We charted changes in the prescriptions issued to opiate addicts in treatment at a London clinic over the first 15 years of operation of one of the new National Health Service (NHS) drug clinics established in 1968. Having located the original handwritten ledger records of prescriptions issued by the drug addict treatment clinic, an SPSS data file was created of the prescriptions given to each of the clinic patients each month over the period 1968-1983 to permit examination of changes over this 15-year period in the drugs prescribed (e.g., heroin, methadone), the form (e.g., injectable or oral), the daily doses, and the extent of multiple items for single patients (e.g., both injectable ampule and oral forms). For each month, a list was available of all current patients detailing which drug(s) they had received during that month, in what form(s) and what dose(s). These items were the basic units of study. We report changes over the 15-year period for which the data were available. For the first 5 years, more than half of the prescriptions were for heroin (diamorphine hydrochloride), with the remainder of the prescriptions mostly comprising oral methadone. After 1973, methadone ampules for injection were increasingly commonly prescribed and thereafter remained at about a quarter of the prescriptions for the remaining 10 years for which data are available, whereas heroin prescriptions declined over the mid-1970s to only 20%. From 1973 onward, oral methadone was increasingly prescribed, rising from approximately one third of prescriptions in the early 1970s to more than two thirds by the early 1980s. Individual patients often received more than one drug or form of drug: From 1969 onward, oral methadone was commonly prescribed as a supplement to heroin prescriptions. This same practice was widespread with prescriptions for methadone ampules prescribed as a supplement to heroin prescriptions. Daily doses of heroin were at a mean of from 160 to 540 mg, in contrast with means of 20 to 40 mg and 25 to 50 mg daily for the injectable methadone ampules and the oral methadone solution, respectively. Major changes occurred in the prescribing practices of this London drug clinic over the first 15 years of operation of the new NHS clinics. However, these changes did not occur overnight and can be seen to have been gradual evolutions of practice with the preservation of heroin prescription through the first 5 years and the subsequent move to increased prescribing of injectable methadone, with both these practices being overtaken by the subsequent increased prominence of oral methadone syrup as the main drug prescribed. Reference to the original records reveals a more interesting picture than might be evident from the potentially selective recall of contemporary or historical commentators.