For the past 20 years, scientific studies have warned that heavy marijuana use can lead to schizophrenic episodes and psychotic disorders.
Now, in the first study of its kind, RTI International in Research Triangle Park suggests marijuana may carry other risks as well.
The research organization said Monday that states with permissive medical marijuana laws have significantly higher rates of serious mental illness. The study, published in the peer-reviewed International Review of Psychiatry, was based on data involving 630,000 people interviewed between 2008 and 2015.
RTI’s study doesn’t explain the causes of the relationship between marijuana legalization and mental illness. It’s possible that states with higher rates of serious mental illness were quicker to legalize medical marijuana, said Lauren Dutra, a social scientist at RTI and the lead author of the study.
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“We know there’s some sort of relationship between marijuana legislation, marijuana use and mental health,” Dutra said. “Your risk of certain mental health problems is higher if you use marijuana, particularly if you use marijuana heavily.”
RTI describes itself as a leader in marijuana research and has been conducting studies for the past four decades. The subject has exploded in recent years, generating more than 100 studies between 2012 and 2016 alone, compared to fewer than 10 studies a year during the 1990s, according to a literature review in the Current Psychiatry Reports journal.
But the link between marijuana and brain disorders remains a mystery, said Paul Armentano, deputy director of The National Organization for the Reform of Marijuana Laws.
“The relationship between marijuana use and mental illness is not well understood and it appears to be fairly complicated,” Armentano said. “It could be that people with mental illness are self-medicating. ... It’s also possible that in some cases, marijuana may exacerbate these latent conditions.”
RTI’s study leaves some questions unanswered. For example, it didn’t look at mental illness rates in states before they passed medical marijuana laws.
A 2016 review in Current Psychiatry Reports journal said that early marijuana use and heavy marijuana smoking is more likely among people with behavioral problems, rather than the cause of those problems. The journal warned against spreading scare stories about permanent brain damage caused by pot use.
“It has been argued that even if we are uncertain that cannabis actually causes psychosis, it is better to err on the side of caution and warn cannabis users, psychiatric patients and the general public about this potential danger of cannabis use,” the article stated. “However, those adolescents most at risk for beginning cannabis use are already suspicious about the official warning messages when it is perfectly clear that cannabis use is not approved by the general society. If we wish scientists to be taken seriously when we do discover real and substantial dangers, then we believe it would be better to avoid behaving like ‘the boy who cried wolf.’ ”
Dutra said that as public acceptance of marijuana increases, more data will be needed to help shape responsible policy regarding the plant’s legal status. North Carolina has a 4.1 percent rate of serious mental illness, in line with the national average. Legislation to legalize medical marijuana has failed in North Carolina repeatedly, and a bill to allow up to 4 ounces for personal use is currently pending in the state legislature.
Nationally, 29 states and the District of Columbia have legalized medical marijuana, and eight states have legalized recreational pot. The RTI study only considers medical marijuana laws without regard to recreational smoking.
The RTI study identified two categories of medical marijuana laws: liberal laws that let people use pot for mild conditions, such as anxiety and sleep problems, and restrictive laws that let people use marijuana only for serious, chronic problems, like managing nausea after chemotherapy and various life-threatening conditions.
RTI found that states with permissive laws had higher rates of serious mental illness, and the rate was about 2 percent higher than states with restrictive medical marijuana laws or states where medical marijuana is illegal.
Serious mental illness refers to a debilitating condition that interferes with one’s ability to work and harms family relationships.
Medical marijuana is gaining acceptance around the country. In 2008, two states had restrictive medical marijuana laws and 10 states had liberal medical marijuana laws, Dutra said. In 2014, nine states had restrictive medical marijuana laws and 14 states had liberal medical marijuana laws.
RTI is a global contract research organization that does work for U.S. federal agencies and other clients. The nonprofit employs 5,000 people globally, including 2,200 in RTP.
Dutra has previous experience studying the effects of substances that are smoked. Her previous research into e-cigarettes concluded that vaping is often not a substitute for tobacco but instead leads young people to smoke the real thing.