"In conclusion, our results suggest that participation in the use of both licit and illicit drugs is price sensitive. Participation is sensitive to own prices and the price of the other drugs. In particular, we conclude that cannabis and cigarettes are complements, and there is some evidence to suggest that cannabis and alcohol are substitutes, although decriminalization of cannabis corresponds with higher alcohol use. Alcohol and cigarettes are found to be complements."
Cameron, Lisa & Williams, Jenny, "Cannabis, Alcohol and Cigarettes: Substitutes or Complements?" The Economic Record (Hawthorn, Victoria, Australia: The Economic Society of Australia, March 2001), p. 32.