Last updated: October 2026
Magnesium oxide packs more magnesium into each tablet than any other common form, about 60% by weight, but it is one of the least well absorbed, so much of it stays in the gut. That’s why it works as a laxative and often causes loose stools. For everyday top-ups, gentler forms such as glycinate or citrate are usually preferred.
Below: what magnesium oxide is, what the absorption studies really show, what it’s used for, how it compares with citrate, glycinate, malate and taurate, how much to take, side effects and who should check first.
What is magnesium oxide?
Magnesium oxide is magnesium bound to oxygen. It’s a white, chalky powder that dissolves very poorly in water, and it’s the cheapest form of magnesium, which is why it turns up in so many budget supplements and multivitamins.
Because oxygen is a light atom, magnesium makes up about 60% of the compound’s weight (24.3 out of 40.3 in its molecular weight). Compare that with roughly 14% for magnesium bisglycinate or 16% for magnesium citrate. So a small oxide tablet can carry a big number on the label.
In the UK, magnesium oxide is also an ingredient in some prescription bowel-cleansing sachets, such as Picolax and CitraFleet, used to clear the bowel before examinations such as a colonoscopy. There it reacts with citric acid to form magnesium citrate. It isn’t one of the oral magnesium salts the BNF lists as licensed for low magnesium: those are magnesium glycerophosphate, aspartate and citrate.
New to the different forms? Our guide to the 12 types of magnesium explains each one.
How well is magnesium oxide absorbed?
Poorly, compared with most other forms. Three studies are quoted again and again, and all three are small, so it’s worth knowing what they actually measured.
- The “4%” figure. In a 2001 study, healthy volunteers took about 21 mEq (roughly 255mg) of magnesium a day from four products. Measured by the rise in magnesium passed in urine, the fractional absorption of magnesium oxide was 4%. Magnesium chloride, lactate and aspartate were absorbed significantly better, and about equally well as each other.
- Citrate vs oxide in the lab and in people (1990). Magnesium oxide was virtually insoluble in water and only 43% soluble even in conditions that mimic peak stomach acid, while magnesium citrate was 55% soluble in plain water. After a single large dose, the rise in urinary magnesium over four hours was far higher with citrate than with oxide.
- A 60-day UK trial (2003). At the University of Reading, 46 healthy adults took 300mg of magnesium a day as citrate, an amino acid chelate (the family glycinate belongs to) or oxide, or a placebo. Citrate gave the highest blood levels, both organic forms were absorbed better than oxide, and oxide was no different from placebo.
A 2021 systematic review of 14 studies reached a similar overall view: inorganic forms such as oxide appear less bioavailable than organic forms such as citrate, and the percentage absorbed depends on the dose. The same review also found that all the supplements studied could maintain normal magnesium levels in healthy people who weren’t low to begin with.
So the honest summary is: oxide reliably loses head-to-head comparisons, but the exact “4%” is one estimate from one small study, not a fixed fact. For more on why cheap tablets disappoint, see why many magnesium supplements are a waste of money.
What is magnesium oxide used for?
Constipation
This is where oxide’s weakness becomes its strength. The magnesium that isn’t absorbed stays in the bowel and draws water in, softening stools. In a 2021 Japanese trial, 90 adults with chronic constipation took 1.5g of magnesium oxide a day, senna or a placebo for four weeks; 68% improved on magnesium oxide and 69% on senna, compared with 12% on placebo. Doses, timing and who should avoid it are covered in magnesium for constipation. Ask a pharmacist before using any magnesium as a laxative.
Heartburn and indigestion
Magnesium oxide is alkaline and neutralises stomach acid. UK antacids more often use other magnesium salts: the NHS lists magnesium carbonate, magnesium trisilicate and magnesium hydroxide among the ingredients to look for. Antacids can stop other medicines working properly, so the NHS advises not taking other medicines within 2 to 4 hours of an antacid.
Migraine
Some migraine trials used magnesium oxide. In a 2019 crossover trial of 63 people, 500mg magnesium oxide tablets taken twice a day performed similarly to the prescription medicine sodium valproate over eight weeks. A 2012 trial of 133 people found reductions in migraine frequency with 500mg a day of magnesium oxide on top of usual treatment, though the authors called the results preliminary. Other trials used citrate. These are small studies, so talk to your GP before relying on magnesium. Our magnesium for migraines guide has the wider evidence.
Leg cramps
Probably not. In a 2017 trial, 94 adults (average age 65) with night-time leg cramps took magnesium oxide or a placebo at bedtime for four weeks. Both groups improved by a similar amount, which the authors put down to a placebo effect. A 2020 Cochrane review concluded that magnesium is unlikely to give meaningful cramp prevention in older adults. More in magnesium for cramps.
Sleep
There is very little research on magnesium oxide specifically for sleep. Because so little is absorbed and it can send you to the loo, it’s an awkward choice for an evening routine. Most people choose glycinate for the evening instead; see magnesium for sleep.
Topping up low magnesium
Oxide is the form least likely to raise your levels per milligram. If you think you may be low, read the signs of magnesium deficiency, look at foods high in magnesium first, and ask your GP about a blood test if you have symptoms.
Magnesium oxide vs citrate vs glycinate (plus malate and taurate)
| Form | Magnesium per gram (approx.) | Absorption evidence | Stomach | Best for |
|---|---|---|---|---|
| Oxide | 600mg (60%) | Poor in head-to-head studies | Most likely to loosen stools | Cheap high-dose tablets; short-term constipation |
| Citrate | 160mg (16%) | Good: highest blood levels in the Reading trial | Can loosen stools at higher doses | Everyday top-up; constipation |
| Glycinate (bisglycinate) | 140mg (14%) | Good: amino acid chelates beat oxide | Very gentle | Evenings; sensitive stomachs |
| Malate | 155mg (15.5%) | Little head-to-head human data | Generally gentle | Daytime routines |
| Taurate | 90mg (9%) | Little head-to-head human data | Generally gentle | Less common; small amounts of magnesium per capsule |
Figures are calculated from each compound’s molecular weight. Hydrated forms and branded ingredients contain a little less, so always check the label for “elemental magnesium” in mg.
Magnesium oxide vs citrate
Citrate carries far less magnesium per gram, but more of it gets in: it won both the 1990 and 2003 head-to-head studies. Both loosen stools, oxide more so. If you want a well-absorbed everyday form, citrate is the better pick. See magnesium citrate.
Magnesium oxide vs glycinate
Glycinate (bisglycinate) is magnesium bound to the amino acid glycine. It’s much gentler on the gut, which is why it’s popular for the evening, though you need more capsules or powder for the same amount of magnesium. See magnesium glycinate, or for the other forms in the table, magnesium malate and magnesium taurate.
How much magnesium oxide should you take, and when?
- Daily needs: the NHS says adults aged 19 to 64 need 300mg of magnesium a day (men) or 270mg (women), and you should be able to get it from a varied, balanced diet.
- Supplements: taking 400mg or less a day from supplements is unlikely to cause harm. More than 400mg for a short time can cause diarrhoea.
- Read “400mg” labels carefully. A tablet of 400mg magnesium oxide gives about 240mg of magnesium. A tablet “providing 400mg magnesium” contains about 660mg of magnesium oxide and already reaches the NHS supplement figure on its own.
- Laxative or antacid products: follow the pack or your pharmacist; they’re meant for short-term use.
- Timing: magnesium oxide needs stomach acid to dissolve, and acid-reducing medicines such as omeprazole and other proton pump inhibitors can weaken its laxative effect. If it upsets your stomach, take it with food and try a smaller dose.
Side effects and who should check first
The most common side effects are diarrhoea, loose stools, stomach cramps and wind. They’re more likely with oxide than with gentler forms, and more likely the higher the dose. If they happen, stop or cut back. For the full picture, see magnesium side effects.
- Kidney disease: your kidneys remove extra magnesium, so it can build up if they don’t work well. Don’t take magnesium supplements, laxatives or antacids without your doctor’s advice.
- Older adults and long-term use: serious cases of high blood magnesium have been reported with prolonged magnesium oxide use, so check with your GP before taking it regularly.
- Medicines: magnesium can bind to some antibiotics (tetracyclines such as doxycycline, and quinolones) and to bisphosphonates for osteoporosis, making them work less well; a review recommends leaving at least 2 hours between them. The NHS lists antacids and indigestion treatments among the things that can affect levothyroxine. Ask your pharmacist how to space your doses.
- Pregnancy and breastfeeding: ask your midwife, GP or pharmacist before using any magnesium, laxative or antacid. We don’t recommend our products during pregnancy or breastfeeding.
- Children: don’t give children adult supplements. Ask a GP or pharmacist, and see magnesium for kids. All Evopure products are for adults (18+) only.
Why our magnesium doesn’t use oxide
Our Magnesium Glycinate Sleep Complex contains no magnesium oxide. It’s a berry-flavoured evening drink made with magnesium bisglycinate and magnesium citrate, giving 148mg of magnesium per serving, plus glycine, L-theanine, chamomile, passionflower, hops and oat straw.
We chose bisglycinate because it’s gentle on the stomach, and citrate because it’s well absorbed. Oxide would have been cheaper and let us print a bigger number, but it’s the form most likely to send you to the bathroom, which is the last thing you want from a drink before bed. Magnesium contributes to the normal functioning of the nervous system and to the reduction of tiredness and fatigue.
Comparing brands? Our guide to the best magnesium supplements in the UK shows what to look for on a label.
Magnesium chloride is also sold as bath flakes; see magnesium flakes.
FAQs
Is magnesium oxide good for you?
It’s cheap and safe for most adults at normal doses, but it’s poorly absorbed compared with forms like citrate and glycinate and more likely to cause loose stools. It’s useful as a short-term laxative, less so for topping up your magnesium.
Is magnesium oxide a laxative?
Yes. Because much of it isn’t absorbed, it draws water into the bowel and softens stools. In a 2021 trial, 1.5g a day helped about two thirds of adults with chronic constipation. Ask a pharmacist before using it this way.
Is magnesium oxide good for sleep?
There’s very little research on magnesium oxide for sleep, and its laxative effect makes it an awkward choice at night. Most people pick a gentler form such as glycinate for the evening.
What is the difference between magnesium oxide and magnesium citrate?
Oxide contains more magnesium by weight, about 60% against roughly 16% for citrate, but citrate dissolves far better and was absorbed better in head-to-head studies. Both can loosen stools, oxide more so.
Is magnesium glycinate better than magnesium oxide?
For a gentle everyday or evening top-up, usually yes: amino acid chelates were absorbed better than oxide in a 60-day UK trial and are kinder to the stomach. Oxide is cheaper and works better as a laxative.
How much magnesium is in a 400mg magnesium oxide tablet?
About 240mg, because magnesium is roughly 60% of magnesium oxide by weight. Check whether the label says 400mg of magnesium oxide or 400mg of magnesium, as they’re different amounts.
Can children take magnesium oxide?
Only on the advice of a GP or pharmacist. Don’t give children adult supplements, laxatives or antacids without checking first. Evopure products are for adults only.
Sources
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res, 2001. PMID 11794633.
- Lindberg JS et al. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr, 1990. PMID 2407766.
- Walker AF et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res, 2003. PMID 14596323.
- Pardo MR et al. Bioavailability of magnesium food supplements: a systematic review. Nutrition, 2021. PMID 34111673.
- Mori H et al. Magnesium oxide in constipation. Nutrients, 2021. PMID 33525523.
- Morishita D et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. Am J Gastroenterol, 2021. PMID 32969946.
- Karimi N et al. The efficacy of magnesium oxide and sodium valproate in prevention of migraine headache. Acta Neurol Belg, 2021. PMID 30798472.
- Tarighat Esfanjani A et al. The effects of magnesium, L-carnitine, and concurrent magnesium-L-carnitine supplementation in migraine prophylaxis. Biol Trace Elem Res, 2012. PMID 22895810.
- Roguin Maor N et al. Effect of magnesium oxide supplementation on nocturnal leg cramps. JAMA Intern Med, 2017. PMID 28241153.
- Garrison SR et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev, 2020. PMID 32956536.
- NHS. Vitamins and minerals: others (magnesium).
- NHS. Antacids and levothyroxine.
- BNF (NICE). Magnesium imbalance.
- emc. Picolax: summary of product characteristics.
- PubChem. Magnesium oxide (CID 14792).
This guide is general information, not medical advice. Evopure makes the Magnesium Glycinate Sleep Complex.
