Quote from Milton Friedman,
"The proper role of government is exactly what John Stuart Mill said in the middle of the 19th century in On Liberty. The proper role of government is to prevent other people from harming an individual. Government, he said, never has any right to interfere with an individual for that individual's own good.
The case for prohibiting drugs is exactly as strong and as weak as the case for prohibiting people from overeating. We all know that overeating causes more deaths than drugs do. If it's in principle OK for the government to say you must not consume drugs because they'll do you harm, why isn't it all right to say you must not eat too much because you'll do harm? Why isn't it all right to say you must not try to go in for skydiving because you're likely to die? Why isn't it all right to say, "Oh, skiing, that's no good, that's a very dangerous sport, you'll hurt yourself"? Where do you draw the line?"