Fewer people are using opioids in states that have legalized medical marijuana, according to a study published Thursday in the American Journal of Public Health that bolsters advocates’ claims that marijuana can substitute for more deadly drugs. (emphasis added)

The study, which examined data from 1999 to 2013, found an association between a state legalizing medical marijuana and a reduction in testing positive for opioids after dying in a car accident, particularly among drivers ages 21 to 40.

Previous studies have found that opioid overdoses went down after medical marijuana laws were enacted, but this study was geared at opioids use more generally. Researchers noted that those who tested positive may have been taking opioids as a doctor prescribed, and the use of the drug was not necessarily a contributing factor to the vehicle crash.