Christopher Ingraham was a reporter on The Post’s business desk from 2014 to 2021. He currently writes the Why Axis, a newsletter about the data shaping and informing our lives.

It’s been seven years since the country’s first legal recreational marijuana shops opened in Colorado and Washington state. What was once a bold social experiment is now commonplace, with 18 states plus D.C. — representing well over one-third of the U.S. population – permitting recreational use. Nevertheless, myths about marijuana use and legalization remain widespread. Some are stubborn artifacts of the long-standing war on drugs, while others reflect the influence of a growing industry looking to attract new customers. Here are a few of the most persistent ones.

Marijuana is a gateway drug.

“Do most people who use other drugs start with marijuana? Most definitely,” reads a fact sheet on the website of Smart Approaches to Marijuana (SAM), a prominent anti-pot organization. “Research demonstrates that 99% of those addicted to other drugs started with alcohol and marijuana,” the group claims. Pot-squeamish politicians – including New York Democratic Gov. Andrew Cuomo, President Joe Biden and Rep. Andy Harris, R-Md., – have cited this “gateway theory” as a reason to keep restrictions in place.

In reality, no credible evidence shows that pot makes people more inclined to use other drugs. That, at least, was the conclusion of an exhaustive 96-page inquiry by the Justice Department and the Library of Congress in 2018. “No causal link between cannabis use and the use of other illicit drugs can be claimed at this time,” the authors wrote.

The shoddy reasoning undergirding the gateway theory is on full display in SAM’s statements. It’s true, for instance, that most people who use opiates and other dangerous drugs have also used marijuana at some point in their lives. However, you can make the same claim about alcohol, cigarettes, caffeine or just about any other common substance. “Ninety-nine percent of those addicted to other drugs started with milk” is a true statement – and also perfectly meaningless.

Marijuana leads to violent crime.

There are indeed well-documented ties between heavy marijuana use and psychosis, particularly among the young. But teasing out the causality is incredibly difficult. Does marijuana use cause psychosis, or are people who develop psychosis already more predisposed toward marijuana use? Researchers say Berenson and others overstate their claims and ignore evidence that marijuana may help some people with psychotic disorders.

Beyond that, scholars have done plenty of rigorous work directly examining the relationship between marijuana use and crime, and for all intents and purposes, they haven’t found one. In 2013, a sweeping review of the evidence conducted by the Rand Corp. on behalf of the White House concluded that “marijuana use does not induce violent crime.”

Potency is an important indicator of quality and safety.

No topic in marijuana produces a greater noise-to-signal ratio than potency, or the concentration of THC – the plant’s main psychoactive compound – in a given product. Legalization skeptics claim that today’s cannabis is stronger than ever before – “not your father’s marijuana,” as they say. Pot advocates, meanwhile, often say stronger weed is better, because you can theoretically consume less of it to achieve the same high.

But drug warriors have been issuing dire proclamations about pot potency since at least the 1940s. Canadian cannabis advocate Dana Larsen dug up decades’ worth of potency warnings and did the math to determine that if they were all true, today’s weed would be roughly 12,600 times stronger than it was in the 1960s. Since THC concentration cannot exceed 100 percent, we know that isn’t true.

That said, marijuana is getting stronger, although it’s hard to say by exactly how much. Most of our longitudinal knowledge of pot potency comes from a government-run monitoring program that’s been marred by shoddy data-collection practices for much of its existence. There’s no question, however, that under both prohibition and legalization, pot growers have been obsessively selecting for higher THC potency in their plants.

That’s not necessarily good. The research on potency, summed up in a 2020 report from the Colorado Department of Public Health and Environment, shows a good deal of evidence that people who routinely use stronger marijuana put themselves at a higher risk for mental health disorders than people who use weaker stuff. Might users of high-potency product consume less? It’s a nice idea in theory, but the real-world data is mixed. A recent review found, for instance, that “users of more potent cannabis products incompletely adjust their THC doses,” which resulted in those users reporting “more negative consequences than users of less potent products.”

Other recent research has shown that high-potency cannabis products don’t necessarily get users more high. And recent investigations have revealed that the potency testing industry is rife with fraud, quality-assurance problems and general incompetence. Those THC labels on the products at the dispensary may not mean that much, in the end.

Different strains produce different intoxicating effects.

Speaking of labels at dispensaries, pot connoisseurs often rhapsodize about their favorite strains the way wine snobs discuss their favorite chardonnays. Product descriptions promise effects from locking you to the couch to motivating you to clean the house.

But scientists who’ve studied the genetics of cannabis strains say those alleged distinctions have little grounding in reality. A 2015 report, for instance, found only minor differences in the genetic makeup of cannabis strains. “A marijuana strain name does not necessarily represent a genetically unique variety,” the authors wrote. In about a third of the plants they studied, individual samples were “more genetically similar to samples with different [strain] names than to samples with identical names.” A 2019 study failed to find meaningful genetic differences even between the two major varieties, indica and sativa, often said to produce very different types of intoxication. An analysis in 2018 concluded that “the concept of a ‘strain’ does not reflect the crop domestication, breeding programs or plant chemistry.”

Cannabis contains hundreds of chemical compounds, most of which are poorly understood. It’s possible that different combinations could produce different intoxicating effects. But the research strongly suggests there’s little correlation between how a strain is labeled and what it actually does.

Opponents of legalization often make their case in apocalyptic terms. Groups like Parents Opposed to Pot, for instance, claim that legalization is “an anti-science Public Health disaster” and an “assault on public safety” that “increases the black market for all drugs, and overdose deaths,” and that “on EVERY measure, marijuana legalization is failed policy.” Medical groups have argued that legalization will lead to increases in teen drug use. Police groups have warned that traffic fatalities will rise.

But one of the most striking facts about the states that have legalized is how little has changed there. This spring, for instance, two economists from the University of Colorado at Denver and Montana State University performed a comprehensive review of the public health consequences of marijuana legalization, encompassing dozens of previously published studies. They found little evidence suggesting that recreational marijuana laws result in greater teen drug use, but strong evidence that teens who do use marijuana are less likely to use alcohol – a net public health win, given what we know about the relative dangers of the two substances. There are also some provisional signs that legal weed is taking a bite out of opioid mortality, directly contradicting prohibitionists’ dire warnings.

On the traffic front, the researchers found that “road safety improves when medical marijuana is legalized” but that the jury’s still out on the effect specifically of recreational pot. At the workplace, multiple studies have shown that legalization reduces sick-day use and absenteeism.

The net post-legalization changes are modest and trending slightly toward positive. This is not terribly surprising: Human societies are massive and complex systems, with thousands of interconnected parts interacting in unpredictable ways. It’s naive to think that altering a single variable, like the legal status of weed, could bring the whole edifice crashing down.