Addiction or problematic use is a difficult concept to measure
Those with drug-related problems tend to be difficult to find, and addiction is difficult to measure. Experts consistently fail to agree on what constitutes an addict, problematic use or problematic user. Estimates as to how many people are experiencing drug problems have to be drawn from different sources, using different ways of measuring.
National Drug Treatment Monitoring System
In the UK, figures for drug users presenting for treatment related to problematic drug misuse are drawn directly from the National Drug Treatment Monitoring System (NDTMS). The NDTMS relates to the process of collecting, collating and analysing information from and for those involved in the drug treatment sector. The NDTMS is a development of the regional drug misuse databases (RDMDs), which have been in place since the late 1980s.
The NDTMS collects data on both those drug misusers presenting for treatment and those in treatment. It does not count the number of people addicted to a drug, but those seeking help for their drug use or associated problems (and subsequently have their information sent to the database) and is therefore not reflective of the community at large.
The latest statistics from the NDTMS show that in 2011-12 there were 197,110 individuals aged 18 and over who were in treatment, with 69,434 of those having started new treatment journeys in that year. Of the 63,020 that exited treatment that year, 29,855 (45%) left after having completed their treatment and overcome their dependency on the drug for which treatment was sought.
Estimating the number of problem drug users, including those not in treatment
The numbers from the NDTMS only count those people who are in contact with structured drug treatment services, which will not be all of the people who are using drugs problematically. The National Treatment Agency for Substance Misuse (NTA) has commissioned research that attempts to estimate the number of people who use opiates and/or crack cocaine, which are the drugs associated with the highest levels of harm.
The research, from 2010-11, uses two types of measurement techniques, the capture-recapture method and the multiple indicator method, which are statistical methods designed to estimate total population sizes from smaller samples. The research estimates that, for the year 2010-11, there were 298,752 opiate and crack cocaine users in England.
References National Treatment Agency for Substance Misuse, ‘Statistics for drug treatment activity in England in 2011-12 from the National Drug Treatment Monitoring System’, 2012
 Hay, G., dos Santos, A. & Millar, T., ‘National and Regional Estimates of the Prevalence of Opiate and/or Crack Cocaine Use 2010-11’, 2013