Denver will become the first city in the United States to decriminalise "magic mushrooms", based on final unofficial results of a ballot initiative about the hallucinogenic drug.
The initiative called for Colorado's capital to end the imposition of criminal penalties for individuals at least 21 years of age for using or possessing psilocybin, widely known as magic mushrooms.
The Denver Elections Divisions will certify results on May 16, but the final count on its website on Wednesday was 50.56 per cent of voters in favour and 49.44 per cent against.
If the initiative is approved, psilocybin would still remain illegal under both Colorado and federal law.
The US Drug Enforcement Administration classifies the drug as a Schedule 1 substance, meaning the agency has deemed that it has a high potential for abuse with no accepted medical application.
Decriminalize Denver, the group behind Tuesday's ballot question, said psilocybin had a wide range of medical benefits.
It has been shown to reduce depression and anxiety and to help in treating tobacco, alcohol and opioid addictions, and with alleviating symptoms of post-traumatic stress disorder (PTSD), according to the organisation.
Some opponents worry that if passed, the initiative would increase the city's image as a haven for drugs, given that Colorado was one of the first states to legalise possession and sale of marijuana for adult recreational use.
Denver District Attorney Beth McCann opposed the initiative. But if the measure were approved, she supported formation of a review panel under the initiative to study the effects of the drug and the impact the ordinance would have on Denver, spokeswoman Carolyn Tyler said.
Denver residents first voted to decriminalise cannabis possession in 2004, years before Colorado voters ultimately approved its legalisation statewide for recreational purposes, establishing a full regulatory framework to license retail outlets and collect sales taxes on cannabis products.