Aims/hypothesis The aim of this work was to compare treatment intensification strategies based on orally administered vs injectable incretin-based antihyperglycaemic agents in patients with type 2 diabetes mellitus on metformin monotherapy. Methods In a 26 week, open-label study, 653 patients (baseline HbA 1c  = 8.2% 66 mmol/mol) were randomised at 111 sites in 21 countries in a 1:1 ratio to a strategy using oral agents (starting with sitagliptin 100 mg/day) or a strategy using the injectable drug liraglutide starting at a dose of 0.6 mg/day, up-titrated to 1.2 mg/day after 1 week. The following patients with type 2 diabetes mellitus were recruited for the study: those aged 18–79 years, on a stable dose of metformin monotherapy ≥1,500 mg/day for ≥12 weeks, with an HbA 1c ≥7.0% (53 mmol/mol) and ≤11.0% (97 mmol/mol) and a fasting fingerstick glucose (FFG) <15 mmol/l (<270 mg/dl) at the randomisation visit, deemed capable by the investigator of using a Victoza pen injection device (containing 6 mg/ml liraglutide; Novo Nordisk, Bagsværd, Denmark). Women taking part in the study agreed to remain abstinent or use an acceptable method of birth control during the study. Randomisation was performed via a computer-generated allocation schedule using an interactive voice response system. After 12 weeks, patients on sitagliptin with HbA 1c  ≥ 7.0% (53 mmol/mol) and fasting glucose >6.1 mmol/l had their treatment intensified with glimepiride; patients on liraglutide with HbA 1c  ≥ 7.0% (53 mmol/mol) had the dose up-titrated to 1.8 mg/day. The primary analysis assessed whether the strategy using oral drugs was non-inferior to that using an injectable drug regarding HbA 1c change from baseline at week 26 using a per-protocol (PP) population and a non-inferiority margin of 0.4%. Results In the PP population (522 patients included: oral strategy, n  = 269; injectable strategy, n  = 253) antihyperglycaemic therapy was intensified at week 12 in 50.2% and 28.5%, respectively. HbA 1c decreased over 26 weeks in both treatment strategy groups, with a larger initial reduction at week 12 in the injectable strategy group. The LS mean change in HbA 1c at week 26 was −1.3% (95% CI −1.4, −1.2) in the oral strategy group and −1.4% (95% CI −1.5, −1.3) in the injectable strategy group; the study met the non-inferiority criterion. Both treatment regimens were generally well tolerated; hypoglycaemia was reported more often with the oral strategy, while nausea, vomiting, diarrhoea and abdominal pain were reported more often with the injectable strategy. Conclusions/interpretation An oral, incretin-based treatment strategy with sitagliptin and, if needed, glimepiride may be a good approach in many patients with type 2 diabetes mellitus for managing inadequate glycaemic control on metformin monotherapy, as compared with an injectable treatment strategy with liraglutide. The oral and injectable strategies had similar effects on HbA 1c and had good overall tolerability. Trial registration ClinicalTrials.gov NCT01296412 Funding The study was sponsored by Merck Sharp & Dohme Corp., a subsidiary of Merck and Co., Inc., Whitehouse Station, NJ, USA.