One would expect to see a significant decrease in welfare spending. This is because millions of people are fired or quit due to drugs. I know lots of tradesmen who were fired because they failed piss tests. I'm talking about highly skilled welders, electricians, and pipefitters making 6 figures.
The people who say they should just stop doing drugs need a grade 5 education on how the brain works. Not all humans are the same. We have lots of studies covering things like gender, race, environmental effects, life stability, and family that correlate with differences in time preference. The people who go into trades, in general, have a very strong preference for things happening right now. For anyone familiar with economics, think of it like having a high discount rate. Something in the future (a job) needs to have a very high value before it trumps things happening right now (doing coke with friends at a party).
I don't think people understand how badly it damages the economy when we lock up the more impulsive part of the population. People like myself who take the most conservative route possible tend to do ok in life, but we never achieve anything great. The really extreme people like Michael Jordan and Steve Jobs are the kinds of people who use drugs and get locked up. The type of person who risks everything to do coke is the same kind of person who drives on frozen lakes in the arctic or goes crab fishing in Alaska. They are an important resource that we should be using instead of throwing away.