The process is driven by estimates of the prevalence and consumption of "regular users," defined as individuals who use a particular drug at least once a week. Data were obtained from the NEW-ADAM program (New English and Welsh Arrestee Drug Abuse Monitoring) as the basis for estimating the number of regular users. Although the NEW-ADAM sample is likely to underrepresent the United Kingdom as a whole, researchers anticipate that the data will improve as the program becomes more established. Estimates derived from NEW-ADAM indicate that in the United Kingdom there are approximately 270,000 regular heroin users. This is higher than previous official estimates, which have been based on data from household surveys and contacts with treatment agencies. Data suggest that regular use of crack is more prevalent than regular use of cocaine. The converse is true for occasional use. NEW-ADAM is also used to identify drug expenditures by regular users. The findings suggest that a regular heroin user spends about 16,500 pounds a year on all drugs, and a regular crack user spends almost 21,000 pounds a year on all drugs. These expenditures are in line with estimates derived from other research. Occasional use of drugs is a relatively minor part of the overall market for heroin, cocaine, and crack. Heavy users and polydrug users dominate these markets. This methodology aims to give plausible results for the prevalence and consumption of heroin users. More research is required to assess the consistency of, for example, estimates of cocaine, where the number of users and the value of use seems small, especially when compared to crack. 17 tables and 17 references