Clearly there are some drawbacks to some of them, but alcohol and tobacco worse than some of them. Just think about it; it all comes down to personal responsibility so why prevent others from being happy? If you don't feel like smoking crack then don't, but don't stop another from wanting to satisfy themselves.
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You seem to misunderstand something OP. You seem to think that if you don't use the drug then it doesn't affect you when the truth is it does. People get injured or killed when using things like meth or heroin and as a result have to go to the hospital. That means the hospital has to spend time and money on them which could be used elsewhere on people that actually contribute to society. Then a lot of the hard drugs out there produce violent tendencies among other things. There are reasons why they are illegal and should stay that way. Also most people are not doctors and do not know the side effects of mixing multiple different drugs together.