Background Surgery is one of the most common indications for a patient’s first opioid prescription, with some patients progressing to unintended long-term use. There is no current data from the United Kingdom on how much patients use of the opioid medication dispensed at discharge from hospital. This study investigates discharge opioid prescribing and usage following common surgical procedures. Methods This cohort study was conducted at the Oxford University Hospitals NHS Foundation Trust and involved 20 of the most commonly performed adult surgical procedures. At least 20 patients per procedure were surveyed using a standardised telephone questionnaire 6–8 days after discharge to establish the amount of used and unused opioids. Opioid doses were converted to oral morphine equivalent (OME) for analysis. Results The amount of opioid given to patients after all types of surgery far exceeded requirement, with often large variations in prescribing practices for the same procedures, most notably in trauma and orthopaedics. For the cohort of 426 patients, a total of 55 080 mg OME was dispensed on discharge, with only 34.4% actually used by patients, leaving a total of 36 108.5 mg OME unused in the community, risking inappropriate opioid use, overdose, or diversion. Conclusions Opioid overprescribing is common after surgery and represents waste, expense, and risk to patients. There is a clear need to develop a procedure-specific evidence-base for discharge opioid prescribing, adopting an “enough but not too much” approach to ensure that patients have adequate analgesia to facilitate functional surgical recovery, but not more than is needed.