Magnesium is one of the most popular pharmacy supplement categories, marketed for everything from night cramps to migraine prevention to sleep quality to "energy". The evidence base is considerably narrower than the shelf suggests: magnesium has well-characterised clinical uses in specific deficiency and laxative contexts, a modest evidence base in migraine prophylaxis, a weaker one in nocturnal leg cramps, and limited or no evidence for most other marketed indications.
This article summarises what UK guidance supports, how the different magnesium salts differ, and where the counter conversation should land.
Magnesium in the body
Magnesium is an essential cofactor for hundreds of enzymatic reactions, including ATP metabolism, neuromuscular function and bone mineralisation. Adult reference intake in UK dietary guidance is 270 mg/day for women and 300 mg/day for men. Most adults on a mixed diet meet the intake — nuts, wholegrains, leafy greens, legumes and dairy are good sources — and clinical magnesium deficiency is uncommon in the general population.
Deficiency is more likely in specific contexts: chronic alcohol use, prolonged diarrhoea, prolonged proton pump inhibitor (PPI) use, some diuretic therapy, malabsorption syndromes and certain genetic disorders.
The different magnesium salts
Most of what distinguishes supplement products is the salt form rather than the magnesium itself. Once absorbed, magnesium is magnesium — the differences are in solubility, bioavailability, osmotic effect in the gut, and whether the co-anion (oxide, citrate, glycinate, malate, hydroxide, sulphate) has its own actions.
| Form | Typical use | Notes |
|---|---|---|
| Magnesium oxide | Low-cost supplement, osmotic laxative | Lower bioavailability; pronounced laxative effect at higher doses |
| Magnesium citrate | Supplement, osmotic laxative | Better bioavailability than oxide; laxative at higher doses |
| Magnesium hydroxide | Antacid, osmotic laxative | Short-acting; in products like milk of magnesia |
| Magnesium sulphate | IV use in hospital settings (eclampsia, arrhythmia); oral laxative (Epsom salts) | Not typically used as a routine supplement |
| Magnesium glycinate / bisglycinate | Supplement | Better tolerated at higher doses; claimed improved absorption |
| Magnesium malate | Supplement | Often marketed for fatigue; limited comparative evidence |
| Magnesium L-threonate | Supplement | Marketed for cognition; limited human evidence |
The comparative bioavailability differences between well-formulated organic magnesium salts and magnesium oxide are real but often smaller than marketing suggests. The main practical difference for most people is gastrointestinal tolerability at higher doses: citrate and glycinate tend to be better tolerated than oxide.
Where the evidence actually supports use
Osmotic laxative
Magnesium hydroxide and magnesium citrate are effective osmotic laxatives and are listed in the BNF for constipation. At laxative doses they draw water into the bowel, softening stool and stimulating evacuation. NICE CKS on constipation positions osmotic laxatives as second-line after bulk-forming or stimulant approaches, depending on clinical context. This is a medicinal use, not a supplement use.
Documented deficiency
Where deficiency is clinically confirmed (blood tests, clinical context) and attributable to a reversible cause, supplementation under clinical supervision is appropriate. Long-term high-dose PPI users with documented hypomagnesaemia are the commonest counter example.
Pre-eclampsia and eclampsia (hospital only)
Magnesium sulphate IV is the first-line treatment for seizure prophylaxis in severe pre-eclampsia and for treating eclampsia. This is specialist care and not relevant at the pharmacy counter beyond awareness.
Migraine prophylaxis (modest evidence)
The American Academy of Neurology and some specialist UK migraine clinicians consider magnesium (typically around 400–600 mg/day of elemental magnesium) as an option for migraine prophylaxis, supported by trial evidence of modest magnitude. NICE CKS on migraine does not list magnesium as first-line pharmacological prophylaxis but acknowledges it among the lifestyle and complementary options some patients find helpful. Gastrointestinal tolerability becomes the practical limit at higher doses.
Where the evidence is weak
Nocturnal leg cramps
Widely recommended but poorly supported. A Cochrane review of magnesium for skeletal muscle cramps concluded that magnesium is unlikely to provide meaningful benefit to older adults experiencing cramps. NICE CKS on leg cramps lists non-pharmacological measures first and reserves quinine sulphate (prescription-only, specialist-supervised) for refractory cases. Counter advice should be honest about the evidence limitation.
Sleep and anxiety
Widely marketed, with a mix of small trials and observational data. Evidence for routine supplementation in adults without magnesium deficiency remains limited. It is reasonable to trial if the patient is informed about the uncertainty.
"Energy" and general wellness
Not supported by high-quality evidence outside of deficiency states.
Athletic performance
Limited evidence for routine supplementation in non-deficient athletes.
Safety and interactions
Oral magnesium is generally well tolerated. Main points:
- Renal impairment: magnesium is renally excreted. Patients with significant chronic kidney disease can accumulate magnesium and develop hypermagnesaemia with high-dose supplementation. Magnesium-containing products should be used cautiously or avoided in advanced CKD without clinical advice.
- Diarrhoea: the commonest side effect, dose-related, more pronounced with oxide and sulphate.
- Interactions: magnesium reduces the absorption of oral bisphosphonates, tetracyclines (doxycycline, lymecycline) and quinolones (ciprofloxacin, levofloxacin) — these should be separated by several hours.
- Upper limit: the European Food Safety Authority has set a tolerable upper intake level for supplemental magnesium of 250 mg/day for adults, excluding magnesium from food and water. Supplements commonly exceed this; products labelled for laxative use operate at higher doses.
What good counter advice looks like
For a patient asking about magnesium for a symptom:
- Ask what they're trying to fix. Constipation, cramps, migraines, sleep, fatigue — these have different evidence bases.
- Be honest about the evidence. Osmotic laxative use — yes, clear effect. Migraine — modest evidence for some patients. Leg cramps — probably not. Sleep — uncertain.
- Check renal function, medication list (especially PPIs, diuretics, bisphosphonates, antibiotics) and any history of GI disease.
- Suggest a reasonable form. For general supplementation, citrate or glycinate is well tolerated. For laxative effect, hydroxide or citrate.
- Stick to licensed products from reputable manufacturers.
- Refer for investigation when symptoms are persistent, severe or suggest a diagnosis that supplementation won't solve.
Caveats
This article summarises BNF, NICE CKS, NHS patient information and major international review guidance current to April 2026. Individual patient decisions should consider comorbidities and concomitant medications.
Sources
- British National Formulary — Magnesium salts
- NICE Clinical Knowledge Summaries — Migraine; Leg cramps; Constipation
- NHS — Vitamins and minerals: others (including magnesium)
- EFSA — Tolerable upper intake level for magnesium
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