I think this might be a good example of a larger structural problem. culturally, we accept that going to a bar with friends or having a few glasses of wine at a restaurant is a reasonable thing for an adult to do. but a large portion of the US is set up in a way that bars and restaurants are not within walking distance of homes, and the public transit is poor or nonexistent. if I, a 145 lb male, have a cocktail and a glass of wine at dinner, I'm pretty likely to be above the legal limit by the time I leave. I don't do this; I either drink less or arrange a different way to get home, but I do think the combination of law, culture, and (lack of) infrastructure pushes people towards committing a crime here.
Many illegal drugs have a potential of causing harm.
Should the harm be purely physical?
Homelessness is a public nuisance, which we can choose to ignore, but so is walking naked in a school yard, which will be seen as a much more controversial subject.
But: - for it to work people should estimate the risks correctly - most people caught DUIing apparently are quite bad in understanding the risks involved
To the user, which is a risk they chose to take.
Now you could argue that the illicit trade is causing harm, but that's the direct result of criminalization and not of people choosing to consume substances.
Drugs, prostitution and being homeless (a.k.a. being poor/ in need of support). Should never've been even considered for crimes. Hence I proposed to have those laws made unconstitutional by some amendment.