By Lisa Rapaport

(Reuters Health) – Legalizing medical marijuana no longer appears to be linked with a drop in fatal opioid overdoses, according to a new U.S. study that calls into question the potential for cannabis to help fix the opioid crisis.

In recent years, many advocates for legalizing marijuana – including some doctors and public health officials – have cited a pivotal 2014 study that found lower rates of fatal opioid overdoses from 1999 to 2010 in the states that legalized medical marijuana.

For the current study, published in the Proceedings of the National Academy of Sciences, researchers used similar methods to take another look at the same period examined in the 2014 study and extend the analysis through 2017, to include many states that only recently legalized medical marijuana.

The new study found a similar result for the same period covered in that 2014 study: about a 21 percent decrease in opioid overdose deaths for every 100,000 people in the population when states legalized medical marijuana.

But when the new study looked over more time – from 1999 to 2017 – they found an almost 23 percent increase in opioid overdose deaths in states with medical marijuana laws.

“With the benefit of a longer time span … we conclude that medical cannabis laws do not seem to have reduced opioid overdose mortality at the population level,” said lead study author Chelsea Shover of Stanford University School of Medicine in California.

Like the 2014 study, the new analysis can’t show whether people are using marijuana instead of opioids for pain relief or recreation, Shover said by email. And these studies also weren’t designed to determine the safety or effectiveness of medical marijuana for any specific health issues.

The current study didn’t find a difference in opioid deaths associated with legalized marijuana based on how permissive or restrictive state laws might be, or whether states allowed only medical pot or also permitted recreational use.

California became the first state to legalize medical marijuana in 1996. Today 47 states permit some version of medical pot.

It’s possible that the connection between marijuana laws and opioid overdose deaths has shifted over time due at least in part to differences in the characteristics of the states that did this years ago and states that did this only recently, the study authors note.

“States that legalized medical cannabis early formed a group that was pretty different from the rest of the U.S.,” Shover said.

The first study was comparing a group of 13 mostly western, politically liberal states to the rest of the country, and the rest of the country also happened to be where the opioid overdose crisis was growing,” Shover added.

“I think that shared characteristics between those 13 states – things like less incarceration of people using drugs, more availability of treatment for opioid use disorder, and more availability of the overdose reversal drug naloxone – explain the association they found,” Shover added.

Now that almost every state has passed some version of a medical marijuana law, “you get to see how it plays out in a more representative group,” Shover said.

Still, the results were a surprise to Brendan Saloner, a co-author of the 2014 study and a researcher at Johns Hopkins Bloomberg School of Public Health in Baltimore.

“I was not expecting this finding, but I think that it could plausibly be explained by the changing nature of the opioid crisis,” Saloner, who wasn’t involved in the current study, said by email.

“Specifically, heroin and fentanyl have been involved in a lot more overdose deaths – including deaths that also involve prescription opioids — and that could reduce the protective effect of medical cannabis,” Saloner said.

“Second, the states implementing medical cannabis laws, and the way these laws have been implemented, has been changing over time, and it may be that they are getting less effective at reducing harmful opioid use,” Saloner added.

SOURCE: Proceedings of the National Academy of Sciences of the USA, online June 10, 2019.