Misleading statement. Hard drugs have not been ever been legal in our modern society so sales of hard drugs and even pot has been confined to shady fellows selling out of their car and other shady places, so you cannot possibly know or say how many MORE people would be using drugs if they were legal and could be bought at a gas station.
You might want to check out http://facultypages.morris.umn.edu/~ratliffj/psy1081/drug_laws.htm
Cocaine, marijuana and heroin were both legal for quite a while in what I would consider modern society. Morphine is a semi-synthetic derivative of heroin which is still widely used. LSD/Acid were legal into the 60's, I believe. MDMA was sold legally for quite a while before it was made illegal. I'm sure with a little more research you could find more specific details and a longer list of drugs that have been sold legally in 'modern society'...