Alcohol and marijuana are related by what’s called the substitution effect: make it harder to get one, and consumers will turn to the other.
A classic report by economists John DiNardo and Thomas Lemieux studied teenagers from 43 states of the USA over the course of a decade. It showed that when the legal drinking age is raised, more teenagers smoke marijuana.
Look it up: DiNardo, John E. and Lemieux, Thomas, “Alcohol, Marijuana, and American Youth: The Unintended Effects of Government Regulation” (November 1992). National Bureau of Economic Research Working Paper No. W4212. Available online at Social Science Research Network.