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.
|Number of pages||7|
|Journal||International Journal of Drug Policy|
|Publication status||Published - Jun 2005|
Bibliographical noteFunding Information:
The authors thank Prison Health at the Department of Health for England for funding this work. Also thanks to everyone at the Communicable Disease Surveillance Centre and the collaborating services for drug users for their contribution towards the undertaking of the Unlinked Anonymous Prevalence Monitoring Programme. Also thanks to the drug users that took part in the surveys. Additional thanks also to Matthew Hickman for helpful comments.