Eight Democrats in the U.S. Senate on Monday inquired into what the Obama Administration is doing to facilitate medical marijuana research at a time when nearly half the states in the country have legalized marijuana for medical purposes.

In a letter addressed to the heads of the Department of Health and Human Services, White House’s Office of National Drug Control Policy and Drug Enforcement Administration, the senators referenced limited research on marijuana’s health benefits in spite of the fact that millions of Americans have the right under state laws to use marijuana for medical use.

As the senators pointed out, 23 states and the District of Columbia authorize marijuana for medical use, while 15 states have laws that explicitly authorize the use of cannabidiol or CBD. Marijuana remains illegal under federal law, though the Obama Administration has largely left alone states with medical and recreational marijuana laws.

“There is no substitute for rigorous preclinical and clinical research on the potential benefits of medical marijuana," the senators wrote in the letter. “With the patient pool of medical marijuana users growing in the United States, we believe that federal agencies have both an opportunity and a responsibility to craft a sensible research and public health strategy that allows us to generate meaningful data and conclusions from this ongoing natural experiment."

Signatories included Sens. Elizabeth Warren and Ed Markey of Massachusetts, Jeffrey Merkley and Ron Wyden of Oregon, Barbara Mikulski (Maryland), Barbara Boxer (California), Cory Booker (New Jersey) and Kirsten Gillibrand (New York). The letter also was forwarded to the heads of the Centers for Disease Control and Prevention, Substance Abuse and Mental Health Services Administration, National Institutes of Health, FDA and National Institute on Drug Abuse.

The senators probed the executive branch on a wide range of issues, from efforts to reduce barriers to obtain permits for marijuana research, to FDA’s timeline for making a recommendation on whether to reschedule marijuana’s status as a Schedule I. drug.

“Many states and localities are moving forward with policies that facilitate the availability of medical marijuana to a greater proportion of the population than ever before," the senators concluded. “All participants in this important debate will benefit from rigorous, scientific research into the impact of these policies on American public health. Relevant federal agencies must play a leadership role in coordinating and facilitating that research if we are to ensure that public policy in this area is supported by our best science."