In many countries, when health systems are examined from the bottom up medicine sellers emerge as critical actors providing care and access to commodities. Despite this, these actors are for the most part excluded from health systems and policy research. In this paper, we ask 'what happens to the conceptualisations of a health system when medicine sellers and their practices are foregrounded in research?' We respond by arguing that these sellers sit uncomfortably in the mechanical logic in which health systems are imagined as bounded institutions, tightly integrated and made up of intertwined and interconnected spaces, through which policies, ideas, capital and commodities flow. They challenge the functionalist holism that runs through the complex adaptive systems (CAS) approach. We propose that health systems are better understood as social fields in which unequally positioned social agents (the health worker, managers, patients, carers, citizens, politicians) compete and cooperate over the same limited resources. We draw on ethnographic research from Uganda (2018-2019) to analyse the responses of different actors to a new policy that sought to rationalise the medicines retail sector and exclude drug shops from urban centres. We examine the emergence of new lobby groups who contested the policy and secured the rights of 'drug shop vendors' to trade on the basis that these shops are increasingly populated by trained nurses and clinical officers, who are surplus to the capacity of the formal health system and so look to markets to make a living. The paper adds to the growing anthropological literature on health systems that allows for a focus on social change and a form of holism that enables phenomena to be connected to diverse elements of the context in which they emerge.