Counting the uncatchable? An epidemiological method for counting drug misusers.

Service provision and treatment outcome for problem drug users are receiving increased attention, although both are hindered by the lack of good epidemiological data. A technique of population enumeration called capture recapture methodology (CRM) is currently being advocated for use in populations that are otherwise hard to count. CRM is explained and some of its limitations discussed. Studies that have used this methodology are examined.