The 2 placebo-controlled constipation trials in this file used magnesium oxide, not citrate: 1.5 g a day, which is about 905 mg of magnesium, or 2.6 times the 350 mg upper limit for supplements. In both, 68–71% of people on oxide counted as responders after 28 days, against 12–25% on placebo.
What the studies measured
Of the 8 papers below, published 2005 to 2022, 5 involve magnesium citrate: 3 are one-day bowel preparations before colonoscopy, 1 is a 4-week course mixed with a stimulant, and 1 is a case of magnesium overload. Bowel-preparation studies measure one large dose meant to empty the colon, so their results do not carry over to a daily capsule.
A 2019 trial in 34 women with mild-to-moderate chronic constipation measured overall response, bowel movements and colon transit time after 1.5 g of magnesium oxide a day (0.5 g three times) for 28 days: 70.6% responded, against 25.0% on placebo. Complete spontaneous bowel movements did not differ (P = 0.76).
A 2021 trial in 90 adults with chronic constipation compared 1.5 g of magnesium oxide, 1.0 g of senna and placebo for 28 days: 68.3%, 69.2% and 11.7% responded, and no severe side effects were reported.
A 2012 study in 17 analysed patients whose constipation had not responded to tegaserod or polyethylene glycol measured weekly complete bowel movements on half a sachet of Picolax (sodium picosulfate plus magnesium citrate) 3 times a week for 4 weeks: the count rose from 0.5 to 2.4 a week. There was no control group, and picosulfate is a stimulant.
| Year · journal · PMID | People | Salt and dose | Length | Measured | Result | Limitation |
|---|---|---|---|---|---|---|
| 2019 · J Neurogastroenterol Motil · 31587548 | 34 women, mild-to-moderate | Oxide, 1.5 g a day (about 905 mg magnesium) | 28 days | Overall response, bowel movements, transit time | 70.6% vs 25.0% responded; complete movements did not differ | Small; women only; Japan |
| 2021 · Am J Gastroenterol · 32969946 | 90 adults, 93% women | Oxide 1.5 g vs senna 1.0 g vs placebo | 28 days | Overall response, bowel movements, quality of life | 68.3% oxide, 69.2% senna, 11.7% placebo | Dose 2.6 times the supplement limit |
| 2014 · Clin Gastroenterol Hepatol · 24342746 | 244 women with functional constipation | Sulfate-rich mineral water, 0.5 or 1 L a day | 4 weeks | Share with less constipation, stool form | Week 2: 37.5% on 1 L vs 21.1% on low-mineral water; 0.5 L not significant | Magnesium per liter not in abstract; one author from the water's maker |
| 2012 · Hong Kong Med J · 23018066 | 17 analysed, constipation not responding to other drugs | Picolax: sodium picosulfate plus citrate, half sachet 3 times a week | 4 weeks | Complete bowel movements a week | 0.5 to 2.4 a week | No control group; contains a stimulant |
| 2011 · AJR Am J Roentgenol · 21512073 | 2,525 patients; 102 on citrate | 300 mL citrate solution plus bisacodyl | One-day prep | Residual stool and fluid, polyp detection | Citrate left more fluid; polyp detection similar | Bowel prep, not daily use |
| 2021 · Cureus · 35070543 | 51 patients | Citrate capsules plus bisacodyl vs 4 L GoLYTELY | One-day prep | Colon cleanliness, satisfaction | 100% adequate in both arms; satisfaction favored capsules | Small; capsule dose not in abstract |
| 2022 · R I Med J · 35211711 | 19,173 colonoscopies, 2010–2016 | Citrate, 2 or 3 bottles | One-day prep | Adequate cleansing | 2 bottles: 98.94% of screening exams | Retrospective, one practice |
| 2005 · Intern Med · 15942092 | 1 woman aged 76 with bowel obstruction | 34 g of citrate in one day | Case report | Blood magnesium, heart rhythm | Blood magnesium 16.6 mg/dL; very low blood pressure | Single case |
The arithmetic of citrate and oxide doses
Matching the 905 mg of magnesium in the oxide trials with citrate would take 5.6–8.0 g of magnesium citrate, or 8–11 size 00 capsules a day. This compares magnesium content only, not how the two salts act in the gut.
| Magnesium | Oxide needed | Citrate needed | Size 00 capsules of citrate |
|---|---|---|---|
| 200 mg (common serving) | 332 mg | 1,238–1,770 mg | 2–3 |
| 400 mg (common label figure) | 663 mg | 2,475–3,540 mg | 4–5 |
| 905 mg (oxide trials) | 1,500 mg | 5,600–8,007 mg | 8–11 |
Oxide is 60.3% magnesium. Citrate is 11.3–16.2%: trimagnesium dicitrate sits at the top, dibasic and hydrated forms at the bottom, and labels rarely say which. Capsule counts assume pure salt at 0.8 g/mL with no filler.
On the shelf, 135 of 212 on-market NIH DSLD labels that name citrate name it as the only magnesium salt, and 39 also name magnesium oxide (snapshot 2026-08-10). 5 of 62 capsule labels naming citrate alone declare more magnesium per capsule than pure citrate could fit even in a size 000 capsule; the label audit lists them.
Best magnesium citrate for constipation, by measurement
Within citrate, the cheapest ranked label is Best Naturals Magnesium Citrate 200 mg at $0.0344 per 100 mg of magnesium, against a median of $0.1159 across 12 ranked offers (prices observed 2026-10-26 to 2026-11-02). "Best" names a measurement here, not a bowel result.
| Measurement | Label | Number |
|---|---|---|
| Lowest price per 100 mg, within citrate | Best Naturals Magnesium Citrate 200 mg | $0.0344; $2.06 for 30 days at 200 mg |
| Lowest price with no oxide or other salt named | Best Naturals Magnesium Citrate 200 mg | $0.0344 per 100 mg |
| Fewest capsules or tablets for 200 mg | Best Naturals Magnesium Citrate 200 mg | 1 a day |
| Lowest price per 100 mg, within oxide (the trials' salt) | Nutricost Magnesium Oxide | $0.0148, among 5 ranked oxide offers |
Each price is compared only within its own salt; see the citrate ranking and the oxide ranking.
Safety
The upper limit for magnesium from supplements is 350 mg a day for adults, set in 1997 and listed by the NIH Office of Dietary Supplements; the oxide trials gave about 2.6 times that under study supervision. The 34 g of citrate in the 2005 case carried about 3.8–5.5 g of magnesium, and the woman had no kidney disease before. Talk to a clinician if constipation is new or lasting, comes with pain, blood or weight loss, or if you have kidney disease or take prescription medicines.