Marijuana advocates say weed is a natural miracle drug that can relieve pain and much more. But rarely do they advance the argument that marijuana saves lives by preventing traffic accidents. Now they will.

A study conducted at Columbia University’s Mailman School of Public Health found a reduction in deaths of drivers who tested positive for opioid use after a car accident in states with medical marijuana laws after legalization.(Although the researchers were unable to test whether the opioid use was directly causing impaired driving, they could determine whether drivers had taken opioids prior to driving.) The upshot, according to the authors of the study: substituting weed for opioids to treat pain could save lives.

The research, published in the American Journal of Public Health in November, was initiated over concerns about high levels of opioid use, particularly prescription pill addiction, an epidemic that the National Institutes of Health says afflicts more than 2 million people in the US. The study is one of the first to assess a link between state medical marijuana laws (MMLs) and opioid use in individuals.

The study was limited to only those places that tested for drugs in a vast majority of crash victims very quickly after death: Reviewing data from the 1999-2013 Fatality Analysis Reporting System from 18 states that tested for alcohol and other drugs within one hour of crashing in at least 80 percent of drivers who died, the researchers concluded that the legalization of medical marijuana was linked to a reduction in traffic fatalities showing opioid use, but only among 21- to 40-year-olds. They posit that widespread legalization could lead to an even more significant reduction in traffic deaths, as well as fewer opioid overdose fatalities.

Other age groups did not reflect any significant difference in opioid-positive driver fatalities after the legalization of medical marijuana, however. The trend is likely due to age requirements that restrict medical marijuana access to patients 21 and older, and the fact that most medical marijuana patients are younger than 45, the researchers theorized.

“This study is about the possible substitution relationship between marijuana and opioids. The toxicological testing data for fatally injured drivers…supports the substitution hypothesis in young adults, but not in older adults,” Columbia professor of epidemiology Dr. Guohua Li, the study’s senior author, said in a statement.

The number of unintentional overdose deaths from prescription pain relievers has soared in the US, more than quadrupling since 1999, according to the US National Institutes of Health. In a 2014 address to the senate caucus on international narcotics control, Nora Volkow, director of the National Institute on Drug Abuse, said about 100 million people in the US suffer from chronic pain ”and for some of them, opioid therapy may be appropriate…But long-term studies demonstrating that the benefits outweigh the risks have not been conducted.”

National health agencies are not promoting marijuana as an opioid substitute for pain relief, or anything else for that matter. Weed is still illegal at the federal level, a schedule one substance deemed to have “no currently accepted medical use.” That’s not likely to change during Donald Trump’s presidency.