Smoking. It's literally a drug and causes lots of health issues like increased lung cancer risk, but the worst part is that if someone smokes near you then you also inhale some of the toxins even if you yourself don't smoke. And in my country it's common to see people smoking on the streets. Combine this with air pollution and yikes
Reality proves enough that forbidding drugs doesn't work though, it just creates a lot of other problems. Forbidding some parts of it like ads about them, selling to minors etc do have effect however