Magnesium: what it is used for and what evidence there is: Cramps, sleep, stress, migraine, constipation: each use with its level of evidence

What it is

Magnesium is one of the most searched supplements, and almost everything said about it mixes three different levels: what has an authorised health claim in the EU, what has been studied with mixed results, and what is only a popular idea.

Authorised claims (EFSA): it contributes to the reduction of tiredness and fatigue, to electrolyte balance, to normal energy-yielding metabolism, to normal functioning of the nervous system and muscles, to normal psychological function, to the maintenance of normal bones and teeth, to protein synthesis and to cell division. They apply from 56.25 mg per dose and do not mention cramps, sleep or anxiety.

This pill orders the uses by level of proof so you can answer honestly when asked "does it work for…?".

How to sort what you hear

Four levels, from most solid to least:

WITH AN AUTHORISED CLAIM: tiredness and fatigue, muscle and nerve function, bones, electrolyte balance.
STUDIED WITH PROMISING BUT LIMITED RESULTS: migraine (neurology guidelines mention it as an option), occasional constipation (osmotic effect of certain salts), slightly raised blood pressure (small drops in meta-analyses), premenstrual syndrome.
MIXED OR LOW-QUALITY RESULTS: sleep, stress and anxiety, low mood, glycaemic control. It helps more when there is a prior deficit.
NO SOLID PROOF: nocturnal cramps in general (reviews find no clear benefit in older adults; in pregnancy the evidence is weak), "detox", weight loss, sports performance in people without a deficit.

At the counter

When to recommend it

A diet low in leafy vegetables, legumes and nuts, or increased losses (heavy sweating, diuretics, alcohol)
Interest in the claim on tiredness, muscle function or electrolyte balance, within the dose
Occasional constipation, knowing that what acts is the laxative effect of certain salts, not a "mineral supply"
A trial of 4-8 weeks with a single form, to see whether they notice anything instead of stacking supplements

When not to

Kidney failure or low filtration without checking first
Expecting it to replace migraine, sleep, blood pressure or glucose medication
Supplemental dose above 250 mg of elemental magnesium a day without medical indication (EFSA tolerable upper level)
Cramps that appear suddenly, with weakness or dark urine: refer, do not mask

What to say at the counter. When asked "does it work for sleep / cramps / anxiety?", answer by levels: "For tiredness and muscle function there is an authorised claim; for the rest it has been studied with mixed results, without firm proof". And always ask whether they take omeprazole, diuretics, levothyroxine or antibiotics.

Warnings

Important NO CLAIM for sleep, anxiety, cramps, migraine or blood pressure: they cannot be promised. If you mention it, say it has been studied with mixed results.
Important KIDNEY FAILURE: magnesium accumulates and can cause weakness, low blood pressure and arrhythmias. Ask about the kidneys before offering high doses.
Caution MIGRAINE: some guidelines mention 400-600 mg a day, above the supplement tolerable upper level. That is your doctor's decision, not counter advice.
Caution CRAMPS: those that repeat or come with weakness, swelling or medication (statins, diuretics) are referred.
Worth knowing A normal blood magnesium does not rule out a deficit: serum magnesium is a tiny part of the total.

Pharmacokinetics

Absorption: Small intestine; absorbs more when there is little and less when a lot is taken at once.
Distribution: Mostly in bone and muscle; less than 1 % is in blood, which is why a normal blood test does not rule out a tissue deficit.
Metabolism: Not metabolised: it is an ion.
Elimination: The kidney regulates the surplus; if it fails, magnesium accumulates.

What can be said about each use

Aspect	Can be said	Cannot be said
Tiredness and fatigue	It contributes to reducing them (authorised claim)	That it removes exhaustion on its own
Muscle function	It contributes to normal muscle function	That it stops night cramps
Sleep and stress	Studied with mixed results	That it helps you sleep or calms anxiety
Migraine	Guidelines mention it in medical regimens	That it replaces prescribed therapy
Constipation	Certain salts have an osmotic laxative effect	That it is for daily use without stopping

What has a claim is said with its sentence; what has been studied, with "mixed results"; the rest is not promised.

Self-assessment

Three questions. When you check your answers you will see the explanation for each one.

A customer asks whether magnesium "cures cramps". What do you answer? Take double There is no solid proof it prevents them; if they repeat, better to check Yes, it is the best for that
Which claim does magnesium have? Contributes to the reduction of tiredness and fatigue Improves deep sleep Lowers blood pressure
With kidney failure, what do you do? Change the salt Ask and refer: magnesium may accumulate High doses to be safe

Training content. It does not replace the summary of product characteristics or clinical judgement.

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