The basis is 30-plus years of observing. The basis is also the coming to the fore of studies such as the United Kingdom study I just quoted—published only this year, having terminated in 2016, in which they followed kids from age 15 to 18 and saw a definite correlation in abuse of the three substances, nicotine, alcohol, and illicit drugs.
On September 13th, 2017. See this statement in context.