Sen. Cory Booker took former vice president Joe Biden to task during Wednesday’s presidential debate for opposing marijuana legalization over concerns it might be a “gateway drug,” despite scant evidence to support that theory.

“I have a lot of respect for the vice president — he swore me into my office, he’s a hero,” Booker, a New Jersey Democrat, said. “This week, I hear him literally say that ‘I don’t think we should legalize marijuana’ — I thought you might’ve been high when you said it.”

Marijuana in the U.S. “is already legal for privileged people, and it’s why the war on drugs has been a war on black and brown people,” the senator added.

Biden, in response, said he believed marijuana should be decriminalized and that people with marijuana-related offenses “should be let out of jail, their records expunged, be completely zeroed out.”

“But I do think it makes sense, based on data, that we should study what the long-term effects are for the use of marijuana — that’s all it is,” he added.

During a Las Vegas town hall last weekend, Biden had expressed support for medical marijuana and decriminalization of marijuana possession, but said that states should be able to make their own judgments when it comes to legalizing recreational use of the federal Schedule I substance.

There has still “not nearly been enough evidence” to determine whether or not marijuana is a gateway drug, Biden added.

“It is not irrational to do more scientific investigation to determine — which we have not done significantly enough — whether or not there are any things that relate to whether it’s a gateway drug or not,” he said.

Two in three Americans support legalizing marijuana, according to a report last week by the Pew Research Center. The District of Columbia and 11 states, most recently Illinois, have legalized recreational marijuana use. The share of American adults who are against legalization is now 32%, down from 52% in 2010, Pew found.

Generations of students learned from anti-drug abuse programs like D.A.R.E. (Drug Abuse Resistance Education) that marijuana was a “gateway drug” that could lead users to explore other more dangerous substances such as heroin and cocaine.

Research does not appear to support the “gateway” hypothesis. While some research has found that a large share of people who use marijuana proceed to use other illegal drugs, there’s little evidence to suggest this relationship is causal. “The majority of people who use marijuana do not go on to use other, ‘harder’ substances,” says the National Institute on Drug Abuse (NIDA).

An alternative hypothesis, NIDA says, posits that people who are more vulnerable to taking drugs are merely starting off with drugs like marijuana, alcohol and tobacco, which are more easily obtainable, “and their subsequent social interactions with others who use drugs increases their chances of trying other drugs.” “It is important to note that other factors besides biological mechanisms, such as a person’s social environment, are also critical in a person’s risk for drug use,” the institute said.

A 2002 report from the RAND Corporation’s Drug Policy Research Center (DPRC) similarly acknowledged that marijuana users were more likely than non-users to graduate to hard drug use; that nearly all people who had used both hard drugs and marijuana had used marijuana first; and that greater marijuana-use frequency was associated with greater likelihood of later using hard drugs.

But an alternative explanation to the gateway hypothesis is that “those who use drugs may have an underlying propensity to do so,” the think tank said, “that is not specific to any one drug.”

“The new DPRC research thus demonstrates that the phenomena supporting claims that marijuana is a gateway drug also support the alternative explanation: that it is not marijuana use but individuals’ opportunities and unique propensities to use drugs that determine their risk of initiating hard drugs,” the RAND authors wrote. “The research does not disprove the gateway theory; it merely shows that another explanation is plausible.”

Marijuana has, however, been associated with dependence; increased hostile behaviors; and “negative mood,” when consumed in high doses. A Lancet Psychiatry study published in March showed that daily marijuana consumption, particularly that of high-potency marijuana, was linked to an increased risk of psychotic disorder. Some research has raised concerns about the drug’s potential impact on adolescent cognitive development.

Meanwhile, a 2018 study examining insurance claims and police reports in four states that had legalized recreational marijuana (Colorado, Nevada, Oregon and Washington) found an up to 6% rise in the number of vehicle crashes compared to neighboring states where recreational marijuana hadn’t been legalized. But the study, conducted by the nonprofit Insurance Institute for Highway Safety, did not establish causation.

A JAMA Psychiatry study published last week also found that “problematic” cannabis use had risen among adults 26 and older in states that had legalized recreational cannabis, compared to those in states where it wasn’t legal.

“This study’s findings suggest that possible increases in the risk for cannabis use disorder among adolescent users and increases in frequent use and cannabis use disorder among adults after legalization of recreational marijuana use may raise public health concerns and warrant ongoing study,” the authors wrote.

On the other hand, there is evidence that cannabis and/or cannabinoids can help with conditions like pain, multiple sclerosis-related muscle spasms and chemotherapy-related nausea, according to a 2017 research review by the National Academies of Science, Engineering and Medicine. A vast majority of people who support marijuana legalization cite the drug’s benefits to those who use it medicinally as a “very important” reason they favor legalization, according to a Gallup poll earlier this year.

Many advocates for the legalization of recreational cannabis stress the importance of achieving social justice and equity for communities disproportionately impacted by the United States’ war on drugs, including measures like expungement of prior drug offenses, cannabis equity programs and reinvestment in those marginalized communities.

Black people are nearly four times as likely as white people to be charged with marijuana possession, despite similar usage rates, according to the American Civil Liberties Union. Criminal-justice reform advocates say the Biden-backed 1994 crime bill contributed to mass incarceration; Biden, for his part, has defended the law’s legacy by blaming states’ tougher sentencing laws and moves to build more prisons.

“Marijuana should be legalized, and drug consumption should be decriminalized,” Rep. Alexandria Ocasio-Cortez (D-N.Y.), a surrogate for Biden rival Bernie Sanders, tweeted a day after the former VP’s comments. “These are matters of public health.”

This story was originally published Nov. 19, 2019, and has been updated.