In recent years in the UK there has been a shift towards doctors practising preventative medicine. Research suggests, however, that doctors are more comfortable in their traditional role, and may be reluctant to engage in discussion of lifestyle issues with patients. In this article, we use data from GPs' consultations about smoking, recorded prior to the availability of Nicotine Replacement Therapy on NHS prescription, to demonstrate how they attempt to negotiate behaviour change. Using a discursive analytic approach, and drawing particularly on some of the conversation analytic literature on advice giving, we suggest that there are two kinds of difficulties for doctors to overcome: an ambiguity about the interactional endpoint of a discussion about smoking; and the inability to offer 'expert' medical help. As a result, doctors struggle with following through their advice to stop in terms of talking about how to do it. We suggest that the efficacy of nicotine addiction treatments may be due not only to their clinical effects, but also because their prescription legitimizes the difficulty in stopping reported by most smokers as an appropriate problem for medical treatment. We discuss the implications of these findings for the management of smoking and other lifestyle issues within primary care consultations.