Magnesium Benefits: It Lowers Blood Pressure. It Does Not Fix Your Cramps.

Magnesium is one of the few supplements where something real survives the trials. It is also one where the two things it is most often bought for — sleep and leg cramps — are the weakest claims on the shelf.

What it actually does: blood pressure

This is the finding that holds up, and it holds up unusually well for a mineral supplement.

A 2016 meta-analysis pooled 34 randomised, double-blind, placebo-controlled trials in 2,028 adults — both people with high blood pressure and people without. At a median dose of 368 mg a day for a median of three months, magnesium lowered systolic blood pressure by 2.00 mmHg and diastolic by 1.78 mmHg. [1]

Two things make that more convincing than a typical supplement result. The blood-pressure drop was accompanied by an actual rise in blood magnesium — so the thing you swallowed demonstrably got in, and the outcome moved with it. And the authors were willing to write the sentence most meta-analyses avoid: “Our findings indicate a causal effect of Mg supplementation on lowering BPs in adults.” [1]

They also found that around 300 mg a day, or a single month of taking it, was enough to shift both serum magnesium and blood pressure.

Is two points worth anything?

It sounds like nothing. It is small, and it is not nothing.

Two millimetres of mercury will not move you out of a diagnosis or replace a medication. What a couple of points does do is matter at the scale of a population — blood pressure risk is continuous, so shifting a whole distribution down slightly prevents more events than the size of the shift suggests it should.

For one person deciding whether to buy a bottle, the honest framing is: this is a real effect, roughly the size you would get from a modest change in salt intake, and it is not a treatment. The authors themselves note that “residual heterogeneity may still exist” — the trials did not all agree perfectly — which is part of why we rate this Supported rather than Established.

Leg cramps: this one is finished

Magnesium has been sold for cramp for decades. Cochrane went and looked, across 11 trials and 735 people. [2]

For idiopathic cramps — the ordinary night-time kind, in adults averaging around 62 to 69 years old — nothing moved. Cramps per week differed by 0.18, with a confidence interval running from −0.84 to +0.49. Cramp intensity: no difference. Cramp duration: no difference.

And one endpoint deserves singling out. The proportion of people who got a 25% or better reduction in their cramp rate came out at a risk ratio of 1.04, confidence interval 0.84 to 1.29 — and Cochrane graded that high-certainty evidence. [2]

High certainty is the strongest grade they issue. It does not mean nobody should ever look again. It means that further trials of this kind are unlikely to change the answer, and the answer is no.

The trials also agreed with each other almost completely — heterogeneity of 0% to 12% — which is what a real null looks like as opposed to a muddle.

One genuine gap: the five trials in pregnancy-related leg cramps could not be pooled, and all five were judged at high risk of bias. So the pregnancy question is unanswered rather than answered negatively.

Sleep: three trials and 151 people

Thirty-three thousand people a month search whether magnesium helps them sleep. Here is the entire evidence base for older adults with insomnia, as of the most recent systematic review: three randomised trials, 151 participants, three countries. [3]

Pooled, people fell asleep 17.4 minutes faster than on placebo, which is a real difference if you are lying there. Total sleep time improved by 16 minutes — and that one was not statistically significant.

The reviewers were blunt about what they were working with. Every trial was at moderate-to-high risk of bias, the outcomes were supported by “low to very low quality of evidence”, and their own summary is that “the quality of literature is substandard for physicians to make well-informed recommendations.” [3]

They still ended up mildly in favour, and their reason is worth repeating because it is honest rather than scientific: magnesium is very cheap and widely available, so the evidence “may support” trying it. [3] That is a cost-benefit argument, not a demonstration that it works.

If you are taking magnesium for sleep and it seems to help, nothing here says stop. What it says is that you are ahead of the evidence rather than behind it.

Which form? Less settled than the marketing suggests

Twenty-two thousand searches a month go to the question of which magnesium to buy, and the honest answer is that the comparison people want has not been done well.

The most-cited study measured four commercial preparations by how much extra magnesium turned up in urine. Magnesium oxide came out at 4% fractional absorption. Magnesium chloride, lactate and aspartate were all significantly higher, and equivalent to each other. [4]

So the common advice to avoid oxide has something behind it. But notice what that study did not test: glycinate and citrate — the two forms most people are actually choosing between. It cannot rank them, and neither can we.

The paper’s own conclusion cuts against the usual framing too: “Inorganic magnesium salts, depending on the preparation, may have bioavailability equivalent to organic magnesium salts.” [4] The organic-versus-inorganic story is not the clean divide it gets sold as.

On timing — another popular query — the trials on this page did not test it. Anyone telling you there is a correct hour is going beyond the evidence.

One more, briefly

In people with type 2 diabetes, nine double-blind trials covering 370 patients found fasting glucose lower on magnesium and HDL cholesterol slightly higher — but HbA1c, the three-month average of blood sugar, did not significantly change. [5] Interestingly, blood pressure did not move in that population either, though that is a 2006 analysis of nine trials against the 34 in the newer general-population one.

The verdict

Magnesium lowers blood pressure. That is Supported — 34 double-blind trials, a dose-response, a matching rise in blood magnesium, and authors willing to use the word causal. The effect is about two points, which is small and real.

It does not fix leg cramps, and that question is as close to closed as this field gets. The sleep evidence is three trials and 151 people, graded low to very low, and the reviewers said so themselves. The forms question has a partial answer that does not cover the two forms you are probably choosing between.

Which makes magnesium an unusual entry in this corpus: a supplement that does something measurable, sold mostly for two things it does not do.

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