Abstract Introduction Most English hospitals provide out-of-hours pharmacy support via an ‘on-call pharmacist’.[1] There is scant published literature characterising these services. Prior research[2] suggests a significant number of calls may be inappropriate requests for non-urgent medication or information readily available elsewhere. Delaying while the on-call pharmacist is unnecessarily contacted may avoidably compromise patient care. Handling inappropriate calls may decrease pharmacist job satisfaction and compromise rest between shifts. Dissatisfaction with the on-call commitment is regularly raised at local departmental meetings and exit interviews. Servicing such calls is not sustainable. Aim To explore and quantify appropriateness of calls to the on-call pharmacy service at a single, large, acute English hospital, and to explore the influencing factors behind any inappropriate calls using a theoretically informed approach. Methods A retrospective review of existing on-call records quantified and characterised service use in 2021. Missed calls, callers who did not want the on-call pharmacist and internal pharmacy handovers were excluded. As per previous work, calls were considered inappropriate where an available resource would have provided a complete solution/answer at the caller’s level.[2] Service users were invited to semi-structured interviews via email to explore their capability, opportunity, and motivation to handle pharmaceutical issues. Interviews were conducted and recorded on Microsoft Teams before being transcribed and analysed using a framework based on the COM-B model of behaviour. Quantitative data were analysed in Excel and SPSS v27. Pearson’s chi-square test of independence was calculated for several pre-defined variables with p<0.05 considered significant Results There were 1139 calls in 2021, with 32 meeting exclusion criteria. Of the 1107 remaining, 410 (37%) were inappropriate and 697 (63%) appropriate. Eighty-one (20%) inappropriate calls occurred overnight (22:00-08:30). Almost 90 hours were spent on inappropriate calls, generating £3,144.96 in additional overtime payments to pharmacists. Appropriateness was found to be significantly higher on weekdays (66%) versus weekends (58%), p=0.008, from surgical wards (67%) versus medical wards (58%), p = 0.042 and from doctors (74%) versus nurses (54%), p<0.001. All respondents (five doctors) were interviewed. Analysis found limited training and familiarisation led to low awareness of available resources. Access was often restricted by a cumbersome Trust intranet rather than lack of time. Participants were highly motivated to resolve issues themselves, believing this to be best practice. Conclusion The high proportion, high volume and high cost of inappropriate calls warrants further investigation and intervention. Unfortunately, no nurses responded to interview invitations; this is a significant limitation as nurses were more likely to place inappropriate calls. Future work should consider how nurses can be recruited more effectively. However, analysis of calls combined with elucidation of doctors’ behavioural determinants makes it possible to propose informed interventions. Doctors would benefit from improved awareness of/ability to use resources and improved access to them. Therefore, a searchable electronic “signpost” highlighting resources relevant to common on-call queries is proposed. This would leverage the doctors’ high motivation and the rich data collected in a sustainable, evidence-based intervention designed around service user needs.