U.S. Sen. Rob Portman is passionate about trying to prevent opioid overdoses, a problem that has become increasingly prominent across the country in recent years.

But in a floor speech on Monday — his 32nd on the topic in the last year, he said — the Ohio Republican trotted out the “gateway drug” myth about marijuana that has repeatedly been shown to be false.

And he said that marijuana in his state is being laced with the powerful opiate fentanyl, a claim that appears to be unsupported.

Portman first told the story of a man he recently met.

“He’s a very charismatic, young guy,” he said. “For him the gateway drug was marijuana and alcohol in high school. He ended up overdosing on heroin three times. He was convicted of 13 felonies. He went to detox 28 times.”

The idea that marijuana could be a “gateway drug” to three heroin overdoses is a familiar talking point from legalization opponents, but the science doesn’t support it.

Back in 1999, the Institute of Medicine conducted a thorough review of the scientific research and found that there is “no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs.”

While it is true that many people who use other drugs have previously consumed marijuana, the vast majority of those who use cannabis do not go on to try heroin, cocaine or other substances. And as the National Institute on Drug Abuse points out, alcohol and nicotine “are, like marijuana, also typically used before a person progresses to other, more harmful substances.”

The evidence to support the “gateway theory” is so poor that even the Drug Enforcement Administration (DEA) recently deleted documents that cited it from its website.

But more than not being a gateway drug, evidence is mounting that cannabis might actually help to reduce the use of deadly opioids.

Research has shown that states with legal marijuana access see rates of overdose deaths go down. A 2014 JAMA Internal Medicine study, for example, found that states with medical marijuana laws had 25 percent fewer opioid overdose deaths than states that don’t allow medical cannabis.

But Sen. Portman didn’t leave it at the “gateway” myth during his floor speech. He also said something else dubious about marijuana. Speaking about issues related to the powerful synthetic opioid fentanyl, he claimed:

“Drug traffickers are lacing other drugs with it. I was told by the DART task force in Toledo that they are actually putting fentanyl in marijuana now, and people are showing up in the emergency room and overdosing on marijuana because it is sprinkled with fentanyl. It is more addictive. So the traffickers like it. It is more deadly.”

But that doesn’t appear to have actually happened in Toledo.

Kevin Smith, Portman’s communications director, said in an email that the senator “heard this from the Lucas County Sheriff’s Office DART Unit, which he met with several weeks ago.”

But Lt. Bobby Chromik of the Lucas County DART told MassRoots in a phone interview that, “I personally have not heard” of marijuana being laced with fentanyl in the county. “That stuff is usually reported directly to me.”

Chromik did cite a state highway patrol intelligence briefing which he said noted that potential fentanyl-laced marijuana from Pennsylvania could be “working its way westward.”

It’s “one of the fears we have, [that] it’s coming down the line,” he said.

MassRoots could find no news reports about marijuana-tainted fentanyl in Pennsylvania, and Chromik said the intelligence report he cited is law enforcement sensitive, so he was not able to share it. The lieutenant was not present when Portman met with other members of the unit, he said, so he wasn’t sure what might have been conveyed to the senator.

Smith, the Portman staffer, said in a followup email to MassRoots that a Toledo police detective “recounted an event the week prior where a teenager had used marijuana laced with fentanyl and overdosed. If he was relayed inaccurate information that would be unfortunate.”

Recent media reports of marijuana being laced with fentanyl in Ohio and Canada were later walked back.

Last month, for example, Cleveland.com reported that three people overdosed in a 12-hour period after “smoking marijuana laced with an unknown opiate,” which a local fire department believed to be fentanyl or heroin.

But four days later the story was updated with this note:

“Officials have determined the marijuana was not laced with opiates, and that the three people used crack cocaine and other drugs.”

The local police department in question also clarified in a Facebook post that, “Lab results found no evidence of laced marijuana.”

And in Canada, the premier of British Columbia claimed late last year that police were “finding fentanyl in marijuana.”

But the Vancouver Police Department clarified that the official was referring to an old alert that was later shown to be false. It “may have been the belief at the time,” police told the media in an email, “but it has shown not to be the case and we continue to correct any misinformation.”

Police officials in other parts of Ohio say they aren’t aware of any instances of marijuana being laced with fentanyl.

“We’ve not seen anything like that in the city,” Chillicothe Police Chief Keith Washburn told the media last month.

But last year, police in Connecticut said that a man they arrested admitted to selling marijuana-laced fentanyl.

It is unclear why dealers in the illegal marijuana market would lace marijuana with a powerful opioid at added expense to themselves and increased risk for their customers.

“One anecdote does not equal a trend, and in this case the anecdote can’t even be verified,” Paul Armentano, deputy director for NORML, told MassRoots in a phone interview.

He added that one way to make sure marijuana isn’t tainted with other drugs would be to legalize it.

“If Senator Portman is truly concerned regarding potential tampering with these products, then the logical solution would be for him to advocate in favor of a legal, regulated market so that the production and dispensing of marijuana were to be put in the hands of licensed businesses and not in the hands of criminal entrepreneurs,” he said.

In any case, Portman is sticking to the “gateway” theory for now.

Smith, the senator’s communications director, said in an email that “anyone who doesn’t think marijuana is a gateway drug to heroin and other drugs is deluding themselves. The senator hears it time and time again from those in treatment and recovery.”

This story has been updated to include comment from Portman’s staff.