By Doina Chiacu and Bill Berkrot

WASHINGTON/NEW YORK (Reuters) - The U.S. Drug Enforcement Administration on Thursday denied requests to stop classifying marijuana as a dangerous drug with no medical use, leaving users and businesses in limbo after many states have legalized it for medical or recreational purposes.

The DEA though did relax certain restrictions on growing marijuana for research purposes.

For decades, marijuana has been listed as a "Schedule I" drug, placing it on par with heroin. The government has repeatedly rejected appeals for reclassification.

"Marijuana shouldn’t be listed as Schedule I," U.S. Representative Earl Blumenauer, a Democrat from Oregon, said in a statement. He said the decision left "patients and marijuana businesses trapped between state and federal laws."

Thursday's DEA decision was a response to a 2011 petition by two former state governors who had urged federal agencies to re-classify marijuana as a drug with accepted medical uses.

In a letter to the petitioners, the DEA said it had asked the Department of Health and Human Services for a scientific and medical evaluation.

"HHS concluded that marijuana has a high potential for abuse, has no accepted medical use in the United States, and lacks an acceptable level of safety for use even under medical supervision," the letter said.

That assessment comes amid statistics showing zero overdose deaths due to marijuana each year at a time of an alarming rise of heroin-related deaths in the United States as politicians debate remedies for exploding opioid abuse.

Twenty-five states have sanctioned some forms of marijuana use for medical purposes. Alaska, Washington, Oregon, Colorado and the District of Columbia now allow recreational use for adults, while California and eight other states have recreational or medical marijuana proposals headed for their 2016 ballots.

On Thursday, Arizona became the latest state to certify a ballot initiative to legalize recreational marijuana for its November ballot.

The position of the U.S. government on marijuana has impacts for states that have legalized its recreational or medical use. Doctors and businesses growing, selling or prescribing cannabis may be operating legally within a given state but still acting in opposition to federal law, risking arrest and sanctions.

Legal marijuana businesses have difficulty finding banks that will work with them, also due to concerns about breaking federal law.

Some experts have argued that medical marijuana could help cut opioid use.

Taylor West, deputy director of the National Cannabis Industry Association, said the DEA's decision would curtail research since marijuana would remain a criminal product.

“Research institutions are going to be somewhat hesitant if they think they will potentially jeopardize other research funding,” she said. "This decision by the DEA really flies in the face of objective science."

Smart Approaches to Marijuana, an alliance of doctors, policy makers and treatment professionals who oppose legalization, took a different view.

"I think it was a balanced decision and isn’t surprising to the scientific community," said President Kevin Sabat.

Meanwhile, the DEA will allow more growers to apply for certification by the agency to help supply researchers "with a more varied and robust supply of marijuana." Now the University of Mississippi is the lone such supplier.

Britain’s GW Pharmaceuticals <GWP.L>, which is developing a promising cannabis-derived epilepsy treatment, has said it will take longer to reach the U.S. market even with Food and Drug Administration approval because it would then have to be separately rescheduled by the DEA.

(Additional reporting by Natalie Grover in Bangaluru, Jilian Mincer in New York and Sharon Bernstein in Sacramento; Editing by Lisa Von Ahn and Diane Craft)