Hilary Putnam’s influential analysis of the ‘division of linguistic labour’ has a striking application in the area of doctor–patient interaction: patients typically think of themselves as consumers of technical medical terms in the sense that they normally defer to health professionals’ explanations of meaning. It is at the same time well documented that patients tend to think they are entitled to understand lay health terms like ‘sickness’ and ‘illness’ in ways that do not necessarily correspond to health professionals’ understanding. Drawing on recent philosophical theories of concept possession, the article argues that this disparity between medical and lay vocabulary implies that it is, in an important range of cases, easier for doctors to create a communicative platform of shared concepts by using and explaining special medical expressions than by using common lay expressions. This conclusion is contrasted with the view that doctors and patients typically understand each other when they use lay vocabulary. Obviously, use of expressions like ‘sickness’ or ‘illness’ does not necessarily lead to poor communication, but it is important that doctors have an awareness of how patients interpret such terms.