DENVER (Reuters) - Denver voters will decide in May whether to decriminalize possession of small amounts of the hallucinogenic drug psilocybin, which would make it the first U.S. city to halt prosecution of people caught with psychedelic mushrooms.

The citizen-driven proposal, which election officials said this week reached the required number of signatures to be on the city’s municipal ballot, would not legalize so-called “magic mushrooms,” but rather make them a low priority for law enforcement, according to its language.

Decriminalize Denver, the group behind the ballot question, said the drug has medical benefits that could reduce psychological stress and opioid dependence.

“Nationally, Denver and the state of Colorado have represented the first movers in a revised understanding of the potential benefits of naturally-occurring psychoactive medicines,” the group said on its website.

Some opponents worry that if passed the ordinance would further tarnish the city’s image, given that recreational marijuana is already allowed under Colorado law, and another proposal by the city to create the country’s first safe injection site for intravenous drug users was approved by the city council in November.

“Denver is quickly becoming the illicit drug capitol of the world,” Jeff Hunt, director of the Colorado-based Centennial Institute, a conservative think tank, said in a statement. “High potency pot, proposed needle injection sites, and now an effort to decriminalize mushrooms.”

The safe injection site pilot program would need the approval of the state legislature, which has not yet taken up the issue. Federal authorities have warned that such a facility would be illegal.

Kevin Matthews, 33, campaign director for Decriminalize Denver, said worries about expanded drug use under the measure are unwarranted.

“Nothing on our ballot question would do anything to increase access – it does not allow for distribution and sale,” Matthews told Reuters in a phone interview, adding that mushrooms have helped treat his depression.

Mayor Michael Hancock told the Denver Post that he opposes the mushroom question.

Psilocybin is illegal under both Colorado and federal law. The U.S. Drug Enforcement Administration classifies the drug as a Schedule 1 substance, meaning the agency has deemed that it has a high potential for abuse and currently has no accepted medical use.

In 2004, Denver voters voted to decriminalize marijuana possession, years before Colorado voters voted to approve its legalization for recreational use and establish a full regulatory framework.