Background Optimal management of hypertension in older patients with multimorbidity is a cornerstone of primary care practice. Despite emphasis on personalised approaches to treatment in older patients, there is little guidance on how to achieve medication reduction when GPs are concerned that possible risks outweigh potential benefits of treatment. Mindlines – tacit, internalised guidelines developed over time from multiple sources – may be of particular importance in such situations. Aim To explore GPs’ decision-making on deprescribing antihypertensives in multimorbid patients over 80 years old, drawing on the concept of mindlines. Design Qualitative interview study. Setting English general practice. Methods Thematic analysis of face-to-face interviews with a sample of 15 GPs from 7 practices in the East of England, using a chart-stimulated recall approach to explore approaches to treatment for older multimorbid patients with hypertension. Results GPs are typically confident making decisions to deprescribe antihypertensive medication in older multimorbid patients when prompted by a trigger, such as a fall or adverse drug event. GPs are less confident to attempt deprescribing in response to generalised concerns about polypharmacy, and work hard to make sense of multiple sources (including available evidence, shared experiential knowledge, and non-clinical factors) to guide decision-making. Conclusion In the absence of a clear evidence base on when and how to attempt medication reduction in response to concerns about polypharmacy, GPs develop ‘mindlines’ over time through practice-based experience. These tacit approaches to making complex decisions are critical to developing confidence to attempt deprescribing, and may be strengthened through reflective practice.