In my experience a lot of drug use is self medication for relieving emotional or psychological pain, people who just don't feel good for whatever reason and want to feel better. Making drugs illegal won't really helps these people and if anything can do great damage to their future if they run into legal problems. People should have a right to their own bodies anyway even if they might cause themselves harm
A perfect example is amphetamines. Without them, I don't get stuff done, sure I will get the one thing done that I am interested in, but the dishes will sit in the sink for 2 weeks before I can even remotely begin to think about them, and then it is like pulling teeth to get started. The task just seems insurmountable. When I take my medicine, I do them as a way to take a break from thinking. I agree, I think a lot of people self-medicate mental health issues. I know I like opiates, so I always take them as prescribed when they are given to me. Never had an addiction problem with them, but I was on them for a period of time due to a back injury. I think the risk of amphetamine addiction is overblown, but I certainly see how opiates can be addictive and very quickly. They make you pretty much not give a crap about anything.
