Only a few days ago, millions of Americans probably had never heard of psilocybin, the active agent in psychedelic mushrooms, but thanks to Denver, it is about to get its moment in the political sun. On Tuesday, the city’s voters surprised everyone by narrowly approving a ballot initiative that effectively decriminalizes psilocybin, making its possession, use or personal cultivation a low-priority crime.

The move is largely symbolic — only 11 psilocybin cases have been prosecuted in Denver in the last three years, and state and federal police may still make arrests — but it is not without significance. A measure legalizing psilocybin therapy is likely to be on the ballot in Oregon in 2020, and activists in California are mounting a second campaign to get a decriminalization measure on the ballot there. For the first time since psychedelics were broadly banned under the 1970 Controlled Substances Act, we’re about to have a national debate about the place of psilocybin in our society. Debate is always a good thing, but I worry that we’re not quite ready for this one.

No one should ever be arrested or go to jail for the possession or cultivation of any kind of mushroom — it would be disingenuous for me to say otherwise, since I have possessed, used and grown psilocybin myself. Like many others, I was inspired to do so by the recent renaissance of research into psychedelics, including psilocybin.

Scientists at places such as Johns Hopkins, New York University, Harbor-U.C.L.A. Medical Center and Imperial College in London have conducted small but rigorous studies that suggest a single psilocybin trip guided by trained professionals has the potential to relieve “existential distress” in cancer patients; break addictions to cigarettes, alcohol and cocaine; and bring relief to people struggling with depression. Psychiatry’s current drugs for treating these disorders are limited in their effectiveness, often addictive, address only symptoms and come with serious side effects, so the prospect of psychedelic medicine is raising hopes of a badly needed revolution in mental health care.