Honestly? Alcohol. I used to work security at a rehab, and it was always the worst addiction. The withdrawls are horrible, up to and including death. Yes, even worse than heroin.
Read up on US prohibition and how it funded the Mafia. It just changes the form of the societal disease.
The answer to addiction is having support and care on place for those that fall to it so society helps pick them up again. You can't stop the abuse of substances unless you fix why people are crawling into a hole to avoid the world. Lack of mental health is a disease of society as well as the individual.
Its so mad that we have such a literal example of exactly what happens, due to prohibition, yet society refuses to see like for like. The mafia simply used the exact same routes to smuggle heroin. They didn't disappear or die out, due to alcohol prohibition ending. They got into bed with the CIA, under operation gladio. What they did with crack wasn't the first or the biggest example.
Like you said, you can't people abusing substances. They remain illegal because somewhere some very powerful people are making too much money from them remaining so.
I am in my late 30s. Drank in college with friends at parties. I dont anymore just not into it. I like things that make me faster, smarter, or stronger. I dont understand why all TV shows and movies seem to be centered around drinking when its a social scene. (I live in north america). Nothing good comes from drinking alcohol. They make it seem like if you're relaxing or want to have fun you need alcohol. I just need a good brisket for both those.
Why? Simply because this was actually tried in America? All I'm doing is answering the question. Just because this country failed at making it illegal does not mean it still shouldn't be illegal.
People are allowed to make their own decisions, even if they're bad decisions. And it shouldn't be illegal because it has been proven that making it illegal only makes everything worse.