Answer

I haven’t examined the issue in depth. In general, I tend to agree that something which does not harm others should not be prohibited. Even clear harm to the person himself can be prohibited only if society is not prepared to fund treatment, assistance, and the like for him afterward (which is why fattening food or cigarettes are not prohibited). The factual question of whether there is indeed harm to others here—the slippery slope from marijuana to heavier substances—is the heart of the issue. From what I’ve understood from people, that apparently is not true. Another question is whether people act while under such influences and what damage that causes, but here too people are not prohibited from drinking alcohol (only from driving under its influence), even though it too is dangerous for driving.