Main features of the program were that users had to be at least 20 years of age, have been addicted to heroin for at least 2 years, had signs of deterioration in health and/or social relations due to drug use, and been engaged in conventional drug treatment without success despite two or more attempts. Those prescribed heroin were assigned to special clinics that also offered a wide array of medical and social assistance. Heroin could be obtained only at the clinic and it had to be injected on the spot. An evaluation of the impact of the program on heroin users showed large reductions in the use of illicit drugs and in drug-related crime. It appeared that drug users who were admitted to the program accounted for a substantial proportion of the consumption of illicit heroin and that removing them from the illicit market damaged the market's viability. Before involvement in the program, a large proportion of heroin users sold drugs to finance their own use, since the illicit drug market in Switzerland relies heavily on users for retail drug selling. The program seemed to disrupt the function of the illicit drug market by removing retail workers because workers no longer sold drugs to existing users and no longer recruited new users into the market. The authors recommend further research to study the potential effects of substitution programs on drug markets and drug market responses to reduced drug prices. 25 references, 6 notes, and 7 tables