Magnesium oxide, what it is, how well it's absorbed and side effects
How much magnesium magnesium oxide really holds, what absorption studies found, and how it compares with citrate and glycinate.
- Written by
- Ben L.
- Last updated
- Reading time
- 6 min read
Overview
Magnesium oxide is about 60% magnesium by weight, so 500 mg holds roughly 300 mg of magnesium. It dissolves poorly, and short studies found less absorbed than with other forms such as citrate, though a 24-week trial found similar rises in blood magnesium. The NHS says 400 mg or less a day from supplements is unlikely to cause harm.
Key points
- Magnesium makes up about 60% of magnesium oxide's weight, so check whether a label gives the compound or the magnesium.
- Short studies found less magnesium absorbed from oxide than from other forms, including citrate.
- Over 24 weeks, oxide, citrate and sulfate raised blood magnesium by a similar amount.
- Magnesium oxide is also used as an over-the-counter laxative.
- The NHS says 400 mg or less of magnesium a day from supplements is unlikely to cause harm.
A tub that says "magnesium oxide 500 mg" doesn't give you 500 mg of magnesium. About 60% of magnesium oxide's weight is magnesium, so 500 mg of the compound holds roughly 300 mg of the mineral [1].
That high share is the main reason magnesium oxide is so common in tablets. The catch is that it doesn't dissolve well, and studies have found less of it is absorbed than with forms such as citrate.
What is magnesium oxide?
Magnesium oxide is a simple salt of magnesium and oxygen. It's an inorganic form. In citrate or glycinate, magnesium is bound to an acid or an amino acid instead.
Inorganic salts such as magnesium oxide pack in a lot of magnesium but dissolve poorly, while organic forms dissolve well but carry less magnesium per gram [1]. That's why an oxide tablet can show a big number on the label in a small tablet.
How much magnesium is in magnesium oxide?
About 60% by weight [1]. Here's a worked example. A 750 mg magnesium oxide tablet works out at roughly 450 mg of magnesium, which is the tablet one study tested [1].
Check whether the label gives the compound or the magnesium. The magnesium figure is the one to compare between products. Our guide to how much magnesium per day explains what that figure should be.
Is magnesium oxide well absorbed?
Less well than some other forms, in most studies. The trials are small, and each one measured it in a different way.
| Study | What it found | Evidence |
|---|---|---|
| Healthy volunteers, 2001 | About 4% of the magnesium in oxide was absorbed, lower than chloride, lactate or aspartate [2] | Early |
| 46 adults, 60 days | Oxide did no better than placebo, while citrate raised blood magnesium most [3] | Early |
| 30 adults, one dose | A 450 mg oxide tablet raised blood magnesium less than a 196 mg mixed tablet [1] | Early |
| 164 adults, 24 weeks | Blood magnesium rose by a similar amount with oxide, citrate and sulfate [4] | Early |
In a 60-day UK trial, 46 healthy adults took about 300 mg of magnesium a day as citrate, an amino acid chelate or oxide. Citrate raised blood magnesium the most, and oxide made no difference compared with placebo [3].
In a Belgian study, a tablet with 450 mg of magnesium as oxide raised blood magnesium less over 6 hours than a 196 mg tablet that mixed oxide with an organic form [1]. In the lab, the same study found big differences between brands. So how a tablet is made matters as well as the form.
The longest trial points the other way. Over 24 weeks, 450 mg a day of magnesium as oxide, citrate or sulfate raised blood magnesium by a similar amount [4]. So oxide is absorbed, just less efficiently per dose in the short studies.
Is magnesium oxide a laxative?
It can be. Magnesium oxide is one of the over-the-counter treatments people use for constipation. A review of trials lasting 4 weeks or more rated the evidence for it as modest, below polyethylene glycol and senna [5].
Magnesium oxide side effects
The NHS says taking 400 mg or less of magnesium a day from supplements is unlikely to cause any harm. Taking more than 400 mg for a short time can cause diarrhoea [6].
In the 24-week trial, people taking oxide reported fewer stomach and bowel symptoms than those taking citrate [4]. That trial was part-funded by a magnesium producer, and split the dose across the day.
Our guide to magnesium side effects covers who should check with a GP first, including people with kidney problems.
Magnesium oxide vs citrate and glycinate
Organic forms such as citrate and glycinate carry less magnesium per gram but dissolve better [1]. Citrate beat oxide on blood magnesium in the 60-day trial [3]. Our guide to magnesium glycinate vs citrate compares those two forms.
In practice, compare labels by the magnesium figure, then think about how many tablets that takes and how your stomach copes.
Where Nooroots fits
Nooroots Triple Magnesium is a food supplement with magnesium glycinate, citrate and malate, in three capsules a day. It's for topping up your daily intake alongside a varied diet.
Magnesium contributes to a reduction of tiredness and fatigue and to normal muscle function [7]. Our guide to magnesium citrate explains one of the forms it uses.
Frequently asked questions
What is magnesium oxide good for?
Magnesium oxide is used to top up magnesium and, at higher amounts, as a laxative. Magnesium contributes to normal muscle function and a reduction of tiredness and fatigue [5][7].
Is magnesium oxide a good form of magnesium?
It carries far more magnesium per gram than citrate, but short studies found less of it absorbed than other forms, including citrate [1][2][3]. Over 24 weeks, it raised blood magnesium as much as citrate [4].
Is magnesium oxide good for sleep?
We found no good trial of magnesium oxide for sleep in healthy adults. Our guide to magnesium for sleep covers what the trials of other forms found.
How much magnesium is in 500 mg of magnesium oxide?
About 300 mg, because magnesium makes up about 60% of magnesium oxide's weight [1]. Check the nutrition table on the label for the exact figure.
What are the side effects of magnesium oxide?
Diarrhoea is the main one, and it depends on the dose. The NHS says 400 mg or less of magnesium a day from supplements is unlikely to cause harm [6].
When should I take magnesium oxide?
Take it at a time you'll remember each day. In the 24-week trial, people split the dose into three, with two capsules each time [4].
Read more in our guide to when to take magnesium.
References
Every study here is listed so you can read it yourself. This guide is for general information and doesn't replace advice from your GP or pharmacist. We check it again at least every six months.
- Blancquaert L, Vervaet C, Derave W (2019). Predicting and testing bioavailability of magnesium supplements. Nutrients 11(7): 1663. PMID 31330811
- Firoz M, Graber M (2001). Bioavailability of US commercial magnesium preparations. Magnesium Research 14(4): 257–262. PMID 11794633
- Walker AF, Marakis G, Christie S, et al. (2003). Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research 16(3): 183–191. PMID 14596323
- Schutten JC, Joris PJ, Groendijk I, et al. (2022). Effects of magnesium citrate, magnesium oxide, and magnesium sulfate supplementation on arterial stiffness: a randomized, double-blind, placebo-controlled intervention trial. Journal of the American Heart Association 11(6): e021783. PMID 35253448
- Rao SSC, Brenner DM (2022). Evidence-based treatment recommendations for OTC management of chronic constipation. Journal of the American Association of Nurse Practitioners 34(9): 1041–1044. PMID 35943487
- NHS. Vitamins and minerals: others (magnesium and zinc sections). Page last reviewed 3 August 2020. Accessed 26 October 2026. nhs.uk
- Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register, spreadsheet of 19 May 2026. Accessed 26 October 2026. gov.uk
