Amid a drug crisis that kills 91 people in the U.S. each day, Attorney General Jeff Sessions has asked Congress to help roll back protections that have shielded medical marijuana dispensaries from federal prosecutors since 2014, according to a letter made public this week. Those legal controls—which bar Sessions’s Justice Department from funding crackdowns on the medical cannabis programs legalized by 29 states and Washington, D.C.—jeopardize the DoJ’s ability to combat the country’s “historic drug epidemic” and control dangerous drug traffickers, the attorney general wrote in the letter sent to lawmakers.

The catch, however, is that this epidemic is one of addiction and overdose deaths fueled by opioids—heroin, fentanyl and prescription painkillers—not marijuana. In fact, places where the U.S. has legalized medical marijuana have lower rates of opioid overdose deaths.

A review of the scientific literature indicates marijuana is far less addictive than prescription painkillers. A 2016 survey from University of Michigan researchers, published in the The Journal of Pain, found that chronic pain suffers who used cannabis reported a 64 percent drop in opioid use as well as fewer negative side effects and a better quality of life than they experienced under opioids. In a 2014 study reported in JAMA The Journal of the American Medical Association, the authors found that annual opioid overdose deaths were about 25 percent lower on average in states that allowed medical cannabis compared with those that did not.

Marijuana can be habit-forming, at least psychologically, but the risks are not in the same league as opioids. A 20-year epidemiological review of studies concluded that more than nine out of 10 people who try marijuana do not become dependent on the drug. The review paper, published in 2014, said the “lifetime risk of developing dependence among those who have ever used cannabis was estimated at 9 percent in the United States in the early 1990s as against 32 percent for nicotine, 23 percent for heroin, 17 percent for cocaine, 15 percent for alcohol and 11 percent for stimulants.”

Also, unlike the case with opioids, it is virtually impossible to lethally overdose on marijuana—because a user would have to consume massive quantities in a prohibitively short time. The National Institute on Drug Abuse (NIDA) says such a fatal result is very unlikely. Meanwhile, heroin-related overdose deaths have more than quadrupled since 2010. The U.S. Centers for Disease Control and Prevention says that from 2014 to 2015 heroin overdose death rates increased by 20.6 percent—causing nearly 13,000 deaths in 2015.

Many heroin users in the U.S. first become addicted to legally prescribed painkillers, and turn to heroin after their pill supply dries up or becomes too expensive. According to the NIDA, nearly half of young people who inject heroin abused prescription opioids first.

And a significant number of pain sufferers would apparently prefer to use medical marijuana instead of prescription painkillers. A study published in July 2016 in Health Affairs explored what happened to Medicare (Part D) painkiller prescriptions after states green-lighted medical marijuana laws, and found that a typical physician in a state with medical cannabis prescribed 1,826 fewer painkiller doses for Medicare patients in a given year—because seniors instead turned to medical pot. There were also hundreds fewer doses prescribed for antidepressants, anti-nausea medications and antianxiety drugs.

The science on the benefits and risks of medical marijuana is far from settled, largely because conclusive research remains so difficult in spite of the drug’s popularity and apparent promise. Sessions’s DoJ oversees the U.S. Drug Enforcement Administration, which has long kept marijuana listed in the nation’s laws as a Schedule I drug, meaning it is officially declared devoid of any currently accepted medical use and has a high potential for abuse. This federal status hobbles researchers’ abilities to obtain marijuana and conduct comprehensive studies on its potential benefits, even though so many states have defied federal prohibition and the cannabis industry is booming. The DoJ did not respond to a request for comment by the time of publication.

Session’s congressional letter, which was dated May 1, was obtained by Massroots.com and also confirmed and reported by The Washington Post on Tuesday. The letter urges lawmakers to remove the legal impediment that keeps his office from spending cash on interfering with state medical marijuana programs, a safeguard for dispensaries formally called the Rohrabacher–Farr Amendment. That provision expires at the end of September, and would have to be renewed to remain the law of the land—a timeline that guarantees medical marijuana will be discussed in Congress in the coming months.

W. David Bradford, a health policy expert at The University of Georgia who studies medical marijuana policies, says failing to renew the provision “would throw a lot of uncertainty into the [medical cannabis] industry and cause disruption for patients.” Bradford, who was the senior author on the Health Affairs study, also links the amendment’s fate to the opioid crisis: “Anything we can do to divert people away from initial opiate use,” he says, “will divert them away from the potential for misuse and death.”