Let me try to clarify: if you have rules prohibiting the creation and sale of substances solely for personal consumption by competent, consenting adults, then you have the primary tool needed to conduct a War on Drugs. I think that it is a mistaken notion to imagine creating this power, and saying "don't worry, we'll only ban
dangerous things, and not
harmless things, because we'll know better." I think that is definitely not the case. Yes, many people want a different mix of things banned and permitted. But people should not fool themselves into thinking that is what they will get, because a power like that is pretty much always used for more things than its creators intend. We cannot simultaneously prohibit things, and not enforce that prohibition through, ultimately, threats of imprisonment. If the maximum penalty for a drug charge is a ticket, then the prohibition on drugs will simply not be effective.
The War on Drugs has involved arresting and imprisoning a lot of people, because a lot of people really want drugs. Once the soft enforcement mechanisms (reprimands, misdemeanors, fines of legitimate businesses) fail, then if you really want to suppress the act, you have to ramp up the level of force. The reason that fewer people are imprisoned under FDA rulings is, I would say, because the FDA is more lenient, and because the things it bans are far more niche and less desired. You can have effective, or you can have gentle, but you cannot have both.