Adult use in Colorado has increased, and as pot becomes mainstream, use will eventually increase all around.
That hasn't been the case in the Netherlands, in Switzerland and in Portugal -- where all drugs have been legalized. Use has gone
down. The reduction doesn't occur immediately but over time. As people become accustomed to the ready availability they tend to lose interest. It's called the
lure of the illicit. People want what they can't have. Make it readily available and they start looking for reasons for not being bothered with it.
Just think of all the special interests the drug war serves: Start with the cartels and lesser dealers. If marijuana, alone, were legal the liquor industry would soon suffer. Same for the pharmaceutical industry, the drug-testing industry, the legal profession, the prison industry, the law-enforcement industrial complex and all the hucksters who make a living by promoting the drug war -- which they know damn well is wasteful and counterproductive.
The War on Drugs has been doing the same things over and over for decades with absolutely no positive effect, which either is one definition of insanity or the greatest con-job in world history.