The US Supreme Court Monday declined to hear an appeal from medical marijuana advocacy groups who had challenged the DEA's decision to maintain marijuana's status as a Schedule I drug under the Controlled Substances Act, the category reserved for the most dangerous substances.

The court denied in summary order a petition for a writ of certiorari from the groups, led by Americans for Safe Access , which had sought Supreme Court review of a DC Circuit Court of Appeals decision upholding the DEA's ruling that a change in marijuana's classification required the Food and Drug Administration's recognition of acceptable medical uses for the drugs.

Advocates of rescheduling marijuana have been trying to do so for more than four decades, but have been thwarted by DEA delays and intransigence. This was the third formal rescheduling effort to be blocked by DEA decision making.

Schedule I drugs are deemed to have no acceptable medical uses and a high potential for abuse. Other Schedule I drugs include LSD, MDMA, and heroin. Despite the fact that there is an ever-increasing mountain of research detailing marijuana medicinal effects and despite the fact that 20 states and the District of Columbia have legalized medical marijuana, the DEA continues to insist that it cannot be down-scheduled.

Joe Elford, lead attorney on the case for Americans for Safe Access, told Law360 that the Supreme Court's denial of certiorari was in line with its reluctance to overturn lower court and administrative decisions on medical marijuana.

"It's disappointing, but not altogether surprising," he said.

A fourth effort to reclassify marijuana led by the governors of the medical marijuana states of Rhode Island and Washington was filed in 2011 and is still awaiting action.

[For extensive information about the medical marijuana debate, presented in a neutral format, visit MedicalMarijuana.ProCon.org.]