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Comparative guide · 5 min read

Magnesium Citrate vs Magnesium Oxide — absorption, bowel effect, cost

Updated 2026-05-13 · Reviewed by SupplementScore editors · No sponsorships

These are the two cheapest magnesium forms on the shelf, and they're routinely mixed up. Magnesium oxide has the highest elemental magnesium content per gram (60%) but the lowest bioavailability — most of it draws water into the colon and produces a laxative effect. Magnesium citrate has lower elemental content (~11%) but better absorption; it still has a laxative effect at higher doses but less than oxide per equivalent elemental dose. For raising serum or tissue magnesium, citrate beats oxide. For occasional constipation relief or as a colonoscopy prep adjunct, oxide is cheaper and more effective at producing bowel movement.

Quick verdict

GoalBetter choiceWhy
Correcting low magnesium / general supplementationCitrateBetter-absorbed; trial-validated for serum magnesium correction.
Acute constipation reliefOxide (short-term)Most of the oral dose stays in the gut and produces an osmotic laxative effect.
Chronic daily use without GI side effectsCitrate (or glycinate)Citrate is tolerable at moderate doses; for chronic dosing without any laxative effect, glycinate is even better.
Migraine preventionCitrate (or glycinate)Trial doses are 400–600 mg/day; oxide at this dose produces diarrhea in most users.
Cost per elemental mgOxide (per gram)Cheapest source if absorption isn't the constraint.
Cost per absorbed elemental mgCitrate (better real value)Per-pill price hides oxide's much lower absorbed fraction.

How they actually work

Magnesium oxide — high elemental, low absorption, osmotic laxative

Magnesium oxide is 60.3% elemental magnesium by weight — the highest of any common supplemental form. But oxide salts are poorly soluble in water and dissociate slowly in the gut, so most of an oral oxide dose remains in the lumen, draws water osmotically, and accelerates colonic transit. Trial bioavailability of oxide is around 4–10% in fasted healthy adults — meaning a 500 mg oxide tablet (containing ~300 mg elemental Mg) might deliver 12–30 mg absorbed. The rest is laxative.

Magnesium citrate — lower elemental, better absorption

Magnesium citrate is approximately 11% elemental Mg by weight. The citrate counter-ion is well-absorbed across the GI tract, and trial bioavailability is roughly 25–40% — substantially better than oxide. A 500 mg citrate tablet (containing ~55 mg elemental Mg) might deliver 15–22 mg absorbed — comparable absolute absorbed magnesium to oxide per pill, but with proportionally fewer GI effects per pill because more is absorbed and less remains in the lumen.

Other common forms compared

Glycinate (bisglycinate): chelated form. ~14% elemental Mg. Best tolerated for chronic dosing; minimal laxative effect at typical doses. Best for "I want to top up magnesium daily without GI side effects" — see magnesium glycinate vs threonate for the deeper dive.

L-threonate: brain-penetrant form for cognition; expensive and over-marketed.

Malate: well-absorbed; mild fatigue / fibromyalgia evidence at high doses.

Sulfate (Epsom salt): rapid laxative; rarely used for nutritional supplementation orally.

Aspartate, orotate, lactate: well-absorbed; not commonly trialed.

Constipation — when oxide is actually the right answer

For occasional constipation relief, 400–500 mg of magnesium oxide once daily for 1–3 days is a cheap and effective osmotic laxative. The Mori 2021 RCT in chronic constipation showed magnesium oxide at 1000–1500 mg/day was superior to placebo and broadly comparable to other osmotic laxatives. Use short-term; chronic high-dose oxide in the elderly or in users with kidney impairment risks hypermagnesaemia.

Migraine, sleep, muscle cramp — when oxide is the wrong answer

Migraine prevention trials use 400–600 mg/day elemental magnesium. At that dose, oxide produces diarrhea in most users; citrate is more tolerable; glycinate is the most tolerable. For these chronic-dosing indications, citrate or glycinate is the right choice — not oxide.

Dose, form, and timing

Citrate: 200–300 mg elemental Mg/day (from ~2000–3000 mg citrate salt) split into 2–3 doses with meals. Allow for some GI adjustment in the first week.

Oxide: 250–500 mg single dose for occasional constipation. Avoid chronic high-dose use unless monitored. Take with water on an empty stomach or as instructed for acute use.

Safety profile

Both: caution in significant kidney impairment (eGFR <30 — risk of hypermagnesaemia). Oxide more risky for hypermagnesaemia at high doses in elderly with even mild renal decline. Both can reduce absorption of tetracyclines, fluoroquinolones, bisphosphonates, levothyroxine — separate by 2–4 hours.

Practical rule. For daily magnesium supplementation, use citrate (or glycinate). For occasional constipation relief, oxide is a cheap, effective short-term osmotic laxative. Don't use oxide as your primary daily magnesium — the per-pill cost looks attractive but the absorbed fraction is small and the GI effects are large.

Who should pick each

Pick citrate if: you want a daily magnesium supplement at moderate cost with reasonable absorption, you tolerate mild laxative effect.

Pick oxide if: occasional constipation, short-term use, lowest cost, and you understand most of the dose stays in the bowel rather than being absorbed systemically.

Pick glycinate if: chronic daily use without any laxative effect — particularly for sleep, migraine, or muscle cramps.

What we'd actually buy

For daily supplementation: magnesium citrate 200 mg elemental at dinner, or magnesium glycinate 200 mg at bedtime if also addressing sleep. For occasional constipation: keep a bottle of magnesium oxide 400 mg tablets in the cabinet for short-term use. Total cost: under $10/month for citrate or glycinate daily use.

Sources

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