Eight U.S. senators are calling on the federal government to facilitate research on the potential benefits of medical marijuana. The senators say the research is needed because millions of Americans are now eligible by state law to use the drug for medical purposes.

In a letter to multiple federal drug and health officials, the senators say the federal government should use existing tools to collect national data, conduct surveillance and perform clinical trials on patients in states with active medical marijuana programs.

They also said the government should collaborate with and support independent scientists, by eliminating barriers to research on marijuana, The Huffington Post reports.

The senators who signed the letter were Elizabeth Warren and Edward Markey (Massachusetts), Jeffrey Merkley and Ron Wyden (Oregon), Barbara Mikulski (Maryland), Barbara Boxer (California), Cory Booker (New Jersey) and Kirsten Gillibrand (New York).

Boxer, Booker and Gillibrand, along with Kentucky Senator Rand Paul, are co-sponsors of a bill to end the federal prohibition on medical marijuana. The Compassionate Access, Research Expansion and Respect States (CARERS) Act would reclassify marijuana from Schedule I to Schedule II under the Controlled Substances Act. Schedule I drugs, which include heroin, have no accepted medical use in the United States. Schedule II drugs have a legitimate medical use but also have a high potential for abuse.

The letter was addressed to the heads of the Department of Health and Human Services, the Office of National Drug Control Policy and the Drug Enforcement Administration. Officials from the Centers for Disease Control and Prevention, the National Institutes of Health, the Food and Drug Administration and National Institute on Drug Abuse were also copied on the letter.