What makes some drugs 'good' and some drugs bad'?

In society, we have open hands towards perscription and OTC drugs, but illegal drugs are seen as just plain evil. But dont ALL drugs have the potential to do good and bad? How can you determine whats exceptable and what is not? EX. Viagra is widely accepted, but im sure a drug that directly enduces an would be seen as evil. Oxycodone is a widely available perscription narcotic, eventhough it has a greater liklihood to be abused than marijuana. Wouldnt it be better to put illegal drugs in the hands of pharmaceuticals so they can determine for themselves whether a drug would be of benefit to a patient? Since, drugs like LSD and marijuana have proven medical(or in LSD's case pshycological) potential benefit.