“…However, by virtue of the particular circumstances which make such research possible, the opportunity for study of impact is only short-term. Influential natural history studies have included the repeated study over 20 years of heroin addicts who were in treatment with heroin maintenance in London in 1969 (Stimson andOppenheimer, 1982;Oppenheimer et al, 1994), Vaillant's long-term study of a US sample of heroin addicts (Vaillant, 1973), and the study by Robins et al (1974) of heroin use amongst US Army personnel during and after service in Vietnam. However, whilst such studies provide valuable information on the range of outcomes seen with these samples, they do not permit identification of the influences that treatment will have had upon the natural history which is then observed.…”