In my view, treatment would have a much greater effect than imprisonment on the extent to which illegal drugs are used in this country. We have been trying imprisonment for decades now without results that are notably good. It seems likely that treatment would be more helpful.
Drugs such as heroin have been illegal in the US for a long time. We have tried to incarcerate large numbers of people for drug crimes. This strategy does not seem to have lowered the rate of drug use in any dramatic way. It is possible to argue that even more people would use drugs if we did not use imprisonment, but it is not clear that this is true. I would argue that our current system does not work.
I would argue that treatment would work better. Many people who are taking illegal drugs are not taking them because they want to. Instead, they are addicted and cannot help but take the drugs. If we could get such people treatment, they would be more likely to kick their habits and would no longer need the drugs as badly. This seems like a much more effective way to reduce drug use than trying to punish people who are addicted.