Making medical marijuana legal may lead to a reduction of opioid use in adults under the age of 40, a new study suggests.

The researchers found that the rates of opioid use decreased in adults ages 21 to 40 in states that had legalized medical marijuana and where residents with prescriptions could obtain cannabis from dispensaries or grow their own, compared to states that had legalized medical marijuana but did not yet have an operational program for people to obtain it.

However, the finding didn’t apply to adults over 40. For this group, opioid use did not decrease in those states with operational medical marijuana programs, according to the findings, which were published online today (Sept. 15) in the American Journal of Public Health. [Healing Herb? Marijuana Could Treat These 5 Conditions]

These findings seem to support the idea that marijuana may offer a substitute for opioids in people ages 21 to 40 who have severe or chronic pain, said lead author June H. Kim, a doctoral candidate in epidemiology at the Columbia University Mailman School of Public Health in New York City.

There is other evidence that medical marijuana may act as a substitute for opioids in states that have passed this legislation: A study published in JAMA Internal Medicine in 2014 suggested that the legalization of medical marijuana in U.S. states appears to be linked with lower death rates from opioid overdoses within that state.

Medical marijuana and opioid use

Opioid use can be difficult to measure in general, but a positive test for opioids in the blood or urine provides a clear indication of prior use, the researchers wrote. However, in the new study, the researchers did not have data that was drawn from the general population showing people who tested positive for opioid use while they were alive, so instead, they used toxicological data from deceased individuals.

In the study, the researchers looked at data from toxicological tests for alcohol and other drugs that were found in the systems of drivers killed in car crashes. Some states collect this information on a yearly basis for the majority of drivers who die in crashes on public roads, according to the study.

Toxicological testing from deceased drivers who crashed in states that did or did not have medical marijuana laws seemed like an interesting data source, and it is an objective way to evaluate prior opioid use, for both medical or recreational purposes, Kim said.

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