As I said in another response, all drugs used over time are dangerous, even legal ones. It is because they change the way a certain part of your body works over time. Most of the debated ones are debated because they affect vital organs over time. Examples are tobacco with the lungs and brain, marijuana with the brain, heroin with various systems, and so forth. Since drugs are also generally addictive, they are also debated because they affect brain chemistry, if nothing else. The only problem with banning them is that some people would be willing to break the law to get them anyways, and some people genuinely need certain drugs medicinally. That is why I argue for regulation, not illegality.