We as a society noticed the bad effects of alcohol and chose to make it illegal. After a period of time people decided to still purchase alcohol legal or illegal. After further thought everybody decided that making alcohol illegal was a bad idea and we should make it legal to folks over a certain age.
As things now stand alcohol is legal, but it can be placed at the roots of many bad accidents and deaths.
In my view pot/marijuana followed in this same tradition with the slight exception that pot was made illegal because it was perceived popular with blacks and Mexicans. Thus it was easy to make rules making the substance illegal.
In a lot of countries today pot is legal to own, but it's still viewed as a possible medical problem. The countries I'm talking about still spend tax money to help folks eliminate or reduce their use of pot, but they don't throw you in the slammer.
We could save a lot of tax dollars by taking the same approach. There are a lot of people in jail in the US merely for having a joint on them. If we spent tax money on educating people about negative consequences we would both help folks and save money.
As for other substances I'm not sure. It seems like coca leaves are ok talking to climbers who've been to Peru, but cocaine is much worse. Crank and meth sound all around bad. Taking a drug by injecting it without a doctors supervision is another bad idea.