Magnesium supplements in the UK: what the guidance actually says
Magnesium is one of the most heavily marketed supplements in Britain and one of the least well understood. Here is the official UK intake, the upper limit for supplements, and what the authorised claims permit.
Published 13 August 2026
Magnesium has become one of the most marketed supplements in the UK, sold for everything from sleep to stress to cramp. Some of that is defensible. A good deal of it is not. Here is the official position.
How much you need
The recommended daily amount in the UK for adults aged 19 to 64 is:
- Men: 300mg a day
- Women: 270mg a day
The Department of Health and Social Care position is that most people should be able to get all the magnesium they need from a varied and balanced diet.
Where it comes from in food
Magnesium is in a lot of ordinary things - spinach and other green leafy vegetables, nuts, wholemeal bread, seeds, pulses and wholegrains.
Notice the overlap with the fibre list. The foods that get most people to 30g of fibre are largely the same foods that carry magnesium, which is a useful argument for fixing the diet before reaching for a tub. We wrote about that in how much fibre a day.
The upper limit for supplements
This is the part that gets skipped in most articles.
NHS guidance is that 400mg or less a day of magnesium from supplements is unlikely to cause any harm. Above that, in the short term, the common effect is diarrhoea. The long-term effects of consistently high doses are not well established.
Worth checking the label, because high-strength products well above that figure are widely sold, and it is easy to stack doses without meaning to if you take more than one product.
What magnesium is authorised to claim
In Great Britain, only claims on the GB Nutrition and Health Claims Register may be used, in wording that does not strengthen them. For magnesium those include that it:
- contributes to normal muscle function
- contributes to a reduction of tiredness and fatigue
- contributes to normal protein synthesis
- contributes to normal functioning of the nervous system
Read those carefully against how magnesium is usually advertised. “Contributes to a reduction of tiredness and fatigue” is not the same statement as “boosts your energy”, and “contributes to normal muscle function” is not a claim about cramp. The register wording is narrow on purpose.
You will also notice what is not on that list. Sleep is the single most common thing magnesium is sold for in the UK, and there is no authorised claim for it. That does not prove anything either way about the underlying science - it means the evidence has not met the bar required to say it on a label, and a brand saying it anyway is telling you something about the brand.
Forms
The differences between forms are real but smaller than the marketing suggests. Magnesium citrate and glycinate are generally better absorbed than magnesium oxide, which is the cheapest and most common form in low-cost products. If you are choosing between them, that is the distinction worth knowing.
The honest summary
Most people in the UK can reach the recommended intake through food, and the food that gets you there is doing several other jobs at the same time. If you do supplement, stay at or under 400mg, check what else you are taking, and judge the brand by whether it was willing to use the flat authorised wording where the rules require it.
Sources
NHS, Vitamins and minerals - Others. Recommended intakes of 300mg for men and 270mg for women aged 19 to 64, and the 400mg guidance for supplements.
General information, not medical advice. If you have a health condition, particularly affecting the kidneys, or take prescription medication, speak to your GP or pharmacist before taking magnesium.
