Magnesium is one of the better-labelled things on a British supplement shelf, in the sense that the information you need is legally required to be present. It is also one of the most commonly misread, because the number people compare is not the number that differs.
The two numbers
The trap is comparing compound weights. A tablet stating a large milligram figure may be stating the weight of the salt; the elemental magnesium it delivers can be a fraction of that. UK labels must give the percentage of the nutrient reference value, which resolves the ambiguity — so the NRV percentage, not the headline milligrams, is the comparable figure.
What may lawfully be claimed
Magnesium has several authorised claims in the GB register, and they are worth knowing because they define the honest ceiling of the category’s marketing. They include contributions to normal muscle function, to the reduction of tiredness and fatigue, to normal psychological function, and to the maintenance of normal bones and teeth, among others.
Those are the claims. They are modest, specifically worded, and conditional on the product containing a significant amount relative to the 375 mg NRV. Anything a pack implies beyond them — sleep quality in particular, which is heavily marketed and is not among the authorised wordings — is not a claim the register supports.
Where form genuinely matters
Oxide carries a high proportion of elemental magnesium by weight, which makes it cheap per milligram delivered. It is also the form most associated with a laxative effect, which is the practical constraint on it.
Citrate is widely used, better tolerated by many people than oxide at comparable elemental doses, and moderately priced.
Bisglycinate and other chelated forms are marketed on tolerability and carry a lower elemental proportion, meaning a larger tablet or more tablets for the same delivered magnesium. They are the expensive end.
We would not tell a reader which to buy. What we would say is that the choice is a tolerability-versus-cost decision, that the NRV percentage tells you what you are actually getting in each case, and that a product not stating its form is withholding the variable that matters.
The caveats
Magnesium supplements can interact with certain medicines, including some antibiotics. Anyone with kidney problems should not be supplementing magnesium without medical advice, since impaired excretion changes the picture entirely. Gastrointestinal effects are the common complaint and are form- and dose-dependent.
Food supplements are regulated as food in Great Britain, not as medicines. They are not intended to diagnose, treat, cure or prevent any disease. This is not medical advice — a pharmacist is the right first call if you take other medication.
A worked comparison, with assumptions stated
To show why the panel matters more than the front of the box, an illustration. These are stated assumptions, not measurements of any product.
Suppose two products both print a large milligram figure. The first states a compound weight and gives an NRV percentage in the low tens; the second states a smaller compound weight and gives a higher NRV percentage. The second delivers more elemental magnesium per serving despite the smaller headline number, because the NRV percentage is calculated against elemental content and the headline was not.
The comparison is therefore always the same operation: find the percentage of the nutrient reference value, compare that, and then look at the salt form to decide whether the delivery suits you. Where a product prints only a compound weight and no percentage, it is not comparable to anything and we would leave it on the shelf on that basis alone.
Pros and cons
Verdict
Our position is that magnesium rewards a careful reader more than almost anything else on this shelf, because the framework hands you the tools: a required NRV percentage, a disclosed salt form, and a published list of what may lawfully be claimed. Use all three. Compare the percentage rather than the milligrams, choose the form on tolerability and price, and treat any pack promising something outside the authorised wordings as having told you about its marketing department rather than its contents.