A popular notion refers to laughter as ''the best medicine,'' and a stream of psychological research encourages treatment providers to use humour as a therapeutic tool. However, remarkably few studies have investigated client perspectives on professionals' use of humour in everyday practice. Drawing on ethnographic data from fieldwork in two Danish drug treatment institutions for young people, this paper begins to fill this gap in the existing research. Analyzing the use of humour in naturally occurring, informal situations wherein professionals joke, use irony and tell funny stories, this paper suggests that the use of humour by professionals -- despite their allegedly good intentions -- may at times be problematic, even offensive for clients. Furthermore, as the young people who turn to drug treatment are socially vulnerable and dependent on the professionals, the paper illuminates how they find their opportunities for objecting to the professionals' use of humour limited. The paper concludes that professionals should handle humour with care, rather than viewing it as a benign, risk-free addition to treatment, and that future research must tackle the methodological challenges of exploring client perspectives with respect to how professionals use humour in institutional settings characterized by asymmetrical power relations.