Marijuana Use as a Gateway to Other Drug Use

"There is no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs."

Source: 

Janet E. Joy, Stanley J. Watson, Jr., and John A Benson, Jr. Marijuana and Medicine: Assessing the Science Base. Division of Neuroscience and Behavioral Research, Institute of Medicine. Washington, DC: National Academy Press, 1999.