Why is magnesium oxide in so many cheap supplements?

Because it is the cheapest way to put a big number on a small pill. MgO has a molecular weight of 40.3 g/mol and magnesium accounts for 24.3 of that, so roughly 60% of the compound is the mineral — compared with about 14% for magnesium bisglycinate and 16% for magnesium citrate. A manufacturer can therefore hit a 400 mg elemental magnesium claim in one modest tablet instead of three large capsules, at a fraction of the ingredient cost. Nothing about that is dishonest on its face; the label is stating elemental magnesium, which is what the panel is supposed to show. What the label does not tell you is that you will absorb a small fraction of it and that the remainder is osmotically active in your gut. Oxide is a cost and density decision by the formulator, not a quality one, which is why it dominates the bottom shelf and is almost absent from the products that compete on testing and transparency.

How much elemental magnesium should I be taking?

Work from total intake, not from the supplement. The RDA is 400 mg a day for men aged 19-30 and 420 mg from 31 onward, and 310 mg for women aged 19-30 and 320 mg from 31 onward — and that includes food, which should be most of it. You absorb roughly 30-40% of the magnesium you eat, and it is abundant in leafy greens, legumes, nuts, seeds and whole grains. On the supplement side, the tolerable upper intake level is 350 mg a day of magnesium from supplements and medications for adults, which exists precisely because higher supplemental doses cause diarrhea, nausea and cramping. In practice that means most people need a supplemental 100-200 mg of elemental magnesium, not 500 mg, and the 'high potency' capsule is usually solving a problem you do not have. Read the elemental figure on the panel and check the serving size, since glycinate servings are often two or three capsules. If you have kidney disease or take prescription medication, this is a question for your doctor, not for a label.

Will magnesium glycinate give me diarrhea?

Much less often than oxide, citrate, carbonate, chloride or gluconate — and that is its main practical selling point. The laxative effect of magnesium salts comes from the osmotic activity of unabsorbed salt in the intestine and colon plus stimulation of gastric motility, so the less magnesium that stays unabsorbed in the gut, the less of that you get. NIH specifically names carbonate, chloride, gluconate and oxide as the forms most commonly reported to cause diarrhea; glycinate's reputation for being gentle is consistent with its better solubility. It is not immune, though. Any form will loosen stools if the dose is high enough, which is why the supplemental upper limit of 350 mg a day for adults exists in the first place. If a glycinate product is bothering you, the usual fixes are splitting the dose, taking it with food, or simply taking less — in that order — before switching brands.

Should I take magnesium glycinate or citrate?

Either is a reasonable daily choice, and the honest difference is tolerance versus evidence and price. Citrate has the better published absorption record: the NIH professional fact sheet lists citrate among the forms absorbed more completely than oxide and sulfate, while glycinate is not named in that discussion at all. Citrate is also cheaper and is mildly laxative, which is a drawback if your digestion is sensitive and a feature if you tend toward constipation. Glycinate is the gentler of the two and the one most people can take nightly for months without a second thought, which matters more than a few percentage points of absorption in a supplement you are supposed to take every day. If you tolerate citrate, it is excellent value. If cheap magnesium has upset your stomach before, go straight to glycinate. We break the form question down further in our glycinate versus citrate comparison, and our magnesium roundup scores specific products from both camps.

Comparison

Magnesium Oxide vs Magnesium Glycinate: Which One Should You Actually Buy?

Q: Does magnesium oxide actually do anything?

Yes, two things. It raises magnesium somewhat — in a 2024 double-blind crossover study in 40 adults, oral magnesium oxide produced a statistically significant rise in plasma magnesium at one hour, while magnesium bisglycinate in the same study did not reach significance at any timepoint. That study was industry-funded and single-dose plasma is a crude measure, but it is enough to retire the claim that oxide is inert. And it works as an osmotic laxative, with real trial evidence: a placebo-controlled trial in 34 women with chronic constipation met its primary endpoint in 70.6% of the oxide group versus 25.0% on placebo, and a 2021 trial in 90 patients with chronic idiopathic constipation reported overall improvement in 68.3% on magnesium oxide and 69.2% on senna against 11.7% on placebo. So oxide is a legitimate purchase for constipation. The argument against it is narrower than the internet suggests: it is a poor vehicle for closing a daily dietary gap, because you are leaning on about 4% absorption and living with the other 96%.

These two forms sit at opposite ends of the same trade-off. Magnesium oxide is the densest common form of the mineral — MgO is one magnesium atom bonded to one oxygen, so about 60% of the compound's weight is magnesium itself, which is why the cheapest bottle on the drugstore shelf can promise a big number on a small tablet. The catch is that it barely dissolves, and magnesium you cannot dissolve is magnesium you mostly do not absorb: the standard reference study on US commercial preparations put fractional absorption from oxide at roughly 4%, against 9-11% for chloride, lactate and aspartate. The NIH Office of Dietary Supplements says the same thing in plainer language — forms that dissolve well in liquid are absorbed more completely than less soluble ones, and oxide and sulfate specifically sit on the wrong side of that line. Magnesium glycinate is the opposite build: each magnesium atom is chelated to two glycine molecules, which drops the mineral content to about 14% by weight but makes it far more soluble and considerably gentler on the gut. What glycinate does not have is the pile of head-to-head absorption data its marketing implies. The NIH professional fact sheet does not even mention glycinate when it lists the better-absorbed forms, and in a 2024 crossover study that measured plasma magnesium after single doses, oxide produced a significant rise at one hour while bisglycinate produced no significant rise at all. Our call: for topping up daily intake, buy glycinate or citrate and pay attention to elemental dose and tolerance, not the label story. Buy oxide on purpose only if what you actually want is an osmotic laxative, which is a job it does well and has real trials behind it.

Reviewed by Stephen V. · Health & fitness enthusiastPublished October 6, 2026

Option A · Magnesium Oxide

Magnesium Oxide

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Option B · Magnesium Glycinate

Pure Encapsulations — Magnesium Glycinate (Capsules)

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The verdict

For daily supplementation, buy magnesium glycinate — or citrate, which is cheaper and has better absorption data behind it than glycinate does. Glycinate's case is that it dissolves, it is well tolerated at doses that would send oxide straight through you, and glycine itself is harmless and mildly useful. That is enough to make it the sensible default, and it is why the glycinate and bisglycinate products are the ones at the top of our magnesium roundup. But be clear about what you are buying: glycinate is the form with the best tolerance record, not the form with the deepest evidence base, and anyone selling it as dramatically superior absorption is going beyond what has actually been measured. Magnesium oxide is not a scam either — it is real magnesium, it is the cheapest elemental magnesium you can buy, and for chronic constipation it has placebo-controlled trials showing roughly a 68-70% response rate against 12-25% on placebo. That is a legitimate use. Using it as your daily magnesium top-up is the mistake: you are relying on about 4% absorption and accepting the loose stools that come from the 96% that stays in your gut. And before either bottle: magnesium is abundant in leafy greens, legumes, nuts, seeds and whole grains, you absorb 30-40% of what you eat, and food should be doing most of this work.

Side-by-side

Attribute	Magnesium Oxide	Magnesium Glycinate
What it is	Magnesium bonded to oxygen (MgO) — an inorganic magnesium salt	Magnesium chelated to two glycine molecules (magnesium bisglycinate)
Magnesium by weight	~60% (MgO is 40.3 g/mol, magnesium is 24.3 of it)	~14% (magnesium diglycinate is 172.4 g/mol)
Solubility in water	Very poor	Good — chelation is what buys the solubility
Measured fractional absorption	~4% in the standard US commercial-preparations study	No equivalent published figure; chelates are assumed better but under-measured
Named by NIH as better absorbed?	No — named as one of the poorly absorbed forms, with sulfate	Not named at all; NIH lists aspartate, citrate, lactate and chloride
GI tolerance	Poor — one of the forms most commonly reported to cause diarrhea	Good — the usual reason people switch to it and stay on it
Best-supported use	Osmotic laxative for constipation (68-70% response vs 12-25% placebo)	Daily intake top-up; one RCT shows a small insomnia-severity benefit
Typical cost	Cheapest magnesium per elemental milligram on the shelf	Several times more per elemental milligram
Pills to hit a given dose	Fewest — the dense form	More, or larger capsules, because the chelate is mostly glycine
Our call	Buy for constipation, not for daily magnesium	Buy this (or citrate) for daily magnesium
Where to buy	Check Magnesium Oxide on Amazon	$27.00Check Magnesium Glycinate on AmazonIn Stock

Prices pulled from Amazon as of October 18, 2026 and are subject to change. The price shown on Amazon at checkout applies.

Who should pick which

Pick Magnesium Oxide

Anyone whose actual goal is an osmotic laxative for constipation — oxide is cheap, it works by the mechanism you want, and it has randomized placebo-controlled trials supporting that use.
Budget buyers who tolerate it fine and are honest with themselves that they are buying 4%-absorbed magnesium cheaply rather than buying the better form.

Pick Magnesium Glycinate

Anyone taking magnesium daily to close a dietary gap — glycinate is soluble, well tolerated, and the form you can actually stay on for months.
People who tried a cheap magnesium and got diarrhea. That is the classic oxide experience, and switching form usually fixes it without lowering the dose.
Anyone taking magnesium in the evening as part of a sleep routine — the one decent bisglycinate RCT is a sleep trial, and glycinate's tolerability is what makes a nightly dose sustainable.
Readers who want a clean, minimal label: magnesium, glycine, nothing else, with the elemental dose stated on the panel.

The trade-off in one line: density versus solubility

Every magnesium supplement is a magnesium atom plus whatever is carrying it, and the carrier decides both how much mineral you get per pill and how much of it you absorb. Magnesium oxide is the most efficient carrier imaginable on paper: PubChem lists MgO at 40.3 g/mol, and magnesium accounts for 24.3 of that, so roughly 60% of the compound's weight is the mineral you want. Magnesium bisglycinate wraps each magnesium atom in two glycine molecules, giving a molecular weight of 172.4 g/mol and dropping the magnesium content to about 14%. Magnesium citrate sits between them at 451.1 g/mol for three magnesium atoms, or about 16%. That is why a drugstore oxide tablet can be small and still claim a large number while a glycinate serving is often two or three capsules. It is also why oxide is cheapest per milligram of elemental magnesium, and that is a real advantage, not a marketing trick. The problem is the second half of the equation. Oxide is barely soluble in water, and absorption in the gut depends on the mineral being in solution. The NIH Office of Dietary Supplements puts it directly: forms of magnesium that dissolve well in liquid are more completely absorbed in the gut than less soluble forms, and small studies have found the aspartate, citrate, lactate and chloride forms absorbed more completely and more bioavailably than magnesium oxide and magnesium sulfate. A dense form you do not absorb loses to a dilute form you do.

How bad is oxide absorption, really?

The number everyone quotes traces to one study, and it is worth knowing what it actually did. Firoz and Graber tested four US commercial preparations in 16 healthy adults — eight men and eight women — giving each roughly 21 mEq of magnesium as oxide, chloride, lactate or aspartate and measuring urinary excretion to estimate absorption. Fractional absorption from magnesium oxide averaged about 4%, while chloride, lactate and aspartate came in around 9-11% and were statistically indistinguishable from each other. The authors' conclusion was that magnesium oxide has relatively poor bioavailability while the other three are greater and equivalent. Two honest caveats, because one small study carrying this much weight deserves them. Sixteen subjects is not a large trial, and urinary excretion is an indirect proxy for absorption. More importantly, 4% of a large dose is not zero: in a 2024 double-blind crossover study in 40 adults that sampled plasma magnesium before and after single doses, magnesium oxide produced a statistically significant rise at one hour — while magnesium bisglycinate, in the same study, produced no significant rise at any timepoint. That study was funded by the maker of a competing microencapsulated ingredient, and single-dose plasma magnesium is a crude measure of what a daily supplement does to body stores, so do not read it as proof that oxide beats glycinate. Read it as evidence that the popular claim 'oxide does nothing' is overstated, and that glycinate's absorption advantage is assumed more often than it is measured.

What glycinate's evidence actually looks like

Glycinate is the form the internet recommends, and we recommend it too — but for tolerance rather than for a stack of absorption studies, because that stack does not exist. Notice what is missing from the NIH professional fact sheet: when it names the better-absorbed forms, it lists aspartate, citrate, lactate and chloride. Glycinate is not in that sentence, and the document does not discuss it anywhere. The best direct clinical evidence for the form is a 2025 randomized, double-blind, placebo-controlled trial in 155 adults aged 18-65 with self-reported poor sleep, which gave 250 mg of elemental magnesium as bisglycinate daily and tracked the Insomnia Severity Index. The supplement group improved more than placebo — a reduction of 3.9 points versus 2.3 — and the difference was statistically significant at p = 0.049, with a small effect size of d = 0.2. The authors noted that improvements looked larger in participants with lower baseline dietary magnesium, which is exactly what you would expect if you are correcting a shortfall rather than drugging a system. That is a real, honest result: a modest benefit in a decent-sized trial, concentrated in the people who were probably low to begin with. It is not the transformation the category sells. What glycinate unambiguously has over oxide is the tolerance profile. The forms most commonly reported to cause diarrhea, per NIH, include carbonate, chloride, gluconate and oxide — and the mechanism is simply osmotic: unabsorbed magnesium salt sitting in the intestine pulls water in. Less unabsorbed magnesium, less of that. If you have ever abandoned a magnesium supplement after four days, this is usually why, and this is what you are paying extra to avoid.

Oxide's real job: it is a laxative, and a good one

The most common mistake in this comparison is treating magnesium oxide as a failed daily supplement rather than a successful drug for a different indication. The same osmotic effect that makes it a poor daily top-up makes it an effective treatment for constipation, and unlike most of this category, that use has placebo-controlled randomized trials. A double-blind placebo-controlled trial in 34 women with chronic constipation found the primary endpoint met by 70.6% of the magnesium oxide group versus 25.0% on placebo, with significant improvements in Bristol stool form and in colonic transit time. A larger 2021 trial randomized 90 patients with chronic idiopathic constipation — mean symptom duration nearly ten years — to senna, magnesium oxide or placebo, and reported overall improvement in 68.3% of the oxide group and 69.2% of the senna group against 11.7% on placebo. Magnesium's laxative role is also why it shows up in over-the-counter products at doses far above the supplemental upper limit: NIH notes that a tablespoon of Phillips' Milk of Magnesia delivers 500 mg of elemental magnesium as the hydroxide, with directions allowing up to four tablespoons a day, and that much of that magnesium is never absorbed precisely because of the laxative effect. So if your problem is constipation, oxide is cheap, evidence-backed and the right tool. If your problem is that your diet runs low on magnesium, using oxide means depending on about 4% absorption and tolerating the consequences of the rest — which is backwards.

The numbers that should actually drive your purchase

Three figures matter more than the form on the front of the bottle. First, the elemental dose. Supplement panels in the US are required to state the amount of magnesium, not the amount of compound, so read the line that says 'Magnesium (as magnesium bisglycinate) 200 mg' and ignore any larger number printed on the front — and check the serving size, because glycinate servings are frequently two or three capsules. Second, your target. The RDA is 400-420 mg a day for adult men and 310-320 mg for adult women, and that is total intake, food included; you absorb 30-40% of the magnesium in food, and leafy greens, legumes, nuts, seeds and whole grains are dense in it. Most people are closing a modest gap, not replacing an entire requirement. Third, the upper limit on the supplemental side: 350 mg a day of magnesium from supplements and medications for adults. That ceiling exists specifically because higher supplemental doses cause diarrhea, nausea and abdominal cramping, which is why a 500 mg 'high potency' capsule is not a better buy than a 200 mg one. Very high intakes can go further than discomfort — NIH lists hypotension, vomiting, difficulty breathing, irregular heartbeat and cardiac arrest among the signs of magnesium toxicity, with a few fatal cases of hypermagnesemia reported. Those are not risks you run from a sensible supplement, but they are the reason the 350 mg number exists and the reason to respect it. If you have kidney disease or take prescription medications, the dose question goes to your doctor rather than to a label.

What we'd actually buy

A glycinate or bisglycinate from a brand that does third-party testing, with the elemental dose on the panel, bought in a quantity you will finish — and food doing the heavy lifting underneath it. Our magnesium roundup's top picks are bisglycinate and glycinate products for exactly the reasons above: they dissolve, they are tolerable at a dose that matters, and their labels are honest about elemental content. Citrate is a completely defensible cheaper alternative with better published absorption data than glycinate, and it is slightly laxative, which some people want and some do not — that trade-off is what our glycinate versus citrate comparison is for. Skip the proprietary 'magnesium complex' blends that list five forms and no individual amounts, because the only way they make sense as a purchase is if you cannot see what you are getting. Skip buying oxide as a daily supplement; buy it if you want a laxative. And do not buy magnesium at all expecting it to fix energy, mood or sleep on its own if you are eating almost no greens, legumes, nuts or seeds. Magnesium is a mineral you are supposed to get from food, a supplement is for closing the remaining gap, and no form of it — however well chelated — substitutes for the diet, the training and the sleep schedule.

Frequently asked questions

For daily supplementation, yes — mainly on solubility and tolerance rather than on a decisive pile of absorption trials. Magnesium oxide is about 60% magnesium by weight but barely dissolves, and the standard study of US commercial preparations put fractional absorption from oxide at roughly 4% against 9-11% for chloride, lactate and aspartate. The NIH Office of Dietary Supplements names oxide and sulfate as the less completely absorbed forms. Oxide is also one of the forms most commonly reported to cause diarrhea, because unabsorbed magnesium salt pulls water into the intestine. Glycinate is about 14% magnesium by weight, dissolves well, and is far easier to stay on for months. Be aware of what is thin, though: NIH does not list glycinate among the better-absorbed forms at all, and a 2024 crossover study found a significant plasma magnesium rise after oxide but not after bisglycinate. Glycinate is the better daily choice; it is not the dramatic absorption upgrade the marketing suggests.

Sources

Pure Encapsulations — Magnesium Glycinate (Capsules)

$27.00 · on Amazon

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Magnesium Oxide

Pure Encapsulations — Magnesium Glycinate (Capsules)

The verdict

Side-by-side

Who should pick which

Pick Magnesium Oxide

Pick Magnesium Glycinate

The trade-off in one line: density versus solubility

How bad is oxide absorption, really?

What glycinate's evidence actually looks like

Oxide's real job: it is a laxative, and a good one

The numbers that should actually drive your purchase

What we'd actually buy

Frequently asked questions

Related reading

Sources