for medical marijuana. Voting members of the Board of Health said there was not enough research to support its use as an effective treatment.

Additional lawmakers who signed Wednesday's letter to McDonald were: Booker, and Sens. Barbara Boxer, D-Calif.; Patty Murray, D-Wash.; Brian Schatz, D-Hawaii; Tammy Baldwin, D-Wis.; Michael Bennet, D-Colo.; Ron Wyden, D-Ore.; and Elizabeth Warren, D-Mass.; and Reps. Sam Farr, D-Calif.; Jared Polis, D-Colo.; Chellie Pingree, D-Maine; Steve Cohen, D-Tenn.; Justin Amash, R-Mich.; and Mark Pocan, D-Wis.

Patricia Kime covers military and veterans' health care and medicine for Military Times. She can be reached at pkime@militarytimes.com