The Obama administration has denied a bid by two Democratic governors to reconsider how it treats marijuana under federal drug control laws, keeping the drug for now, at least, in the most restrictive category for U.S. law enforcement purposes.

Drug Enforcement Administration chief Chuck Rosenberg says the decision is rooted in science. Rosenberg gave "enormous weight" to conclusions by the Food and Drug Administration that marijuana has "no currently accepted medical use in treatment in the United States," and by some measures, it remains highly vulnerable to abuse as the most commonly used illicit drug across the nation.

"This decision isn't based on danger. This decision is based on whether marijuana, as determined by the FDA, is a safe and effective medicine," he said, "and it's not."

Marijuana is considered a Schedule I drug under the Controlled Substances Act, alongside heroin and LSD, while other, highly addictive substances including oxycodone and methamphetamine are regulated differently under Schedule II of the law. But marijuana's designation has nothing to do with danger, Rosenberg said.

In a letter to the petitioners, Rhode Island Gov. Gina Raimondo, Washington Gov. Jay Inslee and New Mexico nurse practitioner Bryan Krumm, Rosenberg said doctors are responsible for treating patients, but the FDA makes decisions about drug safety: "Simply put, evaluating the safety and effectiveness of drugs is a highly specialized endeavor."

Tom Angell, chairman of Marijuana Majority, said in a statement that the decision was disappointing.

"President Obama always said he would let science — and not ideology — dictate policy, but in this case his administration is upholding a failed drug war approach instead of looking at real, existing evidence that marijuana has medical value," he wrote.

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Most Americans support legalization, Angell wrote, and the federal government should at a minimum leave regulatory decisions to the states.

Drug enforcers insist they are supportive of efforts to advance scientific research on marijuana. The DEA said it has "never denied" an application from a researcher to use lawfully produced marijuana in a rigorous medical study, and Rosenberg pointed out that research continues on a variety of subjects, including the effects of smoking marijuana in human subjects.

A spokesman for the FDA said the agency shares "an interest in developing therapies from marijuana and its components and have taken aggressive, coordinated action to do so."

The FDA added that well-controlled clinical trials represent the "most appropriate way" to advance scientific understanding and that the drug approval process gives the agency the important ability to determine whether a product meets the FDA criteria for safety and effectiveness.

In December 2015, federal authorities said, they made it easier for researchers conducting clinical trials on cannabidiol, a component of marijuana. Some scientists are studying whether the substance can help treat childhood epilepsy. "That would be a wonderful and welcome development," the DEA letter said, "but we insist that CBD research, or any research, be sound, scientific and rigorous before a product can be authorized for medical use."

What's more, federal authorities said, they are increasing the amount of marijuana available for legitimate research. They said they will open up new avenues for more people and institutions to manufacture marijuana for scientific purposes. Currently, the University of Mississippi is the only such site in the U.S.

"As long as folks abide by the rules, and we're going to regulate that, we want to expand the availability, the variety, the type of marijuana available to legitimate researchers," Rosenberg said. "If our understanding of the science changes, that could very well drive a new decision."

Forty-two states and the District of Columbia allow some form of medical marijuana use, but the federal government has not taken that step despite prodding from federal lawmakers. Last month, the Democratic National Committee endorsed the idea of loosening federal restrictions on marijuana and "providing a reasoned platform for future legalization" in its platform.

For now, there remain two ways to change the federal government's classification of marijuana: for a host of federal agencies including the DEA and FDA to sign off; or for Congress to pass a law, and for the president to sign it.

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