Trump press secretary Sean Spicer said the Department of Justice may step up enforcement against states that have voted to legalize recreational marijuana, comparing it to the opioid crisis.

While researchers do have questions about marijuana, most data shows it's linked with far fewer risks than opioids.

Marijuana doesn't cause overdoses like opioid drugs, it's not nearly as addictive, and doesn't seem to be a "gateway" to opioid use.

The US Department of Justice may be considering a crackdown on recreational marijuana use in states where citizens have voted to legalize that use, according to comments by Trump press secretary Sean Spicer.

"[T]here's a big difference between the medical use [of marijuana] ... That's very different from the recreational use, which is something the Department of Justice will be further looking into," Spicer said in response to a question on February 23.

In his explanation of why the government might step up that sort of enforcement, Spicer alluded to the deadly opioid crisis in the US.

"I think that when you see something like the opioid addiction crisis blossoming in so many states around this country, the last thing that we should be doing is encouraging people," said Spicer. "There is still a federal law that we need to abide by when it comes to recreational marijuana and other drugs of that nature."

The opioid crisis is a big deal — death rates from heroin and synthetic opioid painkillers continue to rise, according to a newly released CDC report. But comparing marijuana to the opioid addiction crisis doesn't make sense from a scientific standpoint. Spicer didn't elaborate on why he thought the opioid addiction crisis was a reason to crack down on marijuana use, but the two drugs are very different.

Marijuana doesn't kill people with overdoses like opioids do; it's not as addictive or harmful overall, according to public health researchers; and most research shows that it doesn't act like a "gateway" to other drug use, no matter what we may have learned in school. Even though researchers do still have a lot of questions about marijuana they want answered, the opioid comparison doesn't stand up, no matter how you look at it.

In many ways, the legalization of recreational marijuana in states like Colorado and Washington may offer a good deal of reassurance to many who are concerned about potential negative effects of legal weed.

"What we know right away is that the sky didn't fall," Amanda Reiman, a researcher and marijuana policy expert then-working with the pro-legalization Drug Policy Alliance, told me in an interview on how legalization has worked out so far last fall. "We didn't see huge upticks in traffic accidents or people using marijuana." Data from those states supports what she says.

DCMJ's Inaugural #Trump420 Marijuana Rally on January 20, 2017 in Washington, DC. Theo Wargo/Getty

What the science says about marijuana, public health, and opioids

Overdoses and addiction potential:

There's a fair amount of research out there comparing the harms and risks of different sorts of drugs and by most of those measures, marijuana is fairly low risk. In one recent study where researchers compared the overall risk of fatal overdose from various drugs, marijuana was the only substance evaluated to fall into the "low risk" category.

Substances like amphetamines and diazepam (Valium) were medium risk; substances like alcohol, cocaine, and heroin were all high risk. In other research into how addictive drugs are, opioids and heroin are near the top of the chart, above alcohol and substances like benzodiazepines or ketamine. Marijuana (and psychedelics like LSD or psilocybin) were found to be far less addictive.

Effect on public health and opioid use:

While there are a number of factors behind the current opioid epidemic, one thing that many experts agree has played a role is the use of opioid painkillers to treat chronic pain. Using powerful drugs that have a high risk of causing overdose and high addiction potential is risky. Marijuana, which can also treat chronic pain — the most common reason people want to use it medically — is far less risky, scientists say.

And in fact, there are several studies that show that states that allow medical marijuana have fewer opioid deaths. This effect seems to stack over time, with states who pass these laws seeing a "20% lower rate of opioid deaths in the laws' first year, 24% in the third, and 33% in the sixth," according to Stat News. In general, it's hard to say that deaths went down because of medical marijuana legalization and not other reasons; but in this case, researchers think that since the effect gets stronger the longer marijuana has been legalized, that implies that marijuana is a major cause of the decline in opioid deaths.

That's specifically medical marijuana, which Spicer said the administration was less concerned about. It's hard to know exactly how recreational marijuana will affect opioid use. But a recent review by the National Academies of Sciences, Engineering, and Medicine that provides our most up-to-date comprehensive look at marijuana research found no data linking cannabis use to opioid use.

The "gateway drug" effect:

If marijuana doesn't seem to be linked to opioid use, is there anything to that "gateway drug" effect that many of us were taught about in middle school? Many experts don't think so, and even the National Institute on Drug Abuse (NIDA) considers this idea controversial. As the NIDA notes, there is data indicating that people who have addiction problems later in life are more likely to have used marijuana as adolescents. But most research shows that most marijuana users don't go on to use other substances. And those people who do develop addiction problems are also more likely to use other substances as kids.

"An alternative to the gateway-drug hypothesis is that people who are more vulnerable to drug-taking are simply more likely to start with readily available substances such as marijuana, tobacco, or alcohol," NIDA's website states.

A visitor smokes a marijuana cigarrette during the Expo Cannabis fair in Montevideo, Uruguay, Sunday, Dec. 14, 2014. After state regulation of the production and sale of marijuana, Uruguay had its first cannabis expo with stands selling seeds, marijuana growing technology, conferences and cultivation techniques workshops. Matilde Campodonico/AP

Are there any real risks?

None of this is to say that marijuana is absolutely and completely risk-free. Much of the data we have so far indicates that people who begin using cannabis regularly at a young age may suffer negative cognitive effects from that use in the long run (the same being true of young use of alcohol or any other substance), as one example. Some people struggle with marijuana dependency, too, even if it's not as addictive as some other legal substances.

In other words, there are real risks, like with use of any pharmacological substance.

Researchers think we need to better understand those risks a states move forward with legalization, something that's hard to do right now since that requires study and marijuana is particularly difficult to research. But prohibiting research and cracking down on states who have voted to legalize recreational use won't make marijuana safer.

"One of the the big fallacies is that prohibition somehow gives us control of the product," said Reiman. "One of the big fears is that we lose some sort of control [with legalization], and I think actually the opposite is true."

It's not that marijuana is a zero-risk substance. It's just that from what we know so far, marijuana is far less likely to harm users than opioids or even other legal substances like alcohol, making it hard to see how a comparison to the opioid crisis is relevant.