Magnesium citrate is better absorbed than magnesium oxide, and the difference is significant. In head-to-head studies, citrate consistently produces higher blood levels of magnesium and more urinary excretion (the standard measure of how much actually made it into the body). But “better” depends on what you’re using it for, because the two forms have distinct strengths that make each one the right choice in different situations.
How Absorption Compares
The core difference comes down to solubility. Magnesium oxide is virtually insoluble in water and only 43% soluble even in simulated peak stomach acid. Magnesium citrate, by contrast, is 55% soluble in plain water and substantially more soluble than oxide at every level of acid secretion. That gap matters because your body can only absorb what dissolves first.
A 2020 study in healthy men confirmed this plays out in practice. Researchers gave participants 400 mg of magnesium from either citrate or oxide and tracked blood and urine levels over 24 hours. Citrate raised plasma magnesium significantly at every time point measured, while oxide failed to produce a meaningful increase at all. Urinary magnesium excretion, the gold-standard marker of absorption, was significantly higher after citrate than after oxide.
If your goal is to raise your magnesium levels, citrate delivers more of the mineral into your bloodstream per dose.
Why Magnesium Oxide Still Exists
Oxide has one major advantage: it packs far more elemental magnesium into each milligram of supplement. Magnesium oxide is about 60% magnesium by weight, while magnesium citrate is only about 16%. That means an oxide tablet can contain roughly four times more actual magnesium in the same size pill.
This is why oxide remains popular in cheap, widely available supplements. A single 400 mg oxide tablet delivers around 240 mg of elemental magnesium. You’d need a much larger dose of citrate (or multiple capsules) to match that on paper. Of course, the higher elemental content is partially offset by the lower absorption rate, but it does make oxide more practical for people who want fewer pills.
Oxide is also the least expensive option. Generic magnesium oxide tablets run about $0.06 to $0.07 per tablet for a 400 mg dose, and it’s available at virtually every pharmacy and grocery store without needing to visit a specialty supplement aisle.
Digestive Side Effects
Both forms can cause loose stools, but citrate is more likely to do so. In one clinical trial comparing several magnesium forms, about 10% of participants taking citrate (5 out of 48) reported mild diarrhea, compared to roughly 2% taking oxide (1 out of 48). Two participants on citrate dropped out due to gastrointestinal symptoms.
This makes sense given citrate’s higher solubility. Magnesium that dissolves readily in the gut draws water into the intestines through osmosis, which softens stool and speeds transit. This is the same mechanism that makes liquid magnesium citrate a popular bowel prep before colonoscopies. At normal supplement doses the effect is usually mild, but people with sensitive stomachs may notice it.
Oxide, because it’s poorly soluble, tends to pass through the gut without pulling as much water along with it. For people prone to diarrhea, oxide may actually be the gentler option.
Which Form for Specific Health Goals
Correcting a Deficiency
If blood work shows you’re low on magnesium, citrate is the stronger choice. Its superior absorption means more of each dose reaches your bloodstream. You’ll likely need fewer total milligrams to bring your levels up compared to oxide.
Migraine Prevention
The Canadian Headache Society recommends magnesium for migraine prevention at 600 mg per day. Their guidelines specifically recommend citrate (or glycinate) over oxide, citing better absorption and efficacy. Migraine Canada echoes this, noting that oxide’s poor bioavailability limits its usefulness for this purpose.
Constipation Relief
Citrate’s ability to draw water into the intestines makes it the preferred form for occasional constipation. This is a case where the “side effect” is the point. Liquid magnesium citrate typically produces a bowel movement within 30 minutes to 6 hours. Oxide can also have a laxative effect at high doses, but citrate is more reliable for this purpose.
General Daily Supplementation
For people who simply want to fill a dietary gap and aren’t dealing with a specific deficiency, oxide can work. The lower absorption is partially compensated by the higher elemental magnesium content per pill, and the cost savings add up over months of daily use. It’s a reasonable choice if you’re looking for basic maintenance rather than therapeutic dosing.
The Elemental Magnesium Trap
One common source of confusion is the number on the bottle. A supplement labeled “500 mg magnesium oxide” does not contain 500 mg of magnesium. It contains 500 mg of the compound, of which about 300 mg is elemental magnesium. And of that 300 mg, your body may absorb a fraction.
Citrate labels can be equally misleading. A “500 mg magnesium citrate” capsule contains only about 80 mg of elemental magnesium. The rest of the weight is the citrate molecule attached to it. So while citrate is better absorbed percentage-wise, you’re starting from a smaller pool of actual magnesium per capsule. Always check the supplement facts panel for “elemental magnesium” or “magnesium (as citrate)” to see the real number.
Choosing Between the Two
Citrate wins on absorption, and that matters most when you’re trying to correct low levels, manage migraines, or relieve constipation. Oxide wins on convenience and cost, packing more magnesium into fewer pills at a lower price. For someone eating a reasonably balanced diet who just wants a low-cost insurance policy, oxide is fine. For someone with a diagnosed deficiency or a specific condition that responds to magnesium, citrate (or glycinate) is worth the extra cost and the larger pill count.
People who take acid-reducing medications like proton pump inhibitors have another reason to choose citrate. Since oxide depends heavily on stomach acid to dissolve, reduced acid production makes an already poorly absorbed form even less effective. Citrate dissolves well even in water, so it’s less affected by low stomach acid.