"Deviations from normative patterns of drug use initiation that involve the initiation of illicit drug use earlier than usual in the gateway pattern of initiation may carry small risks for dependence, but other factors seem to be more important in the development of drug dependence. Drug use and initiation are clearly nested within a social normative context, yet neither adherence nor deviation from this order signals highly elevated risks of drug problems in and of themselves, although some violations are predicted by pre-existing mental disorders that seem to be more powerful risk factors for subsequent substance dependence. Although a gateway violation might be a marker of such risk factors, their associations with gateway violations are relatively modest. In targeting intervention efforts, it would probably be more productive to screen directly for these factors (i.e. internalizing disorders, early-onset substance use) than to screen for gateway violations."
Degenhardt, L., Chiu, W. T., Conway, K., Dierker, L., Glantz, M., Kalaydjian, A., Merikangas, K., Sampson, N., Swendsen, J., & Kessler, R. C. (2009). Does the 'gateway' matter? Associations between the order of drug use initiation and the development of drug dependence in the National Comorbidity Study Replication. Psychological medicine, 39(1), 157–167. doi.org/10.1017/S0033291708003425.