By Reuters Staff

WASHINGTON (Reuters) - The U.S. Drug Enforcement Administration on Thursday denied requests to loosen the classification of marijuana as a dangerous drug with no medical use.

The decision is the DEA's 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 of the issue.

"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.

For decades, marijuana has been classified that way as a "Schedule I" drug, on par with heroin. The government has repeatedly rejected appeals over the years to reclassify marijuana.

The DEA said on Thursday that it concluded there was "no substantial evidence" that marijuana should be removed from its classification.

Loosening that definition could encourage scientific study of a drug that is being used to treat diseases in several U.S. states despite little proof of its effectiveness.

Twenty-five states have sanctioned some forms of marijuana use for medical purposes. Alaska, Washington, Oregon, Colorado and the District of Columbia have gone even further, allowing its recreational use for adults.