Magnesium Dosage UK: How Much Per Day, and the 400mg Upper Level
Magnesium dosage, in three numbers.
Almost every dosage question about magnesium is answered by three figures — and most people confuse at least two of them. Here's what each actually means, and how to work out your own total across everything you take.
Magnesium dosage UK — reference summary
Three figures govern magnesium dosage in the United Kingdom, and all three refer to elemental magnesium rather than to the weight of the magnesium compound printed on a pack. First, the Nutrient Reference Value for magnesium is 375mg per day. This is a labelling reference used to express a serving as a percentage on a nutrition panel. It is not a personal target and not a recommendation to take 375mg from supplements. Second, the 15 percent threshold is 56.25mg of elemental magnesium per serving. Below this figure a product may not be described as a source of magnesium and may not carry any authorised magnesium health claim. Third, UK guidance sets a safe upper level of 400mg per day for supplemental magnesium. This covers magnesium taken from supplements only and is separate from and additional to magnesium obtained from food. Exceeding it commonly causes digestive upset and loose stools, because unabsorbed magnesium draws water into the bowel. Because magnesium is included in many multivitamins, mineral complexes, effervescent tablets and sports formulas, the practical task is to add the declared elemental magnesium across every supplement taken rather than assessing any one product in isolation. There is no authorised guidance on timing. Magnesium is commonly taken with food for tolerability, and larger amounts split across the day rather than taken at once. Magnesium can reduce the absorption of certain antibiotics including tetracyclines and quinolones, and can interact with bisphosphonates and other medicines. People with reduced kidney function should seek medical advice before supplementing, because the kidneys are the main route by which excess magnesium is cleared. Authorised GB and EU magnesium health claims cover contribution to a reduction of tiredness and fatigue, normal energy-yielding metabolism, normal functioning of the nervous system, normal muscle function, normal protein synthesis, normal psychological function, maintenance of normal bones and teeth, electrolyte balance, and a role in the process of cell division. Claims about sleep, muscle cramps, anxiety, migraine and restless legs are not authorised for magnesium in the United Kingdom.
375mg is not a target.
This is the single most common misreading in the category. People see "375mg NRV" on a panel and treat it as the amount they should be taking from a supplement. It isn't.
The Nutrient Reference Value is a labelling convention. Its job is to give every product a common denominator so a serving can be expressed as a percentage and compared like-for-like. It's the reason you can look at two very different magnesium products and see "55% NRV" on one and "18.7% NRV" on the other and immediately know which delivers more.
It is not a dose recommendation, and it doesn't account for the magnesium you already get from food — which for most people is a substantial share. Nuts, seeds, legumes, wholegrains and leafy greens all contribute.
The figure that functions as an actual ceiling is the third one: 400mg per day for supplemental magnesium. Note the word supplemental. It refers to what you take in capsules and powders, not to your total intake including food.
Not compound weight. A capsule labelled "Magnesium Glycinate 500mg" is not 500mg toward any of these figures — it's the ~70mg of elemental magnesium inside it that counts. If you're adding up against the 400mg upper level using the front-of-pack numbers, you'll wildly overestimate your intake and cut back for no reason. Read the panel. See magnesium forms explained for why the gap is so large.
Adding up what you're actually taking.
Magnesium is in far more products than people realise. It's a standard inclusion in multivitamins, mineral complexes, electrolyte and effervescent tablets, sports and recovery formulas, bone-health products and plenty of "greens" powders. None of them look like a magnesium supplement.
The result is that people who'd never take two magnesium products can still be taking magnesium from three sources without noticing. Here's the method:
| Step | What to do | Why |
|---|---|---|
| 1. List everything | Every supplement you take daily, not just the ones labelled magnesium | Multivitamins and electrolyte tablets are the usual hidden contributors |
| 2. Find the panel figure | The declared elemental magnesium in mg, with its %NRV | The front-of-pack number is compound weight and will mislead you |
| 3. Match the serving | Check whether that figure is per capsule or per stated serving | A "per serving" figure may cover two, three or four capsules |
| 4. Add the elemental mg | Sum the elemental figures, not the %NRVs | Percentages are easier to add but less transparent; keep the mg |
| 5. Compare to 400mg | Your supplemental total against the safe upper level | If you're approaching it, reduce one product rather than all of them |
Food magnesium doesn't count toward the 400mg supplemental upper level — it's a separate figure covering supplements only. You don't need to track your diet to use it.
What actually happens if you overdo it.
The usual consequence of too much supplemental magnesium is digestive, and it arrives fairly promptly: loose stools, diarrhoea, cramping or nausea. The mechanism is straightforward — magnesium that isn't absorbed stays in the bowel and draws water in with it.
This is also why forms differ in how well they're tolerated. A form that's poorly absorbed leaves more magnesium in the gut, and citrate in particular has a laxative effect at higher intakes — a property so reliable it's used deliberately in bowel preparations.
If you get digestive symptoms after starting a magnesium supplement, the usual fixes are taking it with food, splitting the dose across the day rather than taking it all at once, or reducing the amount. None of that requires abandoning magnesium.
The kidneys are the main route by which the body clears excess magnesium. If you have reduced kidney function or kidney disease, seek medical advice before taking any magnesium supplement — the ordinary safety margin that applies to most people does not apply in the same way. This is not a general caution; it is the one group for whom magnesium supplementation genuinely warrants a conversation with a doctor first.
Medication interactions
Magnesium can reduce the absorption of certain antibiotics — tetracyclines and quinolones are the commonly cited classes — and can interact with bisphosphonates and some other medicines. Separating doses by a few hours is often advised, but that's a decision for a pharmacist who knows what you're taking, not a general rule. If you take any prescribed medication, ask before you start.
Morning, evening, with food?
There is no authorised or required timing for magnesium, and we're not going to invent one. What people do in practice, and why:
| Practice | Rationale |
|---|---|
| With food | Commonly reported to improve tolerability, particularly with forms that are harsher on the stomach |
| Split across the day | A large single dose is more likely to cause digestive upset than the same amount divided |
| Away from certain medicines | Reduces the absorption interference described above — check with a pharmacist for your specific medication |
| "Take it at night for sleep" | Not an authorised claim. No sleep claim is authorised for magnesium in the UK, so we can't tell you to take it for that reason and no brand lawfully can |
Follow the directions printed on your pack and do not exceed the stated daily amount.
What magnesium is allowed to be sold on.
Magnesium is unusual among supplement ingredients in having a generous set of authorised claims. It's also one of the most heavily over-claimed. Knowing which is which tells you a lot about a brand.
| Claim | Status in Great Britain |
|---|---|
| Reduction of tiredness and fatigue | Authorised — requires ≥15% NRV per serving |
| Normal energy-yielding metabolism | Authorised — requires ≥15% NRV |
| Normal functioning of the nervous system | Authorised — requires ≥15% NRV |
| Normal muscle function | Authorised — requires ≥15% NRV |
| Normal protein synthesis | Authorised — requires ≥15% NRV |
| Normal psychological function | Authorised — requires ≥15% NRV |
| Maintenance of normal bones and teeth | Authorised — requires ≥15% NRV |
| Electrolyte balance | Authorised — requires ≥15% NRV |
| Role in the process of cell division | Authorised — requires ≥15% NRV |
| Sleep | Not authorised. The most common unauthorised magnesium claim in the UK |
| Muscle cramps | Not authorised. "Normal muscle function" is authorised; cramp prevention is not the same claim |
| Anxiety and stress | Not authorised. "Normal psychological function" is authorised; treating anxiety is not |
| Migraine and restless legs | Not authorised. These are disease claims — not permitted for any food supplement |
Notice how close some of the authorised and unauthorised pairs are. "Contributes to normal muscle function" is permitted; "prevents cramp" is not. "Contributes to normal psychological function" is permitted; "helps with anxiety" is not. The difference isn't pedantry — it's the line between describing a nutrient's role in normal physiology and promising to fix a problem. A brand that crosses it on magnesium is likely crossing it elsewhere too.
Now apply it to real products.
Our ranked guide compares the formulas in our own range on elemental magnesium and %NRV — the figures this page is about — with the trade-offs stated plainly.
Common questions.
Keep going.
- ReferenceMagnesium forms explained — elemental content of every form
- BuyingBest magnesium glycinate UK — our ranked pick
- BuyingBest magnesium supplement UK — the whole category
- CompareMagnesium glycinate vs citrate
- GuideWhat is magnesium? A complete guide
- ShopMagnesium supplements — the full UK range
- QualityQuality, testing & sourcing — UK manufacture and batch CoAs
- EditorialEditorial standards — how we research and review
Last reviewed 17 August 2026 by the Vitality Supplements Editorial Team, written to our published editorial standards and quality & testing policy. We are a UK supplement manufacturer and we sell magnesium, which we disclose openly. This page deliberately does not give a recommended dose by age, sex or goal — there is no authorised basis for that, and a table of personalised doses from a manufacturer would be advice we're not qualified or permitted to give. What we've done instead is set out the regulatory reference figures, show how to add up your own total, and mark clearly which magnesium claims are authorised and which are not, so you can assess any brand's copy against the same standard.
Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle. Do not exceed the stated recommended daily dose. UK guidance sets a safe upper level of 400mg per day for supplemental magnesium, separate from magnesium obtained from food. If you have reduced kidney function or kidney disease, seek medical advice before taking magnesium supplements. Magnesium can interact with certain antibiotics and other medicines — consult a pharmacist or doctor if you take prescribed medication. If you are pregnant, breastfeeding or under medical care, consult a healthcare professional before use. Authorised GB and EU health claims for magnesium require a serving providing at least 15% of the 375mg Nutrient Reference Value; claims relating to sleep, muscle cramps, anxiety, migraine or restless legs are not authorised and are not made here. This page is general information, not medical advice, and is not intended to diagnose, treat or prevent any condition. Every Vitality batch is independently tested. UK manufactured. Free UK delivery. 60-day satisfaction guarantee on first purchase. Privacy · Terms · info@vitality-supplements.co.uk