The “Gateway Theory” can be traced to 1975, with a paper by Kandel reporting on the frequency with which adolescent illicit drug users had previously used marijuana (Science 1975;190:912-914). The difficulty was the failure to demonstrate causality in the relationship. People who use cocaine or heroin have usually been exposed to marijuana or a licit drug; however, the majority of marijuana users partake of no other illicit drug, and many heroin users skip the “steps” of softer drug use (Drug Alcohol Depend 2012;123:S3-S17)
Central to the gateway theory is that the locus of control is external, minimizing or eliminating the known biological contributions to the disease. The mechanism by which one drug purportedly seeds an SUD( Substance Use Disorder) involving another has never been fully explained, yet the gateway theory remains a popular precept, likely because it is a simple explanation.
For example, U.S. Attorney General Loretta Lynch, speaking at the University of Kentucky, said marijuana was clearly not a gateway drug but that prescription opioids were the gateway to heroin (https://www.youtube.com/watch?v=dkO1dx-y8MU). She said, “When we talk about heroin addiction, we usually … are talking about individuals that started out with a prescription drug problem, and then because they need more and more, they turn to heroin. In so many cases, it isn’t trafficking rings that introduce a person to opioids; it’s the household medicine cabinet. That’s the source.”
So it turns out Pain Pills are the Gateway…Not Cannabis..and definitely not Medical Cannabis in which the psychoactivity is highly controlled.
In contrast, common addiction liability can explain the clinical spectrum of SUDs not explained by the gateway theory. It does so by accounting for complex genetic and environmental factors that are activated in the disease of addiction. This concept takes, as its basis, the overlapping genetic and behavioral factors that are common among people with various SUDs and that do not depend on any particular drug exposure or sequence of exposures (Drug Alcohol Depend 2012;123:S3-S17).
State marijuana ballot initiatives are throwing the discussion of gateway exposure back into public consciousness, as policymakers ponder whether marijuana would help ameliorate or worsen the opioid abuse crisis. A New Zealand examination of the gateway question concluded that although marijuana use predates much harder drug use, many additional factors are also at play that include preexisting psychiatric disorders, differences or changes in brain chemistry, family histories of substance abuse (heredity) or other psychopathology, and prior association with illegal sources of drugs (Relihan T. Is Marijuana a Gateway Drug? The Recorder. October 30, 2016).