Estimating the incidence and prevalence of injecting drug use in Glasgow

Previous research indicates that wide scale heroin injecting began in Glasgow in the early 1980s and that by 1990 there were about 8,500 (mainly poly-drug) injectors. Data on year of, and age at, first injection, obtained from 1,425 current injectors in non-treatment settings between 1990 and 1994 are used to address three issues: 1) what was the incidence of drug injecting from, the first reported year of injection, 1967 to 1994, 2) is it possible to predict 1994 period prevalence and 3) how do changes in age at first injection, duration of injecting and age at interview relate to incidence and outcidence rates? Between 1990 and 1994, the mean number of years injected (6.9 to 8.8 years) and age at first injection (17.4 to 18.9 years) increased significantly. The average age of new injectors increased from an average of 17.7 years during 1967-1989 to 21.5 years during 1990-94. These findings could be explained by outcidence outstripping incidence, resulting in decreasing prevalence. Although further work involving simulation of incidence and outcidence rates is required, we tentatively estimate 4,000-7,000 current injectors in Glasgow 1994, based on 3-6% incidence rates and 10-20% outcidence rates between 1990-1994. Given the importance attached to drug prevalence estimates by the recently established European Monitoring Centre for Drugs and Drugs Addiction (EMCDDA), it is hoped that the research reported here will serve a twofold function by helping to clarify the relationship between behavioural and epidemiological data and showing how such data can be utilized in estimating prevalence and incidence of drug misuse.