Possible linkages of influence among classes of drugs in the observed sequential progression from adolescence to young adulthood are investigated through event history analyses. Three stages are examined: initiation to marijuana, to the use of other illicit drugs, and to prescribed psychoactive drugs. The data are based on a follow-up cohort of former adolescents representative of high school students in grade 10 and 11 in New York State who were reinterviewed nine years later at ages 24-25. The sequential order between alcohol and/or cigarettes and marijuana reflects not only the effect of the use of legal drugs on marijuana initiation, but also age effects on onset of these drugs, controlling for individual characteristics measured in adolescence; marijuana use by one's friends in adolescence is an additional important predictor of marijuana initiation. Prior use of marijuana is necessary for progression to other illicit drugs. Multiple factors are involved in the progression to prescribed drugs, with adolescent depressive symptomatology and use of other illicit drugs important for both sexes, and maternal use of psychoactive drugs, dropping out of school, and prior use of marijuana of additional importance for women. Although licit drugs influence initiation into marijuana independently of age effects, it is especially for the progression from marijuana to other illicit drugs that the earlier drug is associated with the progression to a higher stage drug.