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Myth check

Nobody has settled whether magnesium helps pregnancy leg cramps

Across four trials in 332 pregnant women, magnesium changed neither how often cramps came nor how fast they passed. Eight other trials disagreed too sharply to be pooled at all.

Magnesium

Magnesium is close to standard advice for leg cramps in pregnancy. Across four trials in 332 women, it changed neither how often the cramps came nor how fast they passed. Yet in one trial at 300mg a day, 86% of women halved their cramps against 60.5% on placebo, and another at the same dose found no gap, using a different magnesium compound and counting cramps a different way. The honest answer is not that it is a myth; it is that eight trials could not be added together.

The findings behind this

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PainPregnancy leg cramps did not respond to magnesium in 332 women

Pooling four trials in 332 pregnant women, oral magnesium neither reduced how often leg cramps struck nor sped up recovery from them, against placebo.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.CautionFour trials and 332 women leave enough room that a small real benefit could still be hiding inside the result. Cramps were defined and counted differently from trial to trial, which blunts any pooled figure. And this is cramps in pregnancy, not the ordinary night-time leg cramps most people mean.Click to keep this openRelieve painMagnesium
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PainSame dose, opposite answers on cramps in pregnancy

At 300mg a day, one trial had 86% of pregnant women halve their cramps against 60.5% on placebo; another at the same dose found no gap at all. Eight trials could not be pooled.

★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.CautionThe two trials used different magnesium compounds, so the split may be about which form sat in the capsule rather than about magnesium. Both groups improving by a third in the null trial points at expectation and the natural course of cramps doing much of the work. In the oldest positive trial, blood magnesium never actually rose, which makes it hard to credit magnesium with the improvement. And the trials counted cramps so differently that no single number can be drawn across them.Click to keep this openRelieve painMagnesium

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