The US Drug Enforcement Administration on Thursday said it was again denying requests to allow the use of marijuana for medical purposes.
The decision keeps a growing number of US states at odds with federal law, as 25 and the District of Columbia have enacted laws allowing access to cannabis for medical purposes.
However, the federal government will allow an expansion of marijuana research, letting organizations apply for permission to grow marijuana for use in studies.
Currently, only the University of Mississippi is permitted to do so and researchers have complained that supplies are thin.
Marijuana is commonly prescribed to treat conditions including pain and nausea. Patients are also turning to it to treat illnesses such as Crohn's and Alzheimer's diseases, lupus and rhumatoid arthritis.
Chuck Rosenberg, the acting DEA administrator, said in upholding the medical marijuana ban that there was a "lack of accepted safety for its use under medical supervision."
In justifying the move, the DEA cited a medical and scientific evaluation carried out by the Food and Drug Administration and the National Institute on Drug Abuse.
"DEA's decision flies in the face of objective science and overwhelming public opinion," Aaron Smith, head of the National Cannabis Industry Association, a trade body in Washington, said in a statement.
The DEA's decision rejected a petition submitted by the governors of Rhode Island and Washington state as well as a resident of New Mexico. The agency had rejected a similar request in 2011.
The decision comes amid growing pressure from lawmakers to reconsider allowing the medicinal use of marijuana.
A conference of state lawmakers on Wednesday adopted a resolution calling on the federal government to remove marijuana from a list of illegal drugs, including heroin, LSD and peyote, designated as having "no currently accepted medical use and a high potential for abuse."