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Aims: The present study tested the hypothesis that metformin treatment may increase myocardial efficiency (stroke work/myocardial oxygen consumption) in insulin‐resistant patients with heart failure and reduced ejection fraction (HFrEF) without diabetes. Methods and results: Thirty‐six HFrEF patients (ejection fraction 37 ± 8%; median age 66 years) were randomised to metformin (n = 19) or placebo (n = 17) for 3 months in addition to standard heart failure therapy. The primary endpoint was change in myocardial efficiency expressed as the work metabolic index (WMI), assessed by 11C‐acetate positron emission tomography and transthoracic echocardiography. Compared with placebo, metformin treatment (1450 ± 550 mg/day) increased WMI [absolute mean difference, 1.0 mmHg·mL·m‐2·106; 95% confidence interval (CI) 0.1 to 1.8; P = 0.03], equivalent to a 20% relative efficiency increase. Patients with above‐median plasma metformin levels displayed greater WMI increase (25% vs. –4%; P = 0.02). Metformin reduced myocardial oxygen consumption (–1.6 mL O2·100 g‐1·min‐1; P = 0.014). Cardiac stroke work was preserved (–2 J; 95% CI –11 to 7; P = 0.69). Metformin reduced body weight (–2.2 kg; 95% CI –3.6 to –0.8; P = 0.003) and glycated haemoglobin levels (–0.2%; 95% CI –0.3 to 0.0; P = 0.02). Changes in resting and exercise ejection fraction, global longitudinal strain, and exercise capacity did not differ between groups. Conclusion: Metformin treatment in non‐diabetic HFrEF patients improved myocardial efficiency by reducing myocardial oxygen consumption. Measurement of circulating metformin levels differentiated responders from non‐responders. These energy‐sparing effects of metformin encourage further large‐scale investigations in heart failure patients without diabetes. [ABSTRACT FROM AUTHOR]