OBJECTIVES: Dapagliflozin (Forxiga®) is the first sodium-glucose co-transporter-2 (SGLT-2) inhibitor approved by the European Medicines Association, and positively assessed by the National Institute for Health and Care Excellence (NICE) for type 2 diabetes mellitus. This study assesses the costs-effectiveness of dapagliflozin in combination with insulin versus insulin alone for patients who are inadequately controlled despite high doses of insulin. METHODS: The published and validated CARDIFF diabetes model was used to conduct the analysis. Clinical inputs were derived from a randomized clinical trial comparing dapagliflozin add‑on to insulin with insulin regimens. Based on clinical inputs and the United Kingdom Prospective Diabetes Study (UKPDS) equations, the model predicts disease progression and the number of micro‑ and macro-vascular complications, along with diabetes-specific and all‑cause mortality. The perspective of the National Health Service in England and Wales was adopted over a lifetime horizon. Local unit costs and utility data were assigned to the appropriate model parameters to calculate total Quality‑Adjusted‑Life-Years (QALYs) and total costs. Univariate and probabilistic sensitivity analyses (PSA) were conducted. RESULTS: Compared to insulin, dapagliflozin added to insulin was associated with 0.342 incremental QALYs (95%CI: 0.288; 0.480) at an additional cost of £1,813 (95%CI: £1,165; £2,381), resulting in an incremental cost-effectiveness ratio (ICER) point estimate of £5,295 per QALY gained. The univariate analyses showed that no input parameter change inflated the ICER above £15,000 per QALY. At a willingness-to-pay threshold of £20,000 per QALY gained, the dapagliflozin treatment strategy was estimated to have a 100% probability of being cost-effective when compared to the insulin treatment strategy. These findings were shown to be robust with all sensitivity analyses. CONCLUSIONS: Dapagliflozin was shown to be a cost‑effective treatment option in combination with insulin for patients who are inadequately controlled with insulin alone within established UK cost‑effectiveness thresholds.