Toxicity was uncommon with magnesium sulphate. There was no clear difference in the risk of absent or reduced tendon reflexes. Although respiratory depression was rare (52/5344 versus 26/5333), the risk was higher for women allocated magnesium sulphate (RR 1.98, 95% CI 1.24–3.15; NNH 206, 95% CI 100–1000). A quarter of women who received magnesium sulphate had side-effects, compared to 5% of those given placebo. By far the most common side-effect was flushing.