While the biographical dimensions of chronic illness have been well researched, the concrete dimensions of patients' work have not been as thoroughly investigated as yet. With the growing concern for self-management, such research would be timely. This study aims to better understand patients' invisible work by highlighting the causes of unintentional non-adherence as well as strategies for adherence. For this purpose, it defines medical treatment adherence as the repetition of the pattern of tasks through which a patient succeeds, in a technical sense, in taking the right medication at the right time, in the right amount, for the right duration. Applying a failure modes and effects analysis approach to 48 semi-structured interviews with Dutch patients, it shows the negative impact of schedule changes, pressure, positioning changes, lack of backup pills and lack of verification tools. Symmetrically, it highlights the role of anchoring, sequencing, positioning, cueing, correcting and verifying. This result points to the need for an analytical approach of patients' work and treatment adherence that would build on the role of routines in organisations and in the workplace.