Magnesium

What it is

Magnesium is an essential mineral found in leafy greens, nuts, seeds, wholegrains and pulses. Your body uses it in muscle and nerve function, energy production, protein making and keeping bones sturdy.

Most people in the UK get enough from food, though some surveys suggest a share of adults fall below recommended intakes. People take supplements hoping for better sleep, fewer cramps, calmer mood, fewer headaches or better recovery.

The word magnesium on a label tells you little. The form it is bound to changes how much you absorb, what it does to your gut, and how much actual magnesium is in each capsule.

How it works

Magnesium acts as a helper for hundreds of enzymes, including those that make and use ATP, the cell's energy currency. It also helps regulate how calcium moves in and out of nerve and muscle cells, which is why it is linked with muscle relaxation and nerve signalling. Supplements raise blood levels only a little, because the kidneys remove extra.

What the evidence says

• Strong Correcting low magnesium
  If you are genuinely deficient (for example from long-term diuretics, heavy drinking or certain gut conditions), replacing it works; a clinician usually confirms this.
• Moderate Constipation (citrate, oxide)
  Poorly absorbed forms draw water into the bowel and act as a mild laxative.
• Weak Leg cramps
  Reviews generally find little or no benefit for ordinary cramps in older adults.
• Limited Sleep quality
  Small studies hint at modest improvement, mostly in older adults with poor intake; results are inconsistent.
• Limited Mild anxiety and low mood
  Early research only; not a substitute for proper support if you are struggling.
• Moderate Migraine frequency
  Some guidelines mention magnesium as an option, but effects are modest and doses used were fairly high.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Common supplement doses give 100-400 mg of elemental magnesium. Check the label, since a 1000 mg tablet of magnesium oxide may contain far less actual magnesium.
• UK expert guidance has suggested that supplemental intakes up to around 400 mg a day are unlikely to cause harm; check current guidance, and remember this is on top of food.
• Splitting the dose, or taking it with a meal, reduces diarrhoea. Start low and build up.
• Bedtime use is popular but there is no firm proof that evening dosing works better.

Forms and what to look for

• Glycinate or bisglycinate: well tolerated, gentle on the gut, often chosen for sleep and calm. Often more expensive per milligram.
• Citrate: absorbed reasonably well but loosening for the bowels.
• Oxide: cheap and high in elemental magnesium but poorly absorbed; mostly acts as a laxative.
• Malate: often marketed for energy and muscle; evidence for those claims is thin.
• L-threonate: marketed for brain use because of a claimed ability to raise brain levels; human evidence is very limited and it contains little elemental magnesium per capsule.

What to expect, and when

• If you were low, you might feel better in a few weeks. Many people notice nothing.
• Gut effects (loose stools) show up within hours to days of too high a dose.
• For sleep or mood, give it about four weeks at a sensible dose; if nothing changes, it probably is not doing much for you.

Side effects

• Diarrhoea, loose stools and stomach cramps, especially with oxide, citrate and chloride
• Nausea or bloating
• Drowsiness in some people
• Very high intakes can cause low blood pressure, a slow heartbeat, confusion and muscle weakness

Interactions

• Can reduce absorption of some antibiotics (tetracyclines, quinolones such as ciprofloxacin); separate doses by a few hours
• Reduces absorption of bisphosphonates for osteoporosis and levothyroxine; space them apart as advised by your pharmacist
• Can combine with diuretics, certain heart medicines and calcium channel blockers to affect blood pressure or heart rhythm; ask a clinician
• Taking with iron, zinc or calcium at the same time may compete for absorption

Who should avoid it

• Anyone with kidney disease or reduced kidney function, because magnesium builds up
• People with heart rhythm problems or heart block without medical advice
• Anyone with myasthenia gravis
• Pregnancy and breastfeeding: talk to a midwife or pharmacist before adding high doses
• Children, unless advised by a clinician

Risks and safety

• Toxicity from supplements is uncommon with healthy kidneys but possible at very high doses, particularly from laxatives and antacids that contain magnesium.
• Labels often show the compound weight rather than elemental magnesium, which can mislead.
• Not on the WADA prohibited list.
• Magnesium taken regularly for years should be mentioned to your GP if you have other conditions.

Buying well: quality and testing

• Look for the elemental magnesium figure per serving on the label, not just the compound weight.
• Third-party seals such as Informed Sport, NSF or USP add confidence about contents and contaminants.
• Be wary of branded blends that hide exact amounts behind a proprietary mix.
• Powder drinks and gummies can have much lower doses than the front label implies.

Bottom line

Magnesium is a reasonable supplement if your diet is thin or a clinician has found you low. If you want a try for sleep or cramps, glycinate at a modest dose is the gentle choice. Do not expect miracles, and skip it, or check with a clinician, if you have kidney problems.