Aim Recording the indication for a medicine in the prescription supports communication. In May 2023 our district hospitals made the free-text indication field in prescriptions compulsory for all medicines in the inpatient prescribing system. This study aimed to evaluate the effect of introducing a compulsory indication field in an inpatient prescribing system. Methods Text in the indication field was manually classified as an indication, ‘other text’, ‘rubbish text’, ‘to be determined’, and ‘blank’. Prescribing data were extracted from the district data warehouse. The change in proportion of prescriptions with an indication was compared for eight-weeks after introduction of a compulsory indication field to an equivalent eight-weeks in 2022. Secondary outcomes included patient outcomes, medication cessation, and indication recording before discharge. Results We analysed 81,634 prescriptions before and 82,726 after indications were made compulsory. The proportion of prescriptions with an indication increased from 29.2% to 75.6%. ‘Rubbish text’ increased from 0% to 2.3%, ‘other text’ from 2.5% to 14.7%, and ‘to be determined’ from 0.0% to 6.6%. Of 5,557 prescriptions with ‘to be determined’ initially, 18.1% were ceased and 2.7% had an indication before discharge. There was minimal change in patient outcomes. Conclusion Introducing a compulsory indication field increased the proportion of medicines with an indication from 29% to 76%, with only a small increase in ‘rubbish text’. The carefully selected compulsory field improved information quality but did not alter medicine use or patient outcomes. Compulsory fields should be combined with improvements in other components of care to improve medicine use.