"Alcohol and marijuana are the two most commonly used substances by teenagers to get high, and a question that is often asked is to what extent does change in one lead to a change in the other. If the substances co-vary negatively (an increase in one is accompanied by a decrease in the other) they are said to be substitutes; if they co-vary positively, they are said to be complements.
"Interestingly, the answer may differ by historical era. Before 2007 patterns of use for the two substances suggested they acted as complements. When marijuana use increased in the late 1970s, so too did alcohol use. Between 1979 and 1992 marijuana use declined and a parallel decline took place in annual, monthly, and daily alcohol use, as well as in binge drinking among 12th graders. As marijuana use increased again in the 1990s, alcohol use again increased with it, although not as sharply. In sum, before 2007 there was little evidence from MTF to support what we have termed “the displacement hypothesis,” which asserts that an increase in marijuana use will lead to a decline in alcohol use, or vice versa.8
"However, since 2007 a new trend has emerged that would be consistent with the “displacement” hypothesis. From 2007 through 2019 alcohol use declined markedly, reaching historic lows in the life of the study. Meanwhile, for most of this time period marijuana use has stayed steady or increased for all age groups. For the first time trends in alcohol and marijuana use are substantially diverging, suggesting that the historical relationship between these two drugs may have changed."
Miech, R. A., Johnston, L. D., O’Malley, P. M., Bachman, J. G., Schulenberg, J. E., & Patrick, M. E. (2020). Monitoring the Future national survey results on drug use, 1975–2019: Volume I, Secondary school students. Ann Arbor: Institute for Social Research, The University of Michigan.