Drugs that are illegal are so because someone noted enough difficulty with them to decide to make them illegal. Many legal and illegal drugs work in the same way, and some drugs that are now illegal were actually developed for medical purposes, and were prescribed by doctors until someone decided that those particular drugs were doing more harm than good. Legal/prescription drugs can be abused also, and can do as much damage as illegal drugs. There are also cases of drugs that were designed for entertainment purposes, that were found to be useful and now are being used legitimately (such as Nitrous Oxide). So, no, it's not really a matter of some drugs just plain being bad, and others good. It depends on who is using them, how, and for what purpose. But the idea is that drugs with the most potential for harm are made illegal.