"Heroin misuse in Switzerland was characterised by a substantial decline in heroin incidence and by heroin users entering substitution treatment after a short time, but with a low cessation rate. There are different explanations for the sharp decline in incidence of problematic heroin use. According to Ditton and Frischer, such a steep decline in incidence of heroin use is caused by the quick slow down of the number of non-using friends who are prepared to become users in friendship chains. Musto's generational theory regards the decline in incidence more as a social learning effect whereby the next generation will not use heroin because they have seen the former generation go from pleasant early experiences to devastating circumstances for addicts, families, and communities later on."

Source

Nordt, Carlos, and Rudolf Stohler, "Incidence of Heroin Use in Zurich, Switzerland: A Treatment Case Register Analysis," The Lancet, Vol. 367, June 3, 2006.