The Edinburgh Prison (Scodand) Drug Reduction Programme was evaluated to assess its impact on clients' drug‐using behaviour during their current prison sentence. Thirty drug users who were being prescribed as part of the Drug Reduction Programme (DRP) and who had completed the programme's educational and groupwork sessions were compared with 30 drug users who had not. Participants were interviewed approximately a month after either completing the programme (intervention group) or after being admitted to prison (control group). A smaller sample of participants drawn from both groups were interviewed again within two weeks of their release from prison. The intervention group had used a significantly lower number of drugs during their current sentence than had the control group, and was less likely to have used cannabis, dihydrocodeine, buprenorphine, temazepam, diazepam, and LSD, or to have used these drugs less frequently and in smaller amounts. When compared with other independent variables, not completing the Drug Reduction Programme was found to be the sole predictor of using cannabis more often and in greater amounts; of being more likely to have used dihydrocodeine and to have used it more often and in greater amounts; of being more likely to have used buprenorphine and diazepam, and a co‐predictor of having used a higher number of drugs and of using diazepam more often. Other independent variables which were co‐ or sole predictors of drug use at the time of the first interview were all related to previous drug use. The previous drug use of DRP clients and the control group was broadly comparable, indicating that completing the DRP is associated with reducing the reinstatement of previous behaviour. Prior to release, the differences in drug use between DRP clients and the control group had largely been maintained, and not completing the programme was still a co‐predictor of having used a higher number of drugs. The Edinburgh Prison Drug Reduction Programme provides a model which could be adopted by other prisons. This follows the principle argued for by the WHO of providing health care in prison comparable to that available in the community, and also enables the development of prison‐based drug services which can have a beneficial effect on client's drug‐usin