There’s ‘no evidence’ marijuana works to treat most mental health disorders, largest review of its kind finds

There’s “no evidence” that medical marijuana is effective in treating anxiety, depression or post-traumatic stress, according to a new study.

The review of 54 international trials published over a span of 45 years – the largest of its kind to date – could change the way marijuana is used medically.

Some experts previously linked using marijuana to reduced stress and anxiety, but research has also shown using the drug can result in heightened symptoms of anxiety, depression and mental disorders, according to the Centers for Disease Control and Prevention.

The new findings indicate the need for greater regulation of medical marijuana use, the Australian authors said.

“Though our paper didn’t specifically look at this, the routine use of medicinal cannabis could be doing more harm than good by worsening mental health outcomes,” The University of Sydney’s Dr Jack Wilson, the study’s lead author, explained in a statement. “For example a greater risk of psychotic symptoms and developing cannabis use disorder, and delaying the use of more effective treatments.”

More than 48 million Americans are battling a substance use disorder, according to federal data from 2024 – the latest year for which data is available. And, over 22 million Americans aged 12 and older battled a marijuana use disorder in the past year, according to the National Survey on Drug Use and Health, rising from 19 million in 2023.

The percentage of Americans who use marijuana has also increased to historic highs, with the number of people who smoke it more than doubling from 2013 to 2024, a Gallup survey conducted that year showed.

Marijuana is the most commonly used federally illegal drug in the U.S., and the CDC says 52.5 million Americans used it at least once in 2021.

Medical marijuana is legal in 40 states and Washington, D.C. The number of Americans who use medical marijuana is much smaller, and researchers estimated in 2021 that tally was around three million and growing.

Many of those people are older adults, who are disproportionately impacted by chronic disease, long-term pain and injuries – even as researchers warn that there are physical and mental risks to regular marijuana use for people over the age of 65 and that more studies need to be done to determine its effects.

Studies from researchers at Stanford Medicine found that regular use was correlated with a 29 percent increase in heart attacks and a 20 percent increase in strokes.

“There is no safe amount of cannabis. Low doses and occasional use are still associated with vascular inflammation,” Dr. Joseph Wu, the director of the Stanford Cardiovascular Institute, said in October. “Abstinence is the safest option for heart health.”

The new analysis found that medical marijuana used for other conditions “may be beneficial” in certain health conditions, such as pain and multiple sclerosis. Two studies published in October showed marijuana could help alleviate back pain.

“But the overall quality of evidence for these other conditions, such as autism and insomnia, was low,” said Wilson, also noting that there is no universal experience of autism and that those results should be treated with “with caution.”

The researchers noted that medicinal marijuana was found to increase cravings for cocaine, as well, among people living with cocaine-use disorder.

For people with marijuana dependency, the analysis showed that an oral form could help “when administered alongside psychological therapy.”

A separate study published by Johns Hopkins Medicine on Tuesday found that young people with marijuana use disorder were more likely than others with different substance use disorders to be diagnosed with a psychiatric disorder. The same was not true for adults.