Magnesium is an essential mineral involved in hundreds of processes in the body — muscle and nerve function, heart rhythm, blood sugar, bone structure and energy production. It is taken as tablets for cramps, sleep, anxiety and constipation, applied as sprays and creams, and given by injection in hospital.
What is Magnesium?
Magnesium is a mineral your body cannot make. It acts as a cofactor for more than 300 enzymes and is central to how muscles contract and relax, how nerves fire, how the heart maintains rhythm, and how cells make and use energy.
Magnesium supplements differ enormously, and the form matters more than the brand. The number on the front of the tub is usually the weight of the compound; the useful figure is the elemental magnesium, which is often far smaller.
| Form | Absorption | Best for |
|---|---|---|
| Citrate | Good | General supplementation; mildly laxative |
| Glycinate / bisglycinate | Good | Sleep, anxiety; gentlest on the gut |
| Malate | Good | General use; fatigue |
| Chloride | Good | Sprays, creams, oral |
| Lactate | Reasonable | General use |
| Oxide | Poor — around 4% | Cheap; mainly a laxative and antacid |
| Sulphate | Epsom salts; injection | Baths; hospital use |
Magnesium oxide is the commonest form in cheap supplements and the worst absorbed. If a product lists a large milligram figure at a low price, it is usually oxide, and most of it passes straight through — which is why it works as a laxative.
Magnesium sprays, oils and creams are widely sold on the claim that magnesium is absorbed through the skin. The evidence for meaningful absorption through intact skin is weak. Many people find them soothing, and that may be worth something, but do not rely on a spray to correct a deficiency. An Epsom salt bath is pleasant; it is not a reliable way to raise magnesium.
Magnesium injection is a hospital medicine, not a supplement — used for eclampsia in pregnancy, severe asthma, certain heart rhythm disturbances, and severe deficiency. It is given by professionals with monitoring.
What is Magnesium commonly used for?
Where the evidence is reasonable:
- Correcting genuine deficiency, which causes cramps, weakness, tremor, palpitations, irritability and, when severe, seizures.
- Constipation, where magnesium citrate or oxide works as an osmotic laxative — a well-established effect.
- Migraine prevention, where magnesium has modest but real supporting evidence.
- Eclampsia and pre-eclampsia, by injection. This use saves lives and magnesium sulphate is a WHO essential medicine for it.
- Severe asthma, by injection in hospital.
- Certain heart rhythm disturbances, particularly with low magnesium.
- Indigestion, as magnesium-containing antacids.
Where it is widely used but the evidence is weaker:
- Muscle cramps, including night cramps. Trials have been largely disappointing, particularly in older adults — which is worth knowing before spending money. Some people do benefit.
- Sleep, where magnesium glycinate is popular. Modest evidence, mostly in older adults with low levels.
- Anxiety and stress, where it is heavily marketed. Some supportive evidence, not strong.
- Restless legs, blood pressure, and blood sugar in type 2 diabetes — small effects in some studies.
- Premenstrual symptoms.
Who is genuinely likely to be low: people taking long-term proton pump inhibitors (omeprazole, pantoprazole) or diuretics, people with poorly controlled diabetes, coeliac or Crohn’s disease, chronic alcohol use, or chronic diarrhoea. In these groups, checking is worthwhile.
Most people eating a varied diet get enough. Green leafy vegetables, nuts, seeds, legumes, wholegrains and dark chocolate are all good sources.
Who commonly looks for Magnesium and why
People with night cramps — by far the largest group, and the one most likely to be disappointed. The evidence is genuinely weak, though it is cheap and harmless enough to try.
People who sleep badly, drawn to glycinate. Worth trying; not a substitute for sleep habits or for investigating something like sleep apnoea.
People who are anxious or exhausted. Fatigue has many causes — anaemia, thyroid disease, sleep apnoea, depression, diabetes. A blood test finds which; a supplement guesses.
Athletes and gym-goers, for recovery and cramping.
People on long-term acid-suppressing medicines, who are among the few with a genuinely elevated risk — this group has the strongest reason to ask about it.
People comparing sprays and tablets, where the honest answer is that tablets work and sprays are mostly pleasant.
What to check on the label before taking Magnesium
- The elemental magnesium content, in milligrams. The only comparable figure.
- The form. Citrate, glycinate or malate for absorption; oxide mainly if you want the laxative effect.
- Whether it is a combination with calcium, zinc or vitamin D — check you are not doubling up.
- The daily amount. Supplemental magnesium above roughly 350–400 mg of elemental magnesium a day causes diarrhoea in most people, and there is little reason to exceed it.
- Sugar or sweeteners, if you have diabetes.
A note if you search the medicine register for “magnesium”: you will get antacid suspensions such as Amgel, Spasmogel and Medigel, and intravenous infusion fluids. Those are unrelated products that happen to contain a magnesium salt — disregard them.
Safety, interactions and when to seek professional advice
For most people with healthy kidneys, magnesium supplements are very safe. The kidneys excrete the excess efficiently, and the usual limiting factor is diarrhoea.
The one situation where it is genuinely unsafe is kidney disease. Do not take magnesium supplements without medical advice if you have reduced kidney function — impaired kidneys cannot clear it, and magnesium can rise to levels causing muscle weakness, low blood pressure, confusion, slowed breathing and dangerous heart rhythm disturbances.
Also speak to a doctor or pharmacist first if you:
- Have heart block or a slow heart rhythm.
- Have myasthenia gravis, which magnesium can worsen.
- Have inflammatory bowel disease or chronic diarrhoea.
- Are elderly, where kidney function declines and is often lower than expected.
Space magnesium by at least two hours from:
- Tetracycline and quinolone antibiotics — doxycycline, ciprofloxacin. Magnesium binds them and stops both working.
- Bisphosphonates for osteoporosis.
- Thyroid medicine (levothyroxine).
- Iron supplements.
- HIV medicines, particularly integrase inhibitors such as dolutegravir. This interaction matters clinically — magnesium substantially reduces absorption.
Also tell your doctor if you take diuretics, digoxin, or muscle relaxants.
Common effects are gastrointestinal: diarrhoea, loose stools, nausea and abdominal cramping. These are dose-related — reducing the dose or switching from oxide or citrate to glycinate usually solves it.
Seek prompt advice for: persistent diarrhoea, muscle weakness, unusual tiredness, flushing, a slow or irregular heartbeat, or confusion — particularly with kidney disease.
Get emergency help for severe weakness, difficulty breathing, fainting, or a very slow heartbeat.
See a doctor rather than self-treating if: cramps are severe, frequent, or associated with weakness or numbness; you have persistent fatigue, which deserves a blood test; you are on long-term acid-suppressing medicines and have cramps, palpitations or tremor; you have palpitations or an irregular heartbeat; or you have kidney disease and are considering any mineral supplement.
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Frequently asked questions
What is magnesium used for in everyday life? Muscle and nerve function, heart rhythm, bone structure, blood sugar and energy production. As a supplement it is taken for cramps, sleep, migraine prevention and constipation.
What are magnesium tablets used for? Correcting deficiency, constipation, migraine prevention, and — with weaker evidence — cramps, sleep and anxiety.
What is magnesium spray or cream used for? Marketed for cramps and muscle aches. The evidence that magnesium is meaningfully absorbed through intact skin is weak — many find sprays soothing, but do not rely on one to correct a deficiency.
What is magnesium injection used for? A hospital medicine — eclampsia in pregnancy, severe asthma, certain heart rhythm disturbances and severe deficiency. Not a supplement.
Which form should I take? Citrate, glycinate or malate for absorption. Glycinate is gentlest on the gut. Oxide is poorly absorbed and mainly useful as a laxative.
Will it stop my night cramps? Possibly, but trials have been largely disappointing, particularly in older adults. It is cheap and harmless enough to try.
Why does it give me diarrhoea? Unabsorbed magnesium draws water into the bowel. Reduce the dose or switch to glycinate.
Can anyone take it? Not with kidney disease. Impaired kidneys cannot clear magnesium, and it can rise to dangerous levels.
Can I take it with my other medicines? Separate it by at least two hours from thyroid medicine, iron, bisphosphonates, doxycycline, ciprofloxacin and HIV integrase inhibitors.