People may smoke weed to "mellow out," but experts say medicinal cannabis and its chemical relatives don’t effectively relieve mental health disorders like depression or anxiety . The evidence just isn't there.

That's according to the most comprehensive analysis of medical cannabinoids and mental health to date.

Cannabinoids, which include any chemical derived from the Cannabis plant that exerts drug-like effects in the body, "are often advocated as a treatment for various mental health conditions," according to the new analysis, published today (Oct. 28) in the journal The Lancet Psychiatry . Despite the hype, little evidence supports the notion that medicinal cannabinoids treat mental health disorders more effectively than placebo pills — treatments designed to do nothing, study co-author Louisa Degenhardt, a professor of epidemiology at the University of New South Wales in Sydney, Australia, said in a statement.

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What little research does exist fails to meet standard criteria required to prove that a given drug is safe and effective for widespread use, the authors noted. "Clinicians and consumers need to be aware of the low quality and quantity of evidence for the effectiveness of medicinal cannabinoids in treating mental health disorders and the potential risk of adverse events," they wrote.

Products like medicinal cannabis, pharmaceutical-grade extracts and the synthetic derivatives THC and CBD have gained popularity among consumers as more countries begin legalizing their use for the treatment of medical conditions, according to the new paper. ( CBD is a compound derived from marijuana that does not produce a "high.") Both mainstream media and the commercial sector have hopped on the bandwagon, spreading the word about the many potential therapeutic uses of cannabinoids. The mounting trend may be ahead of the science, though, as Degenhardt and her co-authors found.

The new analysis pooled data from 83 studies aimed at learning whether medicinal cannabinoids can improve mental health disorders, overall, or alleviate their various symptoms. Of these, most examined the effects of THC, with or without added CBD, and only 40 studies were randomized controlled trials — the "gold standard" for drug testing wherein participants are randomly assigned to either receive the treatment or not. The research included about 3,000 people and focused on six mental health disorders: depression , anxiety, attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, post-traumatic stress disorder (PTSD) and psychosis.

Overall, the results left experts unimpressed.

THC failed to meaningfully improve the major symptoms of any of the six disorders, and many patients on the drug experienced more adverse effects and withdrawal symptoms compared with those who took a placebo. Of the randomized controlled trials that looked at depression and anxiety, most prescribed a cannabinoid for a different health condition entirely, such as chronic pain. Hardly any studies examined the effects of CBD or medicinal cannabis on the various disorders. Given the sheer scarcity of data and the poor quality of what little exists, "there remains insufficient evidence to provide guidance on the use of cannabinoids for treating mental disorders within a regulatory framework," the authors concluded.

The study did uncover some "very-low-quality" evidence that suggests that THC, the main mind-altering compound found in cannabis, may help relieve anxiety symptoms in people with other serious health conditions, such as chronic pain or multiple sclerosis. However, in these cases, it's unclear whether the THC use improved symptoms directly, or if the drug primarily improved patients' underlying health problems, and their anxiety improved as a result.

Singular studies also suggested that pharmaceutical products that contain both THC and CBD may improve overall functioning in people with PTSD , while pharmaceutical CBD alone may do the same for patients with psychosis. But neither drug appeared to alleviate the major symptoms of either disorder, and one study even suggested that THC-CBD may worsen the negative symptoms of psychosis, such as social withdrawal and blunted emotions.

Dr. Scott Krakower, assistant unit chief of psychiatry at Zucker Hillside Hospital in Glen Oaks, New York, who was not involved in the study, told Live Science that the idea that cannabis may worsen certain mental health disorders is well-established. "This is especially true of people with preexisting psychotic conditions," he added, while those with a family history of psychosis could place themselves at risk of developing the disorder by taking cannabinoids.

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In addition, research suggests that repeated or heavy cannabis use can permanently disrupt the body's natural "endocannabinoid system," which produces chemicals that bind to the same receptors as cannabinoids do, though how this disruption affects people with different mental health disorders remains under investigation. Anecdotally, the drugs appear to have a "paradoxical effect" where they help people feel better in the short term, but worsen their symptoms over time, Krakower said. For example, recent research suggests that cannabis may reduce ratings of stress, depression, and anxiety, at first, but chronic use may worsen depressive symptoms over time.

"In light of the results of this comprehensive review and meta-analysis, it would be hard for practitioners to justify recommending the use of cannabinoids for psychiatric conditions at this time," Dr. Deepak Cyril D'Souza, a professor of psychiatry at Yale University School of Medicine, wrote in a commentary accompanying the new study. D'Souza noted that research on cannabinoids may be lacking because the drugs have long been under tight governmental regulation. To better understand the potential role of cannabinoids in mental health treatment, existing "barriers to research" will have to be toppled.

Looking forward, scientists will need to determine exactly how cannabinoids ease the symptoms of mental health disorders, assuming they do at all, D'Souza added. "From a mechanistic standpoint, it is uncertain how or why cannabinoids could be effective in treating depression, ADHD, psychosis, anxiety and post-traumatic stress disorder — conditions that share no obvious common pathophysiology," he wrote.

In short, the experts agree: medicinal cannabinoids should be subjected to the same rigorous scrutiny of every other psychiatric medication.

In the future, Krakower hopes that both the public and health care providers stay informed about the actual science of cannabinoid use and make decisions based on solid data, rather than hearsay.