BACKGROUND Several studies suggest that systemic magnesium reduces postoperative opioid consumption and the intensity of pain, but others report conflicting results. The efficacy and safety profile of intravenous magnesium in noncardiac surgery remain uncertain. OBJECTIVES The aim of this review was to investigate the effect of intravenous magnesium on the consumption of postoperative morphine in the first 24 h in adults undergoing noncardiac surgery. DESIGN Systematic review and meta-analysis with trial sequential analysis. DATA SOURCES MEDLINE, EMBASE, CENTRAL from their inception until January 2019. ELIGIBILITY CRITERIA All randomised clinical trials comparing intravenous magnesium versus placebo in noncardiac surgery were systematically searched in the databases. Observational studies, case reports, case series and nonsystematic reviews were excluded. RESULTS Fifty-one trials (n=3311) were included for quantitative meta-analysis. In comparison with placebo, postoperative morphine consumption at 24-h was significantly reduced in the magnesium group, with a mean difference [95% confidence interval (CI)] of −5.6 mg (−7.54 to −3.66, P < 0.001, I 2 = 92%, level of evidence low). The trial sequential analysis for the effect of magnesium on postoperative morphine consumption was conclusive. Patients who received magnesium had a longer time to the first analgesia request [143 (103 to 183) min, P < 0.001, I 2 = 99%, level of evidence low] and a lower incidence of shivering [0.26 (0.15 to 0.44), P < 0.001, I 2 = 35%, level of evidence very low]. However, no significance differences were demonstrated in postoperative pain scores in the first 24 h (mean difference, 95% CI) −0.30 (−0.69 to 0.09, P = 0.13, I 2 = 91%, level of evidence low), bradycardia (odds ratio, 95% CI) 1.13 (0.43 to 2.98, P = 0.80, I 2 = 35%, level of evidence very low) and postoperative nausea and vomiting (odds ratio, 95% CI) 0.90 (0.67 to 1.22, P = 0.49, I 2 = 25%, level of evidence moderate). CONCLUSION The current meta-analysis demonstrates that the use of intravenous magnesium as part of multimodal analgesia may reduce morphine consumption in the first 24 h after surgery and delay the time to the first request for analgesia in patients undergoing noncardiac surgery. However, the included studies were of low-quality with substantial heterogeneity. TRIAL REGISTRATION CRD42018086846.