http://www.nytimes.com/1989/10/16/opinion/actually-prohibiti...
"John D. Rockefeller, a teetotaler, observed in 1932, 'drinking has generally increased, the speakeasy has replaced the saloon; a vast army of lawbreakers has been recruited and financed on a colossal scale.'"
To avoid a long drawn out argument about which statistics are more likely to be accurate, I'm willing to admit that both conclusions are possible and the actual answer lies somewhere in-between. The first guy estimated 30%, Rockefeller surmised that it increased, so lets go with 15% decrease in drug use because of the drug war. That's even hard to say because the drug war has lasted a lot longer than prohibition.
Unfortunately I couldn't quickly find any information about the repeal of prohibition which is more relevant to the topic since we are currently in a prohibition.
Anyway, I guess the country needs to decide if 15% increase in drug use is worth the cost of the drug war, so for every 20 current drug users, we'd get 3 more. In exchange, we'd get a lot of money in the treasury, respect for the law, return of our freedoms (maybe), much smaller incarceration rate, greatly weakened gangs and cartels, much less justification for surveillance, and an overall less hellishly scary police state.
Unfortunately, the US doesn't debate it rationally in those terms.
The problem I have is that the US is so hard headed, they double down whenever something doesn't work rather than reevaluate. The US has been doubling down on the drug war every administration since the 1970s. The solution has been significantly more harmful than the problem since the 80s, and yet they keep doubling down. Sounds like we just doubled down again with painkillers. It's madness.