Magnesium Oxide: Uses, Absorption, Side Effects and Oxide vs Citrate vs Glycinate (UK)

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.

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

iOxide 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)

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 290mg 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 a reduction of tiredness and fatigue.

Related guides

• 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.
• Wondering whether oxide or citrate suits a bloated, backed-up stomach? See which magnesium is best for bloating.
• Magnesium powder vs tablets compared, including what dissolving oxide in water changes.
• Oxide is best taken in the evening with food; see our guide to the best time to take magnesium for every form.
• If oxide upsets your stomach, you may wonder about magnesium spray instead, but very little gets through the skin.
• Many 12 in 1 blends include oxide, so read our guide to whether a 12 in 1 magnesium complex is good for you before you buy one.
• Vitabiotics Ultra Magnesium, which uses magnesium hydroxide rather than oxide.
• Some products mix oxide with a chelate, such as Solgar Chelated Magnesium.
• Oxide works within hours as a laxative but is slow for anything else; see how long magnesium takes to work by type.
• Oxide also turns up in seawater blends, as in Together Health magnesium, alongside hydroxide, sulfate, carbonate and chloride.
• Oxide often appears in blends: in Bioglan Active Magnesium tablets it supplies two thirds of the magnesium.
• Boots’ plain Magnesium 375mg tablets are oxide; see Boots magnesium explained.
• Superdrug’s own-brand Magnesium Oxide 375mg tablets give 375mg of magnesium per tablet.
• Tesco’s own-brand Magnesium and Vitamin B6 375mg tablets also use magnesium oxide.
• Some blends mix oxide with other forms, as in Viridian High Potency Magnesium.
• Plenty of UK products use oxide; see the forms in Healthspan magnesium for one example.
• Oxide is often part of a blend: in Nutravita’s magnesium complex it is the largest compound by weight.
• Oxide is often the biggest share of multi-form blends; see the label maths of a 12 in 1 magnesium complex.
• Some glycinate products are buffered with oxide; our glycinate vs bisglycinate guide shows how to spot them.
• Count only the elemental figure towards your daily amount; our guide to how much magnesium a day explains the UK numbers.
• Much of an oxide dose is never absorbed and leaves in your stools; see how long magnesium stays in your body.
• Magnesium oxide was the form used in a 2022 restless legs trial; see magnesium for restless legs for what it found.
• Several budget products are oxide; see what form each Home Bargains magnesium supplement uses.
• Many budget magnesium and B6 tablets use oxide, as at Savers.
• Many supermarket own brands use oxide; see Sainsbury’s Magnesium 375mg for a worked label example.

• Some own-label tablets use oxide alone; see Myvitamins magnesium for how its oxide tablets compare with its bisglycinate.

• Licensed UK magnesium tablets on prescription use a different form: see magnesium glycerophosphate.
• Oxide is also used to buffer glycinate in some capsules, as in Heights Magnesium+.
• Some glycinate capsules are buffered with oxide; VitaBright magnesium glycinate is one worked example.