Despite guidelines on the treatment of acute heart failure (AHF), treatment remains heterogeneous, particularly regarding intravenous (IV) nitrate use. This clinical decision-making study assessed the use of IV nitrates and diuretics by 40 UK hospital physicians, each 'treating' the same 10 virtual patients. Semi-structured interviews were performed to investigate participants' decision-making rationale. IV nitrates were prescribed in 37% of clinical decisions. Considerable variability was seen in the administration of IV nitrates among physicians. Interview data revealed polarised opinions regarding the efficacy of IV nitrates and the evidence base supporting their use. Physicians' treatment decisions were more heavily influenced by their perceptions and beliefs regarding IV nitrates than by consideration of the evidence. In contrast, diuretics were prescribed more frequently (78%) and more consistently. In conclusion, where there is a limited evidence base and no strong recommendation from clinical guidelines, prescribing behaviour for AHF therapies such as IV nitrates are likely to reflect variability in physicians' beliefs, experiences, and decision-making styles.