A new 20-year study challenges the recent oft-cited claims that marijuana is a harmless, non-toxic drug. Led by Professor Wayne Hall, a drug advisor to the World Health Organization, the study concludes that many of the claims made by marijuana advocates are false:

1. Long-term users do become physically dependent.

2. Smoking marijuana and driving doubles your risk of being in a fatal car crash.

3. Long-term use of marijuana is associated with various mental disorders and poor performance in school/life.

4. Marijuana really is a gateway drug.

I don’t really have a dog in this race, as such, but I think there are a number of issues that need to be discussed pertaining to this “definitive” study.

First, some of Professor Hall’s findings are susceptible to the causal fallacy. For instance, even if marijuana is associated with certain ill effects, it doesn’t necessarily mean marijuana is causing those ill effects. People who perform poorly in school or who have various mental disorders might just be more likely to use marijuana. And marijuana might be a gateway drug because it’s illegal. It’s very possible that legalizing it would reduce this possibility, as dealers of other illegal drugs would be less connected to its sale.

Second, dependency is a tricky thing. Nicotine, caffeine, and various legal drugs (e.g., pain killers, anti-depressants, and sleeping aids) all have very well-established clinical histories of dependency. Yet all of them are legal.

And that leads me to my final point. Do any of Professor Hall’s claims really support the idea that marijuana should remain illegal? All of the things Hall mentioned are true of alcohol, and truth be told, alcohol is probably far worse. Professor Hall’s study sounds tame in comparison to the things said about alcohol before and during Prohibition:

The temperance movement had popularized the belief that alcohol was the major cause of most personal and social problems and prohibition was seen as the solution to the nation’s poverty, crime, violence, and other ills.

We all know how well that worked out.