Humour research in healthcare has tended to focus on rehearsed as opposed to spontaneous humour. This paper reports an empirical example of spontaneous humour in healthcare interactions: a negative case analysis from a constructivist grounded theory study. Twenty Clinical Nurse Specialist (CNS)–patient interactions and CNS pre- and postinteraction audio diaries provided the baseline data corpus. Follow-up interviews, field notes, focus groups and observations serviced theory generation with a constant comparison approach to data collection and analyses. Interpretative and illustrative frameworks incorporating humour theories, non-laughter humour support, discursive features and prosodical features of speech were applied to all data. This paper is based upon the negative case comprising a 90-minute follow-up interview and 10 hours of field note observations. The negative case – a CNS working with female drug users’ sexual and reproductive health needs – contradicted emerging findings from the baseline data corpus. First, the negative case had greater awareness of humour, deliberately initiated humour and recognised parameters and exclusion zones. Second, a good patient persona was evident in the baseline data corpus but the negative case worked with ‘bad’ patients. Accordingly, a specific type of humour – harsh humour – was evident in the negative case. Harsh humour used areas of potential discord (e.g. drug use) as a focus of humour creation and maintenance. The deliberate initiation of harsh humour enabled the negative case and her colleagues to achieve their aims by engaging effectively with unpredictable, reluctant and recalcitrant patients. The negative case demonstrates how humour can be used to therapeutically enhance healthcare interactions with disenfranchised individuals. Humour is not superficial but integral to the accomplishment of key aspects of interactions. Health and social care workers should consider the potential for therapeutic humour to engage and maintain all patients – disenfranchised or otherwise – in healthcare interactions.