Attorney General Jeff Sessions said Wednesday he doubts the accuracy of scientific research that increasingly links state-legal medical marijuana to reductions in opioid use and overdose deaths.

"I think one study that suggested there’s some sort of inverse relationship between increased marijuana use and reducing of deaths, I did see that," Sessions said during a Senate Appropriations Committee hearing.

"I’ve asked my staff to take a look at it because science is very important, and I don't believe that will be sustained in the long run," he added, citing opposition to pot legalization by medical groups.

Sessions was addressing Sen. Brian Schatz, D-Hawaii, who told him, "There are credible scientific studies that show where medical marijuana is legal, opioid overdose deaths have gone down."

More than two dozen states allow medical marijuana, though the specific qualifying conditions and dispensary rules vary. Nine states and the nation's capital have laws allowing adults 21 and older to use marijuana recreationally.

Sessions, a longtime reform opponent, requested last year that Congress drop a budget rider protecting state medical pot programs. In January, he withdrew a 2013 Justice Department policy shielding state recreational cannabis programs. Pot possession remains a federal crime, but President Trump assured Sen. Cory Gardner, D-Colo., this month that he would not allow a federal crackdown.

Although Sessions expressed doubt, multiple studies have found state pot programs are associated with less prescription painkiller use and fewer fatal opioid overdoses, of which there were roughly 42,000 in 2016, with an uptick driven by cheap synthetic opioids cut into heroin and counterfeit prescriptions.

A 2014 study published in JAMA Internal Medicine found 24.8 percent fewer opioid overdose deaths between 1999 and 2010 in the 13 states that had legalized medical pot.

"In 2010, this translated to an estimated 1729 fewer deaths than expected," the study abstract said, adding that reductions in opioid deaths "generally strengthened in the years after passage" of medical pot laws.

A pair of papers published by JAMA this month found reductions in opioid prescriptions in medical marijuana states among low-income people using Medicaid and among enrollees in Medicare Part D, which covers prescriptions for senior citizens and the disabled.

One of the new studies said a review of "Medicaid prescription data from 2011 to 2016 ... found that both medical and recreational cannabis laws were associated with annual reductions in opioid prescribing rates of 5.88 [percent] and 6.38 [percent], respectively."

The other new study found "[m]edical cannabis laws are associated with significant reductions in opioid prescribing in the Medicare Part D populations." The study found statistically significant drops in hydrocodone and morphine use, and said states with medical pot dispensaries had fewer prescriptions than states that required users to grow their own pot.

A study published in March in the Journal of Health Economics, meanwhile, found that "relatively liberal allowance for dispensaries" under state law was "[t]he key feature ... that facilitates a reduction in overdose death rates."

"As states have become more stringent in their regulation of dispensaries, the protective value generally has fallen. These findings suggest that broader access to medical marijuana facilitates substitution of marijuana for powerful and addictive opioids," the study abstract said. A 2015 working paper from members of the same research group found state-legal medical pot dispensaries were linked to a 15-35 percent drop in admissions to drug rehab centers.

Sessions previously expressed doubt about the link between state-legal marijuana and reductions in opioid use and deaths. In February 2017, he mocked a headline that he said declared, "Marijuana is a cure for opiate abuse.'"

"Give me a break," Sessions told state officials. "This is the kind of argument that's been made out there to just — almost a desperate attempt to defend the harmlessness of marijuana or even its benefits. I doubt that's true. Maybe science will prove I'm wrong."

Sessions told senators on Wednesday that the Justice Department was moving ahead with authorizing additional locations to grow marijuana for research. The University of Mississippi for decades has been the only location that can grow pot for researchers. A 2016 policy change allowed for additional grow sites, but no new licenses have been issued.

"A lot of people didn't know, I didn't know, a treaty, an international treaty of which we are a member requires certain controls in the process and the previous proposal violated that treaty," Sessions said. "We have now gotten language I believe that complies with the treaty and will allow this process to go forward."

Sessions was not directly asked about Trump's affirmation that there would be no crackdown on state-legal marijuana, but told Sen. Lisa Murkowski, R-Alaska, that the Justice Department was focused on drugs like heroin and cocaine that cause overdose deaths. He would not comment on whether he would support new marijuana federalism legislation that the White House backs in principle.