I was looking at the United Nations Office of Drugs and Crime 2014 Wold Drug Report (PDF) and I was happy to see at least some acknowledgement that marijuana legalization could potentially have a positive impact on public health.
Looking at the health impact, it is also important to try to determine if there is a substitution effect whereby cannabis replaces other substances (such as alcohol or more harmful drugs such as heroin) or, conversely, a complementary effect whereby greater use of cannabis leads to greater use of other substances. After drug law reforms in Portugal that decriminalized drug possession for personal use in 2001, referrals for cannabis increased from 47 per cent of referrals in 2001 to 65 per cent in 2005, but referrals for heroin decreased from 33 per cent to 15 per cent, and cocaine remained stable at 4-6 per cent. One study in the United States found that while cannabis-related hospital admissions went up after the decriminalization of cannabis in the period 1975-1978, admissions for other drugs went down.
This substitution or complementary issue is one of the most important public health questions surrounding marijuana legalization and the result should play a big role in shaping the future of marijuana regulation.
Marijuana is relatively much safer than most illegal and legal drugs, so the most significant public health impact might not come directly from its use but how it indirectly effects use of other more dangerous drugs like alcohol and opioids. As the report acknowledges, there are reasons to suspect legalization could reduce the use of more dangerous substances.
Behind these few references we see that in the Netherlands people buying marijuana are far less likely to be offered other drugs at the time (PDF) than anywhere else in Europe, thanks to their quasi-legalization. Legalization of marijuana separates the legal market for marijuana from the black market for other illegal drugs. This offers an explanation for one of the ways that legalization could result in a reduction in the use of other substances.
Of course it is only recently that the first few jurisdictions have actually legalized marijuana so it will be years before we have a real answer to this question, but the interaction will be closely monitored.
Jon Walker is the author of After Legalization: Understanding the future of marijuana policy
Photo by Sonya Yruel/Drug Policy Alliance used with permission