SACRAMENTO, Calif. (AP) — California Gov. Gavin Newsom has vetoed a bill aimed at decriminalizing the possession and personal use of several hallucinogens, including psychedelic mushrooms.

The legislation vetoed Saturday would have allowed those 21 and older to possess psilocybin, the hallucinogenic component in what’s known as psychedelic mushrooms. It also would have covered dimethyltryptamine (DMT) and mescaline.

The bill would not have legalized the sale of the substances and would have barred any possession of the substances on school grounds. Instead, it would have ensured people are neither arrested nor prosecuted for possessing limited amounts of plant-based hallucinogens.

Newsom, a Democrat who championed legalizing cannabis in 2016, said in a statement Saturday that more needs to be done before California decriminalizes the hallucinogens.

“California should immediately begin work to set up regulated treatment guidelines - replete with dosing information, therapeutic guidelines, rules to prevent against exploitation during guided treatments, and medical clearance of no underlying psychoses,” Newsom’s statement said. “Unfortunately, this bill would decriminalize possession prior to these guidelines going into place, and I cannot sign it.”

The legislation, which would have taken effect in 2025, would have required the California Health and Human Services Agency to study and to make recommendations to lawmakers on the therapeutic use of psychedelic substances.

Even if California made the bill a law, the drugs would still be illegal under federal law.

In recent years, psychedelics have emerged as an alternative approach to treating a variety of mental illnesses, including post-traumatic stress disorder. The Federal Drug Administration designated psilocybin as a “breakthrough therapy” for treatment-resistant depression in 2019 and recently published a draft guideline on using psychedelics in clinical trials.

Public opinion on psychedelics, which have been mostly associated with 1960s drug culture, has also shifted to support therapeutic use.

Supporters of the legislation include veterans, who have talked about the benefits of using psychedelics to treat trauma and other illnesses.

“Psilocybin gave me my life back,” Joe McKay, a retired New York City firefighter who responded to the 9/11 attacks, said at an Assembly hearing in July. “No one should go to jail for using this medicine to try to heal.”

But opponents said the drugs’ benefits are still largely unknown, and the bill could lead to more crimes — though studies in recent years have shown decriminalization does not increase crime rates. Organizations representing parents also worry the legislation would make it easier for children and young people to access the drugs.

State Sen. Scott Wiener, who authored the bill, called the veto a missed opportunity for California to follow the science and lead the nation.

“This is a setback for the huge number of Californians — including combat veterans and first responders — who are safely using and benefiting from these non-addictive substances and who will now continue to be classified as criminals under California law,” Wiener said in a statement Saturday. “The evidence is beyond dispute that criminalizing access to these substances only serves to make people less safe and reduce access to help.”

He said he would introduce new legislation in the future. Wiener unsuccessfully attempted to pass a broader piece of legislation last year that would have also decriminalized the use and possession of LSD and MDMA, commonly known as ecstasy.

Lawmakers can override a governor’s veto with a two-thirds vote, but they have not tried in decades.

In 2020, Oregon voters approved decriminalizing small amounts of psychedelics, and separately were the first to approve the supervised use of psilocybin in a therapeutic setting. Two years later, Colorado voters also passed a ballot measure to decriminalize psychedelic mushrooms and to create state-regulated centers where participants can experience the drug under supervision.

In California, cities including Oakland, San Francisco, Santa Cruz and Berkeley have decriminalized natural psychedelics that come from plants and fungi.

Despite Newsom’s veto, California voters might have a chance to weigh in on the issue next year. Advocates are attempting to place two initiatives to expand psychedelic use on the November 2024 ballot. One would legalize the use and sale of mushrooms for people 21 and older, and the other would ask voters to approve borrowing $5 billion to establish a state agency tasked with researching psychedelic therapies.