I theorize that the situation is bad enough in the US that the use of drugs /would/ increase, if only as a means for the general populace to find some joy in our crappy lives.

Alcohol and tobacco consumption might go down a bit though; mostly because the drugs would be free in comparison. (/those/ lobbies would be completely against this experiment)

Focusing on the increased use segments of the population and figuring out why they're in to drugs instead of 'healthy' outlets would probably reveal deep social and societal dysfunction caused by a complex and interacting set of self-enforcing patterns.

Maybe changes in zoning, public goods funding, and worker rights would actually start to happen on a scientific basis trying to address the identified issues.

At the very least though, this would be THE MOST EFFECTIVE means of fighting criminal and terrorist organizations; by cutting off their funding at the source.