Abstract Aims To assess the cost‐effectiveness of dapagliflozin, a sodium‐glucose co‐transporter‐2 (SGLT2) inhibitor, as an adjunct to insulin in adults with type 1 diabetes mellitus (T1DM) inadequately controlled by insulin alone in the UK setting. Methods A cost–utility analysis was conducted to compare dapagliflozin (5 mg or 10 mg) added to insulin versus insulin monotherapy (standard of care) over a lifetime horizon.