This review examined whether taking magnesium supplements could be recommended for treating adults with high blood pressure from no known cause. It reviewed 12 trials enrolling 545 people, which compared magnesium supplementation with a dummy drug (placebo) or no treatment, and measured blood pressure 8 weeks to 6 months later. The results of trials varied a lot: some trials found magnesium lowered blood pressure much more than placebo, while others found little difference between magnesium and placebo. On average, people receiving extra magnesium achieved slightly lower diastolic blood pressure at the end of trials. None of the studies reported any serious side effects of taking magnesium supplements.
However, most included trials were of poor quality, so their results may not be reliable. The trials were not long enough or large enough to measure whether extra magnesium can reduce possible consequences of high blood pressure: death, heart attack or stroke.
The review did not find robust evidence that oral magnesium supplementation reduces high blood pressure in adults. Larger, longer duration, better quality trials are needed to clarify whether magnesium supplementation can lower high blood pressure. —Cochrane 2009