Magnesium Oxide vs Magnesium Citrate: How They Compare
To raise your magnesium, take citrate. For constipation, oxide works. Oxide is 60% magnesium and costs the least, but only about 4% of it was absorbed in one study. Citrate is 16% magnesium and absorbs much better.
The verdict
Same mineral, different partner. Citrate binds magnesium to citric acid and dissolves well. Oxide binds it to oxygen and barely dissolves, so most of it stays in the gut. That is why oxide is a poor way to raise your magnesium but a working laxative. If you want magnesium for general health, buy citrate. If citrate loosens your stools, glycinate is the usual next choice; it is not sold as a laxative, though no trial compares the two for stomach upset.
Informational comparison of label data, research and cost, not medical advice. Prices are each brand’s own list price, checked October 14, 2026; Amazon’s can differ.
Nature Made oxide vs citrate, side by side
One maker, both forms, 250 mg of magnesium a serving each. The form, the pill (1 tablet vs 2 softgels), the price and the testing status differ: the citrate is USP Verified and the oxide is not on USP’s list.
| Oxide | Citrate | |
|---|---|---|
| Magnesium a serving | 250 mg in 1 tablet | 250 mg in 2 softgels |
| Magnesium in the compound | About 60% | About 16% |
| Dissolves in water | Barely | Well (Lindberg 1990) |
| Absorbed | About 4% in one study (Firoz 2001) | More than oxide in every study we read |
| Main use | Constipation (the form with trials) | Raising magnesium intake |
| Testing list | None found (its page shows a USP mark; USP’s list does not carry it) | USP Verified |
| Bottle | 200-count bottle, $11.89 | 120-count bottle, $23.59 |
| Per 300 mg of magnesium | $0.071 | $0.47 |
| Buy | Check price | Check price |
The absorption gap
Oxide is the outlier. Small studies found the citrate, aspartate, lactate and chloride forms absorbed better than oxide and sulfate (NIH). Three studies show how big the gap is:
- About 4% absorbed. Volunteers took four US magnesium products. Only about 4% of the magnesium in oxide was absorbed; chloride, lactate and aspartate did much better and about equally well (Firoz 2001).
- Oxide barely dissolves. In lab tests made to copy the stomach, oxide was almost insoluble in water and only 43% dissolved even in strong acid. Citrate was 55% dissolved in plain water and more in acid. After one dose, citrate raised urine magnesium, a sign of absorption, more than oxide (Lindberg 1990).
- Oxide did no better than placebo over 60 days. 46 adults took 300 mg of magnesium a day. Citrate raised blood magnesium the most; oxide did not differ from placebo (Walker 2003).
What that means for one tablet. A Nature Made oxide tablet holds 250 mg of magnesium. If about 4% is absorbed, as in Firoz 2001, that is roughly 10 mg. Absorption changes with the dose and the person, so treat that as a rough guide, not a fixed number. A 2021 review agreed on the direction: inorganic forms such as oxide are less available to the body than organic ones such as citrate (Pardo 2021).
| Item | Value (mg) |
|---|---|
| On the label | 250 mg |
| Absorbed, at about 4% | ~10 mg |
For constipation, oxide is the one with trials
Poor absorption is exactly why oxide works as a laxative. The magnesium that stays in the gut pulls water into the bowel (Mori 2021).
| Study | Who and how much | Improved overall |
|---|---|---|
| Mori 2019 | 33 women with mild to moderate constipation, Japan; 0.5 g oxide 3 times a day for 4 weeks | 71% on oxide vs 25% on placebo |
| Morishita 2021 | 90 adults, Japan; 1.5 g oxide a day, or senna, for 4 weeks | 68% on oxide vs 12% on placebo (senna 69%) |
| Both pooled (van der Schoot 2023) | Review of supplement trials for constipation | 68% vs 19%; more bowel movements a week |
The catch is the dose. 1.5 g of oxide is about 900 mg of magnesium a day, over twice the 350 mg limit for supplements. A 250 mg Nature Made tablet is far below that. The trial amount is a laxative dose, not a supplement dose. Use it with a doctor’s advice, especially if you are older or have kidney disease (Mori 2021).
Citrate as a laxative comes as a drink, at a much bigger dose than a softgel. MedlinePlus lists it for occasional constipation and to clear the bowel before a colonoscopy, and says not to use it for more than a week without a doctor. Citrate supplements can loosen stools too, but no trial we found tests citrate pills for chronic constipation.
What each costs per 300 mg
| Item | Value |
|---|---|
| Nature Made Magnesium Oxide | $0.071 |
| NOW Magnesium Citrate | $0.24 |
| Natural Vitality Calm | $0.27 |
| Nature Made Magnesium Citrate | $0.47 |
| Nature Made Magnesium Glycinate | $1.10 |
We compare on 300 mg of magnesium at each brand’s list price, the same way on every magnesium page. Oxide wins on the label. It is not a fair fight on what you absorb, but no study gives a citrate absorption figure we could turn into a cost per absorbed mg, so we do not print one.
Which to buy
| If you want | Buy | Serving | Buy |
|---|---|---|---|
| To raise your magnesium, tested | Nature Made Magnesium Citrate (USP Verified) | 250 mg in 2 softgels, $0.39 a day | Check price |
| The lowest-cost citrate | NOW Magnesium Citrate | 400 mg in the 2-tablet serving; 1 tablet gives 200 mg | Check price |
| Constipation, with a doctor’s advice | Nature Made Magnesium Oxide | 250 mg a tablet, $0.06 a tablet | Check price |
| Glycinate instead (our overall pick) | Nature Made Magnesium Glycinate (USP Verified) | 200 mg in 2 capsules, $0.73 a day | Check price |
| A powder to mix in water | Natural Vitality Calm (now MaxCalm: citrate + glycinate) | 350 mg in 2 teaspoons; start with less | Check price |
| Glycinate you dose by the tablet | Vitamin Shoppe Magnesium Glycinate 400 mg | 135 tablets, 133 mg each. Take 1 or 2: the 3-tablet serving (400 mg) is over the limit. Not on a testing list. | Check price |
| A low-cost powder on NSF/ANSI 173 | BulkSupplements Magnesium Glycinate Powder | 300 mg in ¾ teaspoon (2.2 g); its label names glycinate and oxide, so it is a blend | Check price |
Why glycinate is our overall pick, not citrate: glycinate vs citrate. Every form ranked: magnesium forms compared.
Safety and medicines
- If you have kidney disease, ask your doctor before you take magnesium. Weak kidneys cannot clear the extra, so it can build up (NIH).
- Antibiotics: take tetracyclines (such as doxycycline) or quinolones (such as ciprofloxacin) at least 2 hours before or 4 to 6 hours after magnesium (NIH).
- Thyroid pills (levothyroxine): In 15 healthy adults, magnesium aspartate taken at the same time cut the thyroid hormone absorbed by 12%; citrate cut it by 7%, too small to be sure. The authors advise taking levothyroxine apart from magnesium (Attinger 2025).
- More on spacing from osteoporosis drugs and on signs of too much: magnesium side effects.
Frequently asked questions
Is magnesium citrate better absorbed than oxide?
Yes. In a study of US products, only about 4% of the magnesium in oxide was absorbed (Firoz 2001). Citrate dissolved far better and raised urine magnesium more after a single dose (Lindberg 1990), and over 60 days it raised blood magnesium more than oxide, which did no better than a placebo (Walker 2003).
Why does oxide have the most magnesium but absorb the least?
About 60% of magnesium oxide is magnesium, against 16% of citrate. But oxide barely dissolves in water, so most of it passes through the gut unused (Lindberg 1990). A big number on the label is not a big number in your blood.
Which is better for constipation, magnesium oxide or citrate?
Oxide has the trials: in two of them, 68% of people improved on it against 19% on placebo (van der Schoot 2023). Those trials used 1.5 g of oxide a day, about 900 mg of magnesium. That is a laxative dose, so use it with a doctor’s advice. Magnesium citrate drinks are sold for short-term constipation (MedlinePlus).
Is magnesium oxide a waste of money?
Not if you want it for constipation, where it works and costs very little. To raise your magnesium, yes, mostly: at $0.071 per 300 mg at list price it is the cheapest form, but little of it is absorbed. Citrate or glycinate is the better buy for that.
Should I switch from magnesium oxide to citrate?
If you take oxide to raise your magnesium, switching to citrate makes sense: it absorbs better and costs $0.47 per 300 mg as Nature Made’s USP Verified softgels. If you take oxide for constipation and it works, there is no need to switch. Ask your doctor if you take it every day.
Is magnesium citrate or oxide cheaper?
Oxide, by a lot: $0.071 for 300 mg of magnesium at Nature Made’s list price, against $0.24 for NOW citrate and $0.47 for Nature Made citrate. That is per mg on the label, not per mg absorbed.
Is Nature Made magnesium oxide USP Verified?
We did not find it on USP’s public list of verified products (searched October 14, 2026), although its page on Nature Made’s site shows the USP mark. Nature Made’s citrate softgels and glycinate capsules are on the list.
Who should not take magnesium oxide or citrate?
If you have kidney disease, ask your doctor before you take magnesium. Weak kidneys cannot clear the extra, so it can build up (NIH). Both forms need spacing from some antibiotics and osteoporosis drugs (NIH), and from thyroid pills (Attinger 2025).
Related
- Glycinate vs citrate · oxide vs glycinate · all forms compared
- What every citrate costs · best magnesium supplements · how much magnesium per day
- Best magnesium for constipation · magnesium side effects · magnesium guide
Sources
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001. PubMed 11794633
- Lindberg JS, et al. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990. PubMed 2407766
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003. PubMed 14596323
- Pardo MR, et al. Bioavailability of magnesium food supplements: a systematic review. Nutrition. 2021. PubMed 34111673
- Mori S, et al. A randomized double-blind placebo-controlled trial on the effect of magnesium oxide in patients with chronic constipation. J Neurogastroenterol Motil. 2019. PubMed 31587548
- Morishita D, et al. Senna versus magnesium oxide for the treatment of chronic constipation. Am J Gastroenterol. 2021. PubMed 32969946
- van der Schoot A, et al. The effect of food, vitamin, or mineral supplements on chronic constipation in adults. Neurogastroenterol Motil. 2023. PubMed 37243443
- Attinger MC, et al. Drug-drug interactions of levothyroxine with magnesium citrate and magnesium aspartate (ThyroMag trial). Clin Transl Sci. 2025. PubMed 41221788
- Mori H, Tack J, Suzuki H. Magnesium oxide in constipation. Nutrients. 2021. PubMed 33525523
- MedlinePlus. Magnesium citrate (saline laxative).
- NIH Office of Dietary Supplements. Magnesium fact sheet for health professionals (updated January 6, 2026).
- Labels and prices: Nature Made, NOW, Natural Vitality and BulkSupplements pages, list prices checked October 14, 2026. Testing: NSF/ANSI 173, NSF Certified for Sport, USP Verified, Informed Choice and Informed Sport, searched October 14, 2026.