Extends current knowledge of recent trends and demographic patterns in the incidence of drug use in the U.S. by presenting estimates based on the combined 1991-1993 NHSDA1s, a data base consisting close to 90,000 records. The estimates are weighted to represent the civilian, noninstitutional population of the U.S., with comparisons of patterns of drug use initiation across 20th century birth cohorts. All of the analyses are reported separately for 13 major drug categories, including non-medical use of stimulants, analgesics, tranquilizers, and sedatives. Tables, figures and references.