A new study shoots down the notion that medical marijuana laws can reduce opioid overdose deaths, challenging a favorite talking point of legal pot advocates.

Researchers repeated an analysis that sparked excitement years ago. The previous work linked medical marijuana laws to slower than expected increases in state prescription opioid death rates from 1999 to 2010. The original authors speculated that patients might be substituting marijuana for painkillers.

States ravaged by painkiller overdose deaths began to rethink marijuana, leading several to legalize pot for medical use.

When the new researchers included data through 2017, they found the reverse: States passing medical marijuana laws saw a 23% higher than expected rate of deaths involving prescription opioids.

Legalizing medical marijuana "is not going to be a solution to the opioid overdose crisis," said Chelsea Shover of Stanford University School of Medicine. "It would be wonderful if that were true, but the evidence doesn't suggest that it is."

Shover and colleagues reported the findings Monday in Proceedings of the National Academy of Sciences. It's unlikely, they said, that medical marijuana laws caused first one big effect and then the opposite. Any beneficial link was likely coincidental all along.

Pot has been shown to help ease chronic pain, and other studies have suggested that legalizing medical pot may reduce opioid prescribing. So there's still reason to believe that for some people, pot can substitute for opioids as a pain reliever.

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