In a typical year, approximately 20 per cent of prescriptions filled in the U.S. are of the psychoactive (mood-changing) variety: stimulants, including appetite suppressions, and depressants ranging from sleep inducing sedatives to minor tranquilizers are counted in this category. Another class of drugs in common use which may cause addiction are the analgesics (pain-killers). All these drugs act on the central nervous system. They are often over-prescribed, and misuse can lead to dependence.