Everyone recommends it now. The sleep people, the anxiety people, the woman at the counter who was very confident about which one. You tried it for a fortnight, noticed nothing in particular, and stopped.

So here is the whole picture, including the part that might matter for you and almost never gets mentioned.

Start with the fair part. Magnesium is an essential mineral, involved in hundreds of enzyme reactions, and running low on it is a recognised clinical problem with a name, hypomagnesaemia. This is not homeopathy.

Where the evidence is decent, it is narrow. In hospital, given intravenously, it is standard care in some obstetric emergencies. Magnesium salts work as an osmotic laxative, which is why some forms loosen the bowels whether you wanted that or not. For migraine prevention, several clinical guidelines list it as an option worth trying. Those are the strong points, and they are narrow.

Where the evidence is thin is where the marketing is loudest. The sleep and anxiety claims that sell most of the tubs rest on a handful of modest trials. The results are mixed, and a lot is extrapolated. It may help some people. It has not been shown to do what the packaging suggests.

Now the part worth the price of the issue.

Several common medications lower magnesium. Long term use of proton pump inhibitors, the reflux drugs ending in prazole, is a recognised cause. So are some diuretics. Chronic diarrhoea, coeliac disease and inflammatory bowel disease do it too. If you have been on a stomach drug for years, that is a conversation to have, not a supplement to buy on a hunch.

There is a complication in testing it. Most of your magnesium sits inside cells and in bone. The routine blood test measures serum, which can look normal when stores are low. That is a known limitation rather than a conspiracy, and a normal result does not settle the question by itself.

Three cautions. If your kidneys work below capacity, supplements need medical advice, because the body clears magnesium through them. Some supplements interfere with antibiotics and thyroid medication, a timing matter your pharmacist can sort out in a minute. And nothing here is a reason to change anything you have been prescribed.

So the useful question is not the one the internet is answering:

Today’s Practice

A pharmacist can answer most of this for free, without an appointment. Ten minutes, and it is the most underused room in healthcare.

On a hard day, photograph the boxes and take the photograph instead. And if you have muscle cramps alongside palpitations, weakness or numbness, or diarrhoea that will not settle, that is a doctor rather than a supplement aisle. Those combinations get looked at properly.

What the Research Says

The sleep claim deserves its own look, because it is why most people buy it. The trials are small, often run in older adults with sleep complaints, and they measure different things, which makes them hard to pool. Reviews land on the same verdict: low certainty evidence, modest effects where they appear. That is not the same as useless, and it is a long way from what the label promises. If you try it and sleep better, that is a legitimate result, and also the sort a fortnight of attention to your evenings can produce on its own.

From the Old World

The name on the bathroom shelf is a piece of English geography. In the seventeenth century, in Epsom in Surrey, a farmer found his cattle refused to drink from one spring. The water was bitter, and it was soon sold as a purgative, because what was dissolved in it was magnesium sulfate. Epsom became a spa town on the strength of it, and the salts still carry the name. Whether a bath delivers any of it through your skin is another matter, and the honest answer is probably not. A hot bath is pleasant on its own terms.

Come Home

Five lines, and none of them require you to buy anything.

One act for today, if you want one. Put the question to a pharmacist next time you collect a prescription, with your full list in hand.

The mineral is doing something. Most of what it is being sold for is not that.

P.S. Tomorrow, the solar term we are standing in now, the one that cuts the year in half, and what it asks of a body that runs cold.