The Drug Enforcement Administration’s decision on Thursday to not remove marijuana from the list of the nation’s most dangerous drugs outraged scientists, public officials and advocates who have argued that the federal government should recognize that marijuana is medically useful.

Reclassifying marijuana from a Schedule 1 drug to a Schedule 2 drug would have made it easier to get federal approval for studies of its uses and paved the way for doctors to eventually write prescriptions for marijuana-derived products that could be filled at pharmacies, like other Schedule 2 drugs such as Adderall, which is used to treat attention deficit hyperactivity disorder.

Eight Democratic legislators had urged the D.E.A. to reclassify marijuana to a Schedule 2 drug. Senator Elizabeth Warren of Massachusetts voiced her disappointment with the decision on Twitter. Senator Kirsten Gillibrand of New York said in a statement, “It shouldn’t take an act of Congress for the D.E.A. to get past antiquated ideology and make this change.”

Yet in a separate policy proposal also issued on Thursday, the agency handed researchers and advocates a victory in removing a significant roadblock to medical studies of marijuana. The D.E.A. said it will allow universities and even private companies to apply to grow marijuana for scientific research. For many years, the University of Mississippi has had a monopoly on that role as the sole D.E.A.-approved provider of marijuana, and researchers have long complained that the supply of the drug was grossly inadequate, stymying efforts to establish whether marijuana is an effective treatment for many diseases.