Is Cannabis a Gateway Drug?

Research suggests that marijuana use may precede the use of illicit and licit substances, and lead to addiction to these substances. A longitudinal study from the National Epidemiological Study of Alcohol Use and Related Disorders showed that marijuana use was more common in adults who reported it during the first wave of the survey. This means that marijuana use can precede other licit and illicit substances and lead to addiction to other substances.

Early exposure to cannabinoids in adolescent rodents decreases the reactivity of brain dopamine reward centers later in adulthood. To the extent that these findings generalize to humans, this could help explain the increased vulnerability for addiction to other substances of misuse later in life that most epidemiological studies have reported for people who begin marijuana use early in life. It is also consistent with animal experiments showing THC's ability to "prime" the brain for enhanced responses to other drugs. For example, rats previously administered THC show heightened behavioral response not only when further exposed to THC but also when exposed to other drugs such as morphine--a phenomenon called cross-sensitization.

These results are consistent with the notion of marijuana being a "gateway drug". The majority of marijuana users do not continue to use "harder" drugs. Cross-sensitization does not only occur with marijuana. Like marijuana, nicotine and alcohol can also trigger a higher brain response to other drugs. They are often used before the person moves on to more dangerous substances.

Important to remember that there are other factors than biological mechanisms that can impact a person’s likelihood of using drugs. Alternatives to the gateway-drug hypothesis are that people more susceptible to drug-taking are more likely to use readily available substances like marijuana, tobacco, or alcohol. Their subsequent social interactions with other users increases their chances of trying new drugs. This question requires further research.