Gateway drug theory

The gateway drug theory unproven hypothesis is an assertion that the use of "softer" drugs will lead to the use of "harder" drugs. A commonly-assumed pattern is that alcohol or nicotine use will lead to marijuana use, followed by cocaine, methamphetamine, or heroin use, in a manner similar to a slippery slope. Another variation posits that some dealers will spike their supplies of marijuana with the harder drugs, so as to get their customers addicted to the latter before they know they are using them (although most dealers have little reason to do this, as they would be losing money by selling more expensive drugs for the price of marijuana).

Conflicting studies
Studies on the gateway drug theory show conflicting results; some studies support the concept while others bring it into question. One RAND Corporation study failed to identify a significant gateway effect in the case of marijuana. RAND speculated that a common-factor model, in which certain factors (genetic, environmental, social, etc.) determine the likelihood of a particular subject engaging in the use of particular drugs (the "common-factor model"), is another possibility. Hence, it may be a case for which correlation does not equal causation. A large, multifaceted 2017 literature review of marijuana found moderate evidence for a correlation between marijuana use and "the development of substance dependence and/or a substance abuse disorder for substances including, alcohol, tobacco, and other illicit drugs", but could not find enough evidence to address the issue of causation.

Crank arguments
The theory may reflect a mental model based on the increasing availability of certain drugs, particularly since alcohol and nicotine are legal. If, for example, one drinks alcohol 250 times and smokes marijuana 20 times over the course of 5 years, one is more likely to have used alcohol first. Notably, few people claim that caffeine or (most) proper use of pharmaceuticals causes the use of other drugs.

There is little physiological reason to assume that one drug acts as a gateway to others, as different neurological receptors are involved with different drugs, although most psychoactive drugs do result in an increase in the amount of dopamine in the brain.

Many proponents of the gateway drug theory use statistics in a misleading manner, for example citing that a high percentage of heroin users previously used marijuana in an effort to imply that a high percentage of marijuana users go on to use heroin, when the two are not the same at all. Even if it was established that 100% of heroin users had previously used marijuana, it would not follow that their marijuana usage was a causative factor in their progression to heroin, nor does it prove that 100% of marijuana users (or even a statistically significant percentage of marijuana users) later used heroin.

Marijuana
There is an argument that the prohibition of marijuana can lead users to buy from people who may also sell heroin, but this would suggest a problem with the law rather than the drug itself. The same hypothetical effect would be seen if hamburger users were forced to buy their patties on street corners and in back alleys.

There is another argument that, since marijuana is illegal, it has a certain "taboo" mystique about it. When an enterprising young soul finally works up the courage to try this "criminal substance", he might discover that the effects are far less impressive than the mystique about marijuana would suggest. This, in turn, might make him wonder just how bad the other equally-illegal drugs are (though, at least under federal law in the United States, some "hard drugs" like methamphetamine and cocaine are actually i.e. fewer federal legal restrictions, than marijuana).

At least two studies, including one published by the University of Pittsburgh in 2006, and another published by the University of New Hampshire in 2010, show that stressors like unemployment play a bigger role in the use of progressively harder illicit drugs.