People who become heavy drug users often need to find a way to finance their habit. As a result, they often become drug dealers themselves, and try to push drugs onto other people, including children.
Furthermore, once they become dealers, they become exposed to an even wider variety of drugs than before, and they are likely to try drugs that they never would have before because drugs are essentially free to them. And a great way to push drugs on people is to consume drugs with them. So, to make more money, they end up consuming more drugs.
Making drugs legal will stop this vicious cycle. The price of drugs will drop to free market levels, and people will not need to become drug dealers themselves to finance their habit.