When former Vice President Joe Biden asserted over the weekend that marijuana shouldn’t be legal because it might be a “gateway” to hard drug use, pro-legalization critics were quick to paint him as an out-of-touch codger still fighting the last drug war. But the reaction isn’t entirely fair: Yes, the marijuana gateway theory that was omnipresent in the 1980s was at best distorted and at worst dishonest. Nevertheless, gateways between marijuana and other addictive substances are real — and they swing in both directions.

During the heyday of anti-marijuana sentiment in America, fear-based prevention programs warned adolescents that a huge percentage of adults who experienced some horrible drug-related outcome (e.g., becoming addicted to heroin) had used marijuana when they were younger. These statistics were technically accurate, but even as teenagers, most of my classmates and I could see the logical flaws in the implication that marijuana was inevitably a road to ruin. Just because most people who used heroin had previously used marijuana didn’t prove that most people who used marijuana would go on to use heroin.

But strip away that era’s ideological agenda, designed to assign marijuana a unique and powerful role in ruining lives, and a more nuanced underlying truth about gateways reveals itself. People who become users of almost any addictive substance are at higher risk of subsequently using and having problems with other substances. A recent National Academies of Sciences, Engineering, and Medicine report found moderate evidence that this is the case for cannabis, but it’s also true of other drugs, including legal ones such as alcohol and tobacco. At least three causal forces can create such gateways.

First, people can become habituated to particular routes of administering drugs. For example, once you become used to wrapping a piece of paper around dead plant matter, lighting it on fire and sucking the smoke into your lungs, it’s easier to move from your first dead plant to your second.

The tobacco smoking and marijuana smoking populations are increasingly overlapping; taking up one likely increases the chances of taking up the other. The same phenomenon probably occurs with injected drugs as well. Once a person is used to injecting, say, heroin, it’s a smaller jump to also inject cocaine, methamphetamine or benzodiazepines.

Social networks are the second force behind drug gateways. Drug use, like many other behaviors, is very commonly a social activity. This creates gateway effects between drugs in part because classes of behavior (e.g., playing sports, traveling to exotic locales, collecting antiques or, yes, taking drugs) come to seem more normal when your friends all engage in them. Shared social networks can also create opportunities to try new drugs. Hanging out regularly with a group of people who regularly use crack cocaine together raises your odds that someone will offer you a chance to try methamphetamine.

Gateways can also result from users’ desire to combine the effect of a new drug with a familiar one. Established cocaine users sometimes become heavy drinkers (and vice versa) because they find the cocaethylene produced in the body by this drug combination particularly euphoric. Similarly, the fact that many people take the trouble to carve out cigars and fill them with marijuana (known as smoking “blunts”) demonstrates that tobacco and marijuana combined is uniquely reinforcing to some users.

So those who mocked Biden’s claim that marijuana could be a gateway to other drugs thus got the science wrong. There are plenty of ways using the drug can make people more likely to use other substances.

But research also shows that singling out marijuana is wrong; the gateway effect is in fact shared by many substances. If Biden continues to oppose the legalization of marijuana on the grounds that marijuana could lead to other drugs, it is only fair that he should answer another question: Why have we made alcohol and tobacco legal and often subjected them to insufficient restrictions when they are powerful gateways, too?

— Keith Humphreys is a professor of psychiatry at Stanford University. He served as a drug policy adviser in the Bush and Obama White Houses.

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