Magnesium: What It Actually Does, and Who Needs It

Magnesium is currently sold for sleep, anxiety, muscle cramps, migraine, restless legs, constipation, blood pressure and general wellbeing. Some of that has evidence behind it. Much of it does not.

Here is what a GP would actually say.

What magnesium does

Magnesium is a mineral involved in several hundred enzyme reactions — energy production, muscle and nerve function, blood glucose control, blood pressure regulation and bone structure. It is genuinely essential.

Most of it is stored in bone and inside cells, which is why a blood test is a poor measure of body stores. A normal serum magnesium does not exclude depletion, and this is worth knowing before paying for one.

Is deficiency common?

Frank deficiency is uncommon in healthy people eating a varied diet. Marginal intakes are more common — UK dietary surveys suggest a proportion of adults, particularly teenagers and younger women, fall below the recommended intake.

The groups where deficiency is genuinely likely are specific:

• People taking proton pump inhibitors long term — omeprazole, lansoprazole. This is a recognised and under-checked effect, and it can be significant after a year or more of use
• People on loop or thiazide diuretics
• Poorly controlled type 2 diabetes, where magnesium is lost in the urine
• Chronic alcohol use — one of the commonest causes
• Malabsorption — coeliac disease, Crohn's, after bowel surgery

If you are in one of those groups and have cramps, palpitations or fatigue, magnesium is worth checking. If you are not, it is much less likely to be the answer.

Where the evidence is reasonable

Constipation

Magnesium salts work as osmotic laxatives, and this is not controversial — it is a recognised pharmacological effect rather than a nutritional one.

Migraine prevention

There is reasonable evidence for magnesium in migraine prophylaxis, and it appears in several international headache guidelines as an option. It is not first-line in UK practice, but it is a legitimate thing to try.

Eclampsia and pre-eclampsia

Intravenous magnesium is genuinely life-saving here — though that is hospital treatment and has nothing to do with supplements.

Documented deficiency

Obviously. Replacing a genuine deficiency resolves the symptoms it was causing.

Where the evidence is weak

Sleep

This is the biggest selling point and the weakest evidence. Trials are small, short, often in older adults with existing deficiency, and inconsistent. There is no good evidence that magnesium improves sleep in people who are not deficient.

For persistent insomnia, CBT for insomnia is first-line, outperforms medication long term, and is free on the NHS.

Anxiety and low mood

Some small studies, no convincing evidence in people with normal magnesium levels.

Muscle cramps

Systematic reviews have repeatedly found magnesium no better than placebo for cramps in older adults. It may help cramps in pregnancy, where the evidence is slightly more favourable. For most people with night cramps, it does not work — and this is one of the most common reasons people buy it.

Restless legs

Frequently recommended, poorly evidenced. The thing genuinely worth checking in restless legs is ferritin, where the treatment threshold is much higher than for ordinary anaemia — iron replacement helps a great many people who were sold magnesium instead.

Food first, and it is not difficult

Magnesium is abundant in ordinary food: nuts and seeds — particularly pumpkin seeds and almonds — wholegrains, green leafy vegetables, beans, lentils and dark chocolate.

A handful of nuts and a portion of greens most days covers a substantial share of the requirement.

If you do supplement

• Form matters for tolerability. Magnesium oxide is cheap and poorly absorbed, and is the most likely to cause diarrhoea. Citrate, glycinate and malate are better absorbed and gentler
• Diarrhoea is the usual dose-limiting effect, and it is a reasonable signal to reduce
• Care in kidney impairment. If your kidneys do not clear magnesium normally, supplements can accumulate to dangerous levels — do not take magnesium supplements in significant chronic kidney disease without advice
• Magnesium reduces the absorption of some antibiotics and levothyroxine — separate them by a few hours

The honest summary

Magnesium is essential, deficiency is real, and there are specific groups — particularly long-term PPI users — where it genuinely warrants checking.

For a healthy person eating reasonably, a magnesium supplement is unlikely to do much, and the sleep and cramp claims that sell most of it are not well supported.

If you are taking it and it helps, it is cheap and safe in normal kidney function — there is no need to stop. But if you are taking it in place of investigating persistent symptoms, that is worth a conversation instead.