But what we do want to do is send a clear message: Marijuana use is a bad choice.
There are many excellent reasons to avoid marijuana. Marijuana use damages brain development in young people. Heavy users become socially isolated and perform worse in school and at work. Marijuana smoke harms the lungs. A growing body of evidence suggests that marijuana can trigger psychotic symptoms that otherwise would have remained latent.
It's possible to imagine a marijuana rule that tries to respond precisely to such risk factors as happen to be known by the current state of science. Such a rule might say: "You shouldn't use marijuana until you are over 25, or after your brain has ceased to develop, whichever comes first. You shouldn't use marijuana if you are predisposed to certain mental illnesses (most of which we can't yet diagnose in advance). Be aware that about one-sixth of users will become chronically dependent on marijuana, and as a result will suffer a serious degradation of life outcomes. As yet, we have no sure idea at what dosage marijuana will impair your ability to drive safely, or how long the impairment will last. Be as careful as you can, within the limits of our present knowledge!"