Background and objective To test the hypothesis that magnesium sulphate reduces the amount of remifentanil needed for general anaesthesia in combination with propofol and mivacurium, we studied 50 patients undergoing elective pars plana vitrectomy in a double-blind, randomized prospective fashion.

Methods Magnesium sulphate (50 mg kg−1 body weight) or placebo (equal volume of NaCl) was given slowly intravenously after induction of anaesthesia with propofol 1–2 mg kg−1. Anaesthesia was maintained with propofol (using electroencephalographic control), mivacurium (according to train-of-four monitoring of neuromuscular blockade) and remifentanil (according to heart rate and arterial pressure).

Results We observed a significant reduction in remifentanil consumption from 0.14 to 0.09 μg kg−1> min–1 (P> < 0.01). Mivacurium consumption was also markedly reduced from 0.01 to 0.008 mg kg −1min−1 (P < 0.01), whereas propofol consumption remained unchanged. There was a trend towards lower postoperative pain scores, less pain medication requirements in 24 h after surgery and less postoperative nausea and vomiting in the magnesium group but not statistically significant. No side-effects were observed.

Conclusion We can recommend the use of magnesium sulphate as a safe and cost-effective supplement to a general anaesthetic regimen with propofol, remifentanil and mivacurium, although we cannot clearly distinguish between a mechanism as a (co)analgesic agent at the NMDA-receptor site or its properties as a sympatholytic. The effect of a single bolus dose of 50 mg kg−1 on induction lasts for about 2 h. For longer cases, either a continuous infusion or repeated bolus doses might be necessary.