First popularized in the 1980s, the gateway drug theory says that adolescent use of tobacco, alcohol or marijuana increases the risk of using and developing an addiction to other, more harmful substances, such as opioids, cocaine, and methamphetamines.

Data from the National Epidemiological Study of Alcohol Use and Related Disorders found that adults who reported marijuana use were more likely than adults who did not use marijuana to develop an alcohol use disorder within three years. People who used marijuana and already had an alcohol use disorder were at greater risk of their alcohol use disorder worsening. Marijuana use is also linked to other substance use disorders, including nicotine addiction.

Animal experiments have shown the ability of THC (the main active ingredient in marijuana) to “prime” the brain for stronger responses to other drugs. For example, in a phenomenon called cross-sensitization, rats previously given THC show heightened behavioral response not only when exposed to more THC but also when exposed to other drugs like morphine. However, the majority of people who use marijuana do not go on to use other, more dangerous substances. Also, cross-sensitization is not unique to marijuana. Alcohol and nicotine can also prime the brain for a heightened response to other drugs.

A little less than half of Americans over the age of 12 have tried marijuana, while less than 15 percent have taken cocaine and less than two percent have used heroin, according to a National Household Survey on Drug Use and Health.

Studies have found that rats that ingest marijuana are more likely to use heroin when it’s presented to them. The problem with studies done on rats is that most rats don’t want anything to do with THC and have to be forcibly injected. If anything, critics say, all this proves is that stressed rats are prone to addiction.

Skeptics of the gateway idea say that this theory only distracts us from the real reasons why some people are more prone to addiction, such as mental illness or trauma.

A 1999 report from the Institute of Medicine, which is part of the National Academy of Sciences, stated that marijuana “does not appear to be a gateway drug to the extent that it is the cause or even that it is the most significant predictor of serious drug abuse.”

A different theory focuses on the social and cultural reasons that some drugs could be gateways. For example, simply by being around marijuana and the people who use it, an individual might be more likely to end up trying other drugs. There is also the fact that someone who uses marijuana frequently may simply be more likely to engage in risk-taking behavior, and that could explain why they move on to harder drugs.

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