A gateway drug is a term used to describe a psychoactive substance that is believed to increase the likelihood of using further substances. The concept of gateway drugs is based on observations that the sequence of first-time use of different drugs is not random but shows trends. While the phrase "gateway drug" was first popularized by anti-drug activists in the 1980s, the underlying ideas had already been discussed since the 1930s using phrases such as stepping-stone theory, escalation hypothesis, or progression hypothesis.

Marijuana is often considered a gateway drug, as some research suggests that its use is likely to precede the use of other licit and illicit substances and the development of addiction to other substances. However, it is important to note that the majority of people who use marijuana do not go on to use other, "harder" substances. Other substances that are commonly considered gateway drugs include tobacco and alcohol.

It is also important to note that the concept of gateway drugs is based on observations of trends and statistical probability, and the interpretation of these trends is not necessarily indicative of causation. Additionally, other factors besides biological mechanisms, such as a persons social environment, are critical in a persons risk for drug use. An alternative to the gateway-drug hypothesis is that people who are more vulnerable to drug-taking are simply more likely to start with readily available substances such as marijuana, tobacco, or alcohol.