Gut Check looks at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The claim:

When legal medical marijuana dispensaries start operating in a state, deaths from opioid overdoses in that state drop.

Tell me more:

In the United States, 25 states have legalized medical marijuana, including 19 that let patients with a prescription buy pot from dispensaries. Proponents argue that expanding the availability of medical marijuana reduces opioid abuse and overdose deaths because it gives people an alternative for pain relief.

About 3 out of 5 opioid overdoses occur in people with legitimate prescriptions for pain pills. These are the people who might opt for medical marijuana instead.

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Three recent studies support that claim.

In 2014, researchers found that states with any kind of medical marijuana law had a 25 percent lower rate of death from opioid overdoses than other states. The apparent effect grew over time: a 20 percent lower rate of opioid deaths in the laws’ first year, 24 percent in the third, and 33 percent in the sixth, researcher Colleen Barry of Johns Hopkins Bloomberg School of Public Health and her colleagues reported in JAMA Internal Medicine. In 2010 (the last year for which the researchers had data), there were 1,729 fewer such deaths than expected in medical marijuana states.

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A 2015 study drilled down into different aspects of marijuana laws. Medical marijuana dispensaries were associated with a 16 percent (and possibly as high as a 31 percent) decrease in opioid overdose deaths, especially among men, compared to states without dispensaries, said health economist Rosalie Liccardo Pacula of RAND, who led the study, which has been submitted to an economics journal. The decline seemed to be driven by lower rates of illegal opioid use.

And a July study in Health Affairs found that doctors in states with medical marijuana laws wrote fewer prescriptions for pain meds to Medicare patients than their colleagues in other states. That suggested that the reason for the decline in opioid deaths seen in the two earlier studies might indeed be because doctors are prescribing fewer of the drugs.

The three studies “tell a similar story,” said Pacula: After medical marijuana comes to a state, “fewer people are dying” of opioid overdoses. “If it were just one study, or two, I’d be less confident,” she added. “But now there are three.”

Experts said they were not aware of any rigorous study finding that medical marijuana serves as a gateway to opioid abuse.

Really?

The first question about studies like these is whether states with medical marijuana differed in other ways from states without it. Maybe the pot-tolerant states also had tough opioid monitoring programs, imposed strict oversight of pain clinics, or had better addiction-treatment programs — all of which can reduce opioid abuse and overdose deaths.

When researchers in the 2014 study took into account trends in opioid deaths before the medical marijuana laws, the association between the deaths and laws were just barely statistically significant (meaning the odds were less than 5 percent that the association was due to chance). But they squeaked over that crucial threshold.

RAND’s finding of a 16 percent drop in opioid deaths following the legalization of medical marijuana dispensaries also held up even after the researchers “put this through the econometric wringer,” said Pacula: they checked that the results weren’t due to existing trends simply continuing, or to other factors (such as changes in unemployment rates or demographics) that might differ between medical-marijuana states and others. The results of the Medicare study also held up after checking for such confounding factors.

“Overall, we find strong, consistent evidence that medical marijuana dispensaries lead to reductions in opioid-related mortality,” the RAND team concluded. Some people who would otherwise be at risk of abusing opioids, perhaps fatally, must have turned to medical marijuana instead, Pacula said.

Whether more physicians in medical marijuana states discussed the potential benefits of cannabis, or more patients — aware of the dispensaries — sought it out isn’t known, however. “We don’t yet have studies tracking what individuals are actually doing,” said Hopkins’ Barry.

The verdict:

After a state legalizes medical marijuana, and especially after dispensaries start operating, opioid deaths fall.