Yes, we actually can. People like to talk about Prohibition as being a failure, but Prohibition actually resulted in a substantially lower rate of liver cirrhosis. People drank less and suffered less as a result. I'm not saying we need prohibition, but the idea that making drugs illegal doesn't work is absolutely ridiculous.
Yes, if we ignore all the other effects (e.g.: financing of organized crime, bringing people in contact with said organized crime) then sure, the prohibition was a sounding success. If we include all of these it doesn't look so great anymore.
