Not all drugs are illegal, and alcohol, a powerful addictive drug, is often not even recognized as a drug. Excepting their illegality, the illicit drugs, having fewer users, would pose fewer problems. Even the mildest of drugs cause problens. Caffeine contributes to ulcers and sugar to children's hyperactivity. Each drug has Its own effects and dangers, but they can be generalized into problems of health, responsibility, and cost.
Most drugs are also poisons.
If you take a sufficient dose,they will kill you. A dose sufficient to get you high is a mild poisoning. Obviously regular poisoning is unhealthy. For people around the drug user, most problems will be with their failure to meet responsibility. Families suffer greatly for the drug driven decisions of users preferring a high, to fulfilling obligations.