Gateway drug effect

The gateway drug effect (alternatively, stepping-stone theory, escalation hypothesis, or progression hypothesis) is a comprehensive catchphrase for the often observed effect that the use of a psychoactive substance is coupled to an increased probability of the use of further substances. Possible reasons for the connection include environmental influence, impulsive people seeking both soft and hard drugs (meaning people who use one drug are likely to use another drug due to personality and that it's not the initial drug that leads to the use of another drug), alterations in the brain due to earlier substance exposure, as well as similar attitudes of people who use different substances, and therefore experience a "common liability to addiction". A 2018 literature review conducted by the National Institute of Justice, which analyzed 23 peer-reviewed research studies, concluded "that existing statistical research and analysis relevant to the "gateway" hypothesis has produced mixed results", and that "no causal link between cannabis use and the use of other illicit drugs can be claimed at this time." However, in 2020, the National Institute on Drug Abuse released a research report which supported allegations that marijuana is a "gateway" to more dangerous substance use, despite stating that "most people who use or have used cannabis do not go on to use other substances later in life"; one of the peer-reviewed papers cited in the report claims that while "some studies have found that use of legal drugs or cannabis are not a requirement for the progression to other illicit drugs ... most studies have supported the "gateway sequence", although the authors of the study itself conceded that they were "not able to ascribe causality".

While the phrase gateway drug was first popularized by anti-drug activists such as Robert DuPont in the 1980s, the underlying ideas had already been discussed since the 1930s by using the phrases stepping-stone theory, escalation hypothesis, or progression hypothesis.

The scientific and political discussion has intensified since 1975 after the publications of several longitudinal studies by Denise Kandel and others.

In 2019, a research report by the National Institute on Drug Abuse determined that marijuana use is "likely to precede use of other licit and illicit substances", and cited a longitudinal study in which "adults who reported marijuana use during the first wave of the survey were more likely than adults who did not use marijuana to develop an alcohol use disorder within 3 years; people who used marijuana and already had an alcohol use disorder at the outset were at greater risk of their alcohol use disorder worsening." They further claimed that marijuana use "is also linked to other substance use disorders including nicotine addiction." While stating that their "findings are consistent with the idea of marijuana as a "gateway drug"", they conceded that "the majority of people who use marijuana do not go on to use other, "harder" substances", and that "cross-sensitization is not unique to marijuana. Alcohol and nicotine also prime the brain for a heightened response to other drugs and are, like marijuana, also typically used before a person progresses to other, more harmful substances."

On the other hand, the 2018 report issued by the National Institute of Justice states "that the current state of research on this issue is limited, and the studies that have been conducted suffer from difficulties in collecting information and applying the findings to a larger population. The report further notes that although many of the reviewed studies found statistically significant associations between cannabis use and the users' later use of other illicit drugs, there is currently no conclusive evidence that cannabis use caused the later use of harder illicit drugs."

The concept of gateway drug is based on observations that the sequence of first-time use of different drugs is not random but shows trends. On the basis of established techniques of longitudinal studies such trends can be described precisely in terms of statistical probability. As to the interpretation of the observed trends, it is important to note the difference between sequence and causation. Both may – but need not – be coupled, a question which is subject of further research, e.g., by physiological experiments.

From a sample of 6,624 people who had not used other illegal drugs before their cannabis consumption the overall probability of later use of other illegal drugs was estimated to be 44.7%. Subgroup analyses showed that personal and social conditions, such as gender, age, marital status, mental disorders, family history of substance use, overlapping illegal drug distribution channels, alcohol use disorder, nicotine dependence, ethnicity, urbanicity, and educational attainment influenced the height of probability.

A study of drug use of 14,577 U.S. 12th graders showed that alcohol consumption was associated with an increased probability of later use of tobacco, cannabis, and other illegal drugs. Adolescents who smoked cigarettes before age 15 were up to 80 times more likely to use illegal drugs. Studies indicate vaping serves as a gateway to traditional cigarettes and cannabis use.