Aims: This audit aimed to assess the prevalence and completeness of documented medication handover regarding opioid substitution therapy (OST) during transitions of care in to and out of a major tertiary teaching hospital, focusing on surgical admissions. The secondary aim was to evaluate the proportion of handover episodes that involved a clinical pharmacist, and to determine the impact of pharmacist involvement on the completeness of documented OST handover. Method and Results: A 5-year retrospective audit was conducted for surgical patients undergoing OST prior to admission to a major tertiary teaching hospital. Data were collected pertaining to: handover on admission of 13 OST-related metrics deemed important for appropriate ongoing clinical care, whether medication handover to the community OST team was documented on discharge, and the involvement and impact of pharmacists in these processes. Sixty-one admissions were included in the audit. On average, just over half (7.4/13) of the predefined OST-related metrics audited, were documented on admission. Only 57% of patients had OST handover to the community team documented on discharge. Pharmacist involvement on discharge significantly increased completeness of OST handover documentation on admission and increased the proportion of patients with documented OST handover on discharge. Conclusion: Medication handover relating to OST on admission and discharge was frequently incomplete, posing a potential risk to safe and appropriate ongoing care, highlighting the need for education and procedures to guide this process. Pharmacist contribution in handover of OST medication-related information increased completeness of documentation, demonstrating their role in facilitating medication-related communication during transitions of care.