Do 'informed' or 'expert' patients challenge dominant traditions in biomedicine or simply adopt these as conventional ways of thinking about body shape and size, illness and health? This paper examines this question in relation to the use of the weight-loss drug Xenical by participants in an Internet forum for obese and overweight people. Ethnographic and interview data from the forum provides evidence that participants share information and support each other as they use Xenical, and in the process emerge as 'expert patients' in relation to their body shape and its treatment. However, it is argued that while an 'expert patient' can be perceived as desirable, enabling the democratisation of healthcare, it can also be constraining. The exchanges between the users in the forum perpetuate a biomedical model of overweight as a condition to be overcome. The discussion critically considers a number of options for the development of the expert patient, including the emergence of an 'informed consumer'.