Keyes notes that Kandel’s more recent research in rodents does suggest the possibility of a biological gateway, but not for cannabis. “They’ve found that mice who are primed to be addicted to nicotine then find the effects of cocaine, for example, much more reinforcing,” she told me. “When you use these substances early, it primes you to enjoy substances more, and you also tend to have the opportunity to use those substances.” She also notes that the science is not yet in place to determine whether cannabis has a similar biological-priming effect.
Contrary to Keyes’s belief in the soundness of the gateway theory’s basis, Michael Vanyukov, a professor of pharmaceutical sciences at the University of Pittsburgh, interprets Keyes’s findings and their relationship to the gateway theory as further proof that the idea has been flawed all along. “Gateway theory only relates to initiation of drug use, not progression to abuse, and even in that regard it’s about as meaningful as the sequence of dishes served in a dinner,” Vanyukov says. “The order of drug use, in general, is simply opportunistic. What is accessible at the least personal cost is what comes first.” For example, in a 2012 study he led about substance abuse in Japan, more than 82 percent of hard-drug users had never used cannabis.
Instead of the gateway, Vanyukov is a proponent of an alternate theory of substance abuse known as the common-liability theory. It holds that substance use and eventual abuse is best predicted through a particular person’s biological mechanisms of reward. How your brain’s dopaminergic system responds to stimuli determines whether you’re likely to progress to abuse of hard drugs, not whether you smoked weed as a teen.
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To Ethan Nadelmann, the founder and former executive director of the Drug Policy Alliance, the new data illustrate the gateway’s flawed logic. “With this dramatic drop in cigarette use and drinking, if those substances were actually a gateway to marijuana, you’d think we’d see similar decreases in marijuana use,” he says. “But it’s fairly stable among adolescents. You’re not seeing a significant causal relationship.” Nadelmann is right about the numbers: Adolescent cannabis use isn’t moving much, even in places where it’s been legalized recreationally, while usage rates for other psychoactive drugs in both lower and higher stages of the supposed gateway sequence continue to fluctuate in ways that have no obvious relationship to cannabis use.
A potentially important confounding factor that wasn’t discussed in the study is e-cigarettes. Their use among high schoolers rose 78 percent from 2017 to 2018, to more than 3 million high schoolers, but they wouldn’t yet have been a huge issue in adolescent health when the study’s data cut off in 2016. Keyes says that current use isn’t high enough to account for the entire simultaneous drop in youth cigarette consumption, though, and the Food and Drug Administration has begun to implement policy changes it hopes will stymie the burgeoning popularity of vape products like Juul among adolescents.
And despite the recent inroads the tobacco industry has made among minors via the e-cigarette market, there’s cause for optimism in how American institutions might fare in combatting their popularity, and in reducing teen cannabis use even as legalization spreads. “We’ve achieved a remarkable reduction in the harms caused by alcohol and cigarettes by using public-health principles and goals,” Keyes told me. Hopefully, we’ve also learned from our errors.