The U.S. government's classification of cannabis as a Schedule I drug is impeding medical marijuana research, a leading think tank said Tuesday, in what it called a misstep that could impact advancements in health care.

“As medical marijuana becomes increasingly accessible in state-regulated, legal markets, and as others self-medicate in jurisdictions that do not allow the medical use of cannabis, it is increasingly important that the scientific community conduct research on this substance,” said the report by Brookings Institution public policy analysts John Hudak and Grace Wallack.

“However, statutory, regulatory, bureaucratic, and cultural barriers have paralyzed science and threatened the integrity of research freedom in this area.”

Medical marijuana is currently legal in 23 states and Washington, D.C. Researchers have said marijuana can, for example, help reduce appetite loss associated with cancer and AIDS and relieve anxiety and insomnia.

The Drug Enforcement Administration (DEA) has repeatedly rejected calls to change marijuana’s...