Modelling the characteristics of the male injecting drug user population in England and Wales

Estimating characteristics of the injecting drug using (IDU) population is of major health importance. This study proposes a method to determine the age-specific rate at which individuals start injecting drugs, and the rate at which individuals leave the IDU population. A simple age-structured model describing the initiation of injecting and the removal of injectors from the IDU population and their evolution over time was fitted to data by maximum likelihood. The peak age at which males start injecting drugs is 21 years. The rate at which IDUs leave the surveyed IDU population (removal rate) increases linearly with age up to a maximum rate and is constant thereafter. The model suggests that the rate at which IDUs started injecting may have peaked during the early 1980s and has declined since. These results reflect the characteristics of a sample of IDUs in contact with services; they suggest that the incidence of injecting drug use has been broadly stable throughout the 1990s with possibly a slight drop in recent years. The actual IDU population may differ from the surveyed sample (in particular they may have a lower average age and length of career) and this must be investigated. Additional modelling work attempting to clarify the difference between the removal rates proposed here and the true cessation rates as they vary with age should also be undertaken.