People with Medicaid coverage who live in states with medical marijuana laws are less likely to take medications commonly prescribed to treat conditions like nausea, pain, and depression, according to a new study.

The research, published last week in Health Affairs, looked at nine conditions and found reductions in prescriptions of medications used to treat depression (13%), nausea (17%), psychosis (12%), seizure disorders (12%), and pain (11%). There was no statistically significant difference for drugs used to treat anxiety, glaucoma, sleep disorders, and spasticity.

The study examined all fee-for-service Medicaid prescriptions between 2007 and 2014 in the 28 states and the District of Columbia that now have medical marijuana laws, comparing usage rates with  states where medical marijuana is illegal.

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“We’re only able to see what’s happening to the FDA-approved prescription drugs when medical marijuana becomes an option,” said W. David Bradford, the George D. Busbee Chair in Public Policy. He co-authored of the study with his daughter Ashley Bradford, who is pursuing a master’s degree at the University of Georgia.

Last year, they studied prescription-drug usage for the same conditions among Medicare beneficiaries in states where medical marijuana is legal. The findings were similar, with some small differences, Bradford said.

Medicaid beneficiaries tend to be younger and poorer than Medicare beneficiaries. In addition, younger people are more likely to use medical marijuana than individuals in their 60s or older although research from dispensaries has found that one-quarter of customers are older than 51, Bradford noted. Another difference is that people covered by Medicare are more likely to use medical marijuana to address anxiety and sleep disorders than the Medicaid population.

According to Express Scripts’ most recent Drug Trend Report, spending among Medicaid beneficiaries on treatments for mental and neurological disorders fell 21.6% in 2016, while spending on drugs that treat pain and inflammation rose 3.9%. Spending on mental and neurological disorders among Medicare beneficiaries also fell in 2016, by 12.9%. The PBM attributed the decline in spending on drugs that treat mental and neurological disorders to new generic drugs.

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Not only does the legalization of medical marijuana raise questions about utilization — particularly for drugmakers that market therapies that treat these conditions — it also hints at a more complex topic: how people pay for treatment. Patients who opt for medical marijuana pay for that treatment out of pocket, Bradford said. A prescription drug, on the other hand, is more likely than not partially covered by health insurance. “There is a shift in who is paying,” he noted.

That issue may become more prevalent as marijuana policy continues to evolve state by state. In 2007, roughly 12 states had legalized medical marijuana. However, it wasn’t until New Mexico passed legislation in 2009 that states began to allow dispensaries, as opposed to home cultivation alone. By 2014, 23 states had passed medical-marijuana laws.

In addition, some drugmakers like GW have started to study cannabis-based pharmaceuticals.

Bradford said he plans to next study a commercially insured population.