Even assuming that the drugs we're talking about are bad and have a destructive effect on people, should those people be jailed for making a mistake? Should we jail single mothers who give birth out of wedlock? Alcoholics?
Jailing people for making a mistake is done to incentivize not doing the activity leading to jailing or to protect others from people who would do such a thing. I will certainly agree with you that jail is not always beneficial for the person incarcerated. Perhaps an option of some sort of mandatory rehab couple with community service would be better for the users, though we typically don't give users nearly as harsh of sentences as dealers, which makes sense.
As for jailing single mothers, that would be a disastrous idea because it would leave the child parentless and incentivize abortion. I'm not sure what you mean by jailing alcoholics, in order to be jailed people have to be charged and convicted of doing something, not being something.