Quick Facts
- Evidence LevelStrong (pharmacological effect)
- Improvement Rate~70% vs 12-25% placebo
- Form RequiredMagnesium oxide (poorly absorbed = the point)
- Dosage1.5 g/day MgO (0.5g x 3)
- Duration28 days in studies
- Comparable ToSenna (69% vs 68% improvement)
Key Studies
Senna vs Magnesium Oxide for Chronic Constipation
The larger of the two RCTs. MgO 1.5 g/day achieved a 68.3% symptom improvement rate, virtually identical to senna (69.2%), and dramatically higher than placebo (11.7%, p<0.0001). Both treatments significantly improved bowel movement frequency and quality of life. Zero severe adverse events.[1]
Magnesium Oxide in Chronic Constipation
MgO 1.5 g/day (split into 3 doses) showed 70.6% overall improvement vs 25.0% placebo. Significantly improved spontaneous bowel movements, Bristol stool form scores (softer, easier-to-pass stools), and colonic transit time (food moved through the colon faster).[2]
Why Magnesium Oxide — Not Glycinate or Citrate
This is the one application where magnesium oxide's low bioavailability is an advantage. Because MgO is poorly absorbed (~4%), most of it stays in the intestinal lumen where it:
- Draws water into the colon through osmosis (osmotic laxative effect)
- Softens stool by increasing water content
- Stimulates peristalsis through intestinal distension
Well-absorbed forms like glycinate or citrate wouldn't work as well for constipation because they're absorbed into the bloodstream before reaching the colon in significant quantities. Citrate has a mild laxative effect at high doses, but MgO is more reliable for this purpose.
Dosing Protocol
Both RCTs used 1.5 g/day of magnesium oxide split into 3 doses (0.5g each) for 28 days. This is substantially higher than the supplemental UL of 350 mg/day elemental magnesium, but it's important to note:
- The UL applies to absorbed magnesium. Since MgO has ~4% bioavailability, only ~60 mg of elemental Mg is actually absorbed from 1.5g MgO
- Magnesium hydroxide (Milk of Magnesia) is FDA-approved as an OTC laxative at similar or higher doses
- Zero severe adverse events were reported in either trial
Safety
- Main side effect: Diarrhea (which is essentially the intended effect taken further than desired). Dose titration helps.
- Electrolyte concerns: Long-term daily use can affect electrolyte balance. Periodic use is generally safer than chronic daily use.
- Kidney disease: People with impaired renal function should avoid high-dose magnesium — the kidneys can't adequately excrete excess.
- Drug interactions: High-dose MgO can reduce absorption of many medications. Separate by 2+ hours.
The Bottom Line
Magnesium oxide for constipation has strong evidence — two double-blind RCTs showing ~70% improvement vs ~12–25% placebo. It works as well as senna and is available over the counter. This is a pharmacological laxative effect, not a nutritional supplement effect — the mechanism is osmotic, not correcting a magnesium deficiency.
For chronic constipation, it's one of the better-supported OTC options. For occasional constipation, the evidence supports it as one of the better-studied OTC options. For long-term management, discuss with a gastroenterologist to rule out underlying causes.