An amazing book on this topic, a must-read, is Legalize This! by Douglas Husak
He argues that all drugs should be decriminalized (note that this is different from legalized; he is arguing no one should be punished for consumption of any drug). The argument is elegant, simple, and profound.
Anyone who is going to jail requires an explanation for why that is happening. It is easy to provide one for violent crimes (for example). But you can't come up with anything reasonable for drug offenses. In the book he goes through four categories of possible responses and shows that they are all lacking (or prohibit too much if imposed in general). For example: we don't put people to jail for risky behavior like skiing or skydiving; we don't put people to jail for things that hurt their health, like alcohol, or pizza.