Medicinal marijuana – a gateway to increased illegal drug use?

Whether medicinal marijuana should be legalised has attracted debate recently. Central to the discussion both in New Zealand and overseas, is how legalising the drug for medicinal purposes will affect the use of non-medical marijuana and other drugs. Dr Luke Yu-Wei Chu's empirical research provides insight to this question.

Much of the rhetoric surrounding the debate of legalising medicinal marijuana focuses on the flow-on effects to society. Many assume legalising medicinal marijuana will increase recreational use of the drug as access becomes increasingly available. There is further concern that legalising medicinal marijuana will reduce the stigma associated with the drug as well as lowering the public’s perception of its health and legal risks.

Marijuana is dubbed by some as a “gateway drug” that leads to increased use of hard drugs such as cocaine and heroin.

In fact, this is one of the major reasons why federal agencies in the United States (US), such as the Drug Enforcement Administration and the Office of National Drug Control Policy, firmly oppose medical marijuana laws.

Behind this argument is a gaping hole of evidence. The literature on the relationship between marijuana use and other substances is extensive, but a causal link is yet to be proved.

However, my three-year research of drug use in the US, where medicinal marijuana has been legalised, provides an understanding of this relationship. It must be noted, however, that this research best illustrates the effect of legalising medicinal marijuana on heavy users, as the majority of drug arrestees and treatment patients are heavy users.

I analysed the number of drug possession arrests and substance abuse treatment admissions from 1992-2011, which gives a clear picture of drug use, before and after medicinal marijuana was legalised in the states that initiated a law change. I compare the changes in the states where medical marijuana is legal with changes in states without such laws.

If medicinal marijuana truly is a gateway drug, then we would expect to see an increase in the rates of possession arrests and treatment admissions for heroin and cocaine.

My research shows this is not entirely true.

Yes, states that had legalised marijuana did see an increase in both these areas for marijuana by 10-15 percent after legislation was passed. However, it should be noted that the trend was evident only in adults—for teenagers, there was no discernible change.

However, there is no evidence to suggest cocaine and heroin use increased. In fact, there was a decrease of up to 15 percent in the number of arrests for possession of these two drugs combined.

The data did not distinguish between arrests for cocaine, arrests for heroin, or arrests for both, which is why I also analysed treatment data, which separated data for the two drugs.

The data showed a 20 percent decrease in the number of heroin-related admissions to treatment centres, indicating that marijuana, far from a gateway drug, is used by some as a substitute for heroin.

The same cannot be conclusively said for cocaine as there was no significant change in rates of cocaine-related treatment admissions.

When discussing whether New Zealand should legalise marijuana for medicinal purposes, it is vital we take this research into consideration. By better understanding the situation in the US, we will be better informed to make a smart decision for the benefit of our society.