Kevin Matthews believes "magic mushrooms" saved his life.

Matthews, a former cadet at the U.S. Military Academy at West Point, had no choice but to retire from the service because of major depression.

"I struggled tremendously with that, because my life's purpose was to serve as a career army officer. It felt like my life crumbled," Matthews, 33, said.

But when he returned to Denver in 2011, he tried psilocybin mushrooms — a mind-altering experience that he said "lifted the fog" and dramatically alleviated his depression.

He is now leading a citizen-sponsored campaign to effectively decriminalize the psychedelic substance in "magic mushrooms" in Denver. Initiative 301, as it's known locally, goes up for a vote Tuesday.

The initiative would change the city code to say that enforcing laws for possession of psilocybin mushrooms by people 21 or older "shall be the lowest law enforcement priority in the City and County of Denver."

But there are caveats. It would still be illegal to possess the mushrooms even if the initiative passes, the measure would not apply to Colorado as a whole, and sales would still be considered a felony.

A small body of research has found that psilocybin helps reduce the symptoms of anxiety and depression for cancer patients, and advocates have said the substance can help with post-traumatic stress disorder.

"Psilocybin therapy looks like it is a new paradigm in the treatment of psychiatric disorders," said Matthew Wayne Johnson, an associate professor of psychiatry and behavioral sciences at Johns Hopkins University.

"It's very promising, and we are seeing significant effects," Johnson added, although he noted there are risks outside of a controlled setting, including increased anxiety and paranoid thoughts.

It remains illegal nationwide, outlawed by the federal government since 1968 and formally classified as a Schedule I drug with "no currently accepted medical use and a high potential for abuse."

The substance, most commonly associated with the hippie culture of the 1960s, has been used in various religious settings for decades, and it is known to radically alter perceptions or inspire mystical experiences.

But it has also long been a draw for recreational users, and some of the "Decriminalize Denver" campaign's detractors have warned of what they see as a slippery slope.

"Denver is quickly becoming the illicit drug capital of the world," Jeff Hunt, director of the Centennial Institute at Colorado Christian University, a conservative think tank, told the Los Angeles Times.

"The effort to decriminalize is the first step in a movement to totally commercialize the drug," Hunt said.

In response to that criticism, Matthews said "the point of our campaign is protecting individual rights, the right to use something that's a naturally-occurring substance with tremendous medical potential."

The pro-psilocybin campaign has raised roughly $45,000 to date and has encountered relatively little organized opposition in the Mile High City, Matthews said.

NBC News reached out to several local officials Monday to get their thoughts on the initiative.

The office of Denver's mayor, Michael Hancock, did not immediately respond to a request for comment on the initiative. The Washington Post has reported Hancock is opposed to it.

The district attorney, Beth McCann, opposes the initiative, partly because the city is still trying to understand the effects of marijuana decriminalization, according to her spokeswoman Carolyn Tyler.

Denver decriminalized marijuana in 2005, and seven years later Colorado became the first state in the United States to legalize it for recreational use.

The city's police department does not have an official position on the initiative and has not commented on it publicly, according to police spokesman Doug Schepman.

A similar push to effectively decriminalize the use, possession, growth, sale and transportation of "magic mushrooms" in California last year failed to get to the ballot.