The argument that marijuana is a “Gateway Drug” is probably the most popular argument against the legalization of marijuana. There is absolutely no evidence that this is true but yet marijuana prohibitionists trot it out regularly.
Even the DEA has come to admit that, "Little evidence supports the hypothesis that initiation of marijuana use leads to an abuse disorder with other illicit substances." (#1)
The reason that it sounds believable to many people is that people don’t always understand the difference between “correlation” and “causation”. Sure most heroin addicts used to smoke marijuana… but they also used to smoke cigarettes, eat peanut butter and jelly sandwiches and drink alcohol. Almost all heroin addicts drank milk as children; does that make milk a gateway drug? A basic scientific axiom is that “Correlation does not imply Causation”. Just because something came before something else doesn’t mean that it caused it.
‘The gateway theory argues that because heroin, cocaine and methamphetamine users often used marijuana before graduating to harder drugs, it must be a “gateway” to harder drug use. The theory implies that there is a causal mechanism that biologically sensitizes drug users, making them more willing to try—and more desirous of—harder drugs.
Yet the gateway hypothesis doesn’t make sense to those who use marijuana or have used in the past. Research shows that the vast majority of marijuana users do not go on to use hard drugs." (#2)
Even the Institute of Medicine of the National Academy of Sciences wrote: “There is no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs”. (#3) The “Gateway Theory” is raised again and again even though there is no evidence that it holds water. Anyone espousing it is trying to use fear to keep you from supporting marijuana legalization.
In addition to the gateway theory having been debunked, researchers from the Johns Hopkins Bloomberg School of Public Health found that opioid overdose deaths had gone down by an average of 25% in states where marijuana was legally available. (#4) This is because those people at risk for opioid abuse had a safer alternative available to them.
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REFERENCE #4: http://www.nytimes.com/roomfordebate/2016/04/26/is-marijuana-a-gateway-drug/overdoses-fell-with-medical-marijuana-legalization