BACKGROUND: The availability of prison-based drug treatment has increased markedly throughout Europe over the last 15 years in terms of both volume and programme diversity. However, prison drug treatment faces problems and challenges because of the tension between ideologies of rehabilitation and punishment. METHODS: This article reports on a study of four cannabis treatment programmes and four psychosocial drug treatment programmes in four Danish prisons during 2007. The data include the transcripts of 22 semi-structured qualitative interviews with counsellors and prison employees, prison statistics, and information about Danish laws and regulations. RESULTS: These treatment programmes reflect the 'treatment guarantee' in Danish prisons. However, they are simultaneously embedded in a new policy of zero tolerance and intensified disciplinary sanctions. This ambivalence is reflected in the experiences of treatment counsellors: reluctantly, they feel associated with the prison institution in the eyes of the prisoners; they experience severe opposition from prison officers; and the official goals of the programmes, such as making clients drug free and preparing them for a life without crime, are replaced by more pragmatic aims such as alleviating the pain of imprisonment felt by programme clients. CONCLUSION: The article concludes that at a time when prison-based drug treatment is growing, it is crucial that we thoroughly research and critically discuss its content and the restrictions facing such treatment programmes. One way of doing this is through research with counsellors involved in delivering drug treatment services. By so doing, the programmes can become more pragmatic and focused, and alternatives to prison-based drug treatment can be seriously considered.