Or ask how many marriages, now afflicted by alcoholism, will be afflicted by drug abuse when drugs become legal. Or how many pregnancies that now are harmed by fetal alcohol syndrome will be harmed by fetal drug syndrome.
Recall also that most people in drug treatment are there because of some form of coercion. Very few walk in on their own. Take away coercion, and you take away treatment for all but a few burned-out addicts.
John Stuart Mill, the father of modern libertarians, argued that people can only restrict the freedom of another for their self-protection, and society can only exert power over its members against their will in order to prevent harm to others. I think that the harm to others from drug legalization will be greater than the harm—and it is a great harm—that now exists from keeping these drugs illegal.