In a paper titled "Responsive feedback: Towards a new paradigm to enhance intervention effectiveness", Viswanath . argue that dominant models of intervention design do not account for the complexity and unpredictability of implementation challenges.  Particularly in the behavioural sciences, intervention designs need to consider many factors that will be uncertain, or unknown, at the beginning of a new project.   This letter describes how we were able to respond to feedback during the design phase of a proof-of-concept project to create a digital service business for Nigerian patent and proprietary medicine vendors (PPMVs).  Our approach was to create an initial 'Theory of Change' (ToC) based on a similar project with Kenyan shopkeepers.  This ToC was revised following user feedback and a landscape analysis with key stakeholders.  The new ToC required us to access additional funding to create a 'digital ordering' facility for the PPMVs.  Digital ordering provides a mechanism whereby we can reduce the prevalence of counterfeit medicines, offer the PPMVs credit and group-buying facilities, and reduce supply chain costs through co-distribution with fast-moving consumer goods.  An important learning point was that while our focus was on designing a platform to meet users' needs, changes in regulation meant that we spent considerably more time than anticipated meeting the needs of multiple stakeholders. However, the importance of ensuring stakeholders' continued buy-in cannot be underestimated and has likely increased the sustainability of the project in the longer term.  As Viswanath . suggest, for responsive approaches to be widely adopted needs more flexibility than exists in current funding models and project plans.  Both funding bodies and grantees will need to be more responsive to feedback coming from the field.