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Magnesium is five different questions with five different answers

The same word covers a hospital drip that more than halved seizures in 10,141 pregnancies and a mood finding resting on 325 people. How far to trust it depends which claim you mean.

Magnesium

Magnesium gets argued about as though it were one claim. In 10,141 women with pre-eclampsia, a magnesium sulphate drip cut eclamptic seizures by more than half and lowered the mother's own risk of dying. As a daily capsule for leg cramps in pregnancy, it produced opposite answers in two trials at the same dose. Between those sit blood pressure, where the average barely moves, and depression, where a large effect rests on seven small trials. The molecule is the same; the evidence is not.

The findings behind this

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Sexual & reproductiveThe hospital magnesium drip more than halved seizures in pregnancy

Magnesium sulphate given by drip to 10,141 women with pre-eclampsia more than halved eclamptic seizures and cut the mother's own risk of dying. Magnesium capsules are not this drug.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.This is magnesium sulphate at monitored clinical doses through a vein; magnesium sold in capsules has never been shown to do anything for eclampsia risk. The trial ran across 33 countries with very different standards of care, so the background risk it was measured against varied from site to site. And the Cochrane review is built around this same trial, so the two are not independent of each other.Click to keep this openSupport a healthy pregnancyMagnesium
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PainA magnesium drip eased migraine fast; capsules did a different job

In 948 people, an infused dose eased migraine within 15 to 45 minutes and still helped a day later. In 789 others, a daily capsule cut how often migraines came and how hard they hit.

★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.The drip is given in a clinic under supervision, and nothing about that result transfers to a capsule taken at home. Some of the pooled studies assigned participants poorly, by the authors' own account. And each half of this rests on fewer than a thousand people.Click to keep this openRelieve painMagnesium
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Physical & bodyBlood pressure barely moved on average, more at 400mg a day

Averaged over 8,610 people, magnesium lowered the upper reading by 1.25 points. In the slice taking 400mg a day or more for at least 12 weeks it was 6.38, a figure found by looking back.

★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.The larger figures come from a slice of the data examined after the fact rather than the review's main comparison, which makes them the weaker number even though they are the more striking one. The ten pooled reviews overlap, so some of the same original trials are counted more than once. And blood pressure is a stand-in here: none of these trials counted heart attacks or strokes.Click to keep this openSupport heart and blood flowMagnesium
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Mood & wellbeingMagnesium beat placebo on depression scores, in seven small trials

Across seven randomised trials in 325 adults with depression, magnesium lowered depression scores substantially further than placebo. It is a large effect resting on a small pool.

★☆☆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.CautionSeven trials and 325 people is a thin base, and the authors ask for larger, better ones before anybody draws a clinical conclusion. An effect this big out of a pool this small is also the pattern produced when small trials that found nothing never reach publication. Magnesium was compared with placebo, never with a prescription antidepressant.Click to keep this openSupport moodMagnesium
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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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