Best Magnesium for Constipation: Which to Take and How Much
The best magnesium for constipation is magnesium oxide, the only magnesium supplement tested in trials: 68% of people improved on it vs 19% on a placebo, at about 900 mg a day. Our pick is Nature Made Magnesium Oxide. Start with one 250 mg tablet a day.
By Erin Rose Updated October 19, 2026 How we score
Costs are brand prices per 300 mg of magnesium, checked October 14, 2026.
Our pick for: Ongoing constipation
Nature Made Magnesium Oxide
7.0/10 Our score
Oxide is the only magnesium supplement tested against a placebo for constipation
- Magnesium
- 250 mg1 tablet, as oxide
- Start with
- 1 tablet a dayWith water and a meal, as the label says
- In the trials
- 68% vs 19%Improved on oxide vs a placebo
- How fast
- Not timed in trialsThey ran 4 weeks. For today, see the drink below
- Per 300 mg
- $0.07Brand price; Amazon’s can differ
- Skip it if
- You have kidney diseaseUnless your doctor says yes
Why we pick it Oxide is the only magnesium supplement tested against a placebo for constipation, and a 2023 guideline from two US groups of digestive doctors suggests it. It costs the least per mg of any magnesium we price.
What you give up The trials used about 900 mg of magnesium a day (1.5 g of oxide), far more than one tablet. If one tablet is not enough, ask your doctor about taking more. It is not on a public testing list.
Which magnesium is right for you?
Need to go today? Use the citrate drink.
A bottle of magnesium citrate oral solution from the drugstore is a laxative, not a supplement. Its label says it usually works in 30 minutes to 6 hours. A 10 oz bottle holds about 2,800 mg of magnesium, so it is a one-time fix. The label says not to use it for more than 1 week unless a doctor tells you to. If your doctor gave you a bottle to clear your bowel before a colonoscopy, follow those steps instead. That is a different job and a different dose.
Want magnesium citrate you can measure?
BulkSupplements Magnesium Citrate Powder is plain magnesium citrate with nothing added. It is on NSF/ANSI 173, which checks that the bag holds what the label says. A powder lets you start small and add a little at a time. No trial has tested citrate pills or powder for constipation, so treat it as worth a try, not a sure fix. Its label serving (3.5 g) gives 400 mg, over the 350 mg limit, so weigh about 2.6 g on a small kitchen scale for 300 mg.
BulkSupplements Magnesium Citrate Powder
$0.12 per 300 mg at the brand’s price · NSF/ANSI 173
Want citrate in a tested pill?
Nature Made Magnesium Citrate is on USP Verified. Each softgel holds 125 mg, so you can start with one and go up to the label’s 2 a day (250 mg).
Nature Made Magnesium Citrate
$0.47 per 300 mg at the brand’s price · USP Verified
When to call a doctor instead
If you have kidney disease, ask your doctor before you take magnesium. Weak kidneys cannot clear the extra, so it can build up (NIH). Ask first, too, if a doctor has told you to limit magnesium. This page is for adults. If you are pregnant or breastfeeding, ask your doctor before you take magnesium for constipation.
Stop and call your doctor if you see blood in your stool or have no bowel movement after a laxative (drug label). Call too if your bowel habits change suddenly and stay that way for more than 2 weeks (MedlinePlus).
Taking oxide every day for more than about 5 weeks, at high doses or with weak kidneys, made too much magnesium in the blood more likely in one hospital study (Wakai 2019). If you are older or need it that long, check in with your doctor (Mori 2021). Magnesium also has to be taken hours apart from some antibiotics and bone drugs (NIH), and apart from thyroid pills (Attinger 2025): magnesium side effects.
Magnesium for constipation, compared
Three products, our pick first. Costs are per 300 mg of magnesium at each brand’s own price. Scores compare products of one form only, and we do not score the powder.
| Magnesium | Per 300 mg | Why it is or is not the pick |
|---|---|---|
| Nature Made Magnesium Oxide7.0/10 | $0.07 | Our pick: the form with the trials, 250 mg a tablet, the lowest cost |
| BulkSupplements Magnesium Citrate PowderNot scored | $0.12 | Citrate powder on NSF/ANSI 173; no constipation trial |
| Nature Made Magnesium Citrate9.0/10 among citrates | $0.47 | Citrate softgels on USP Verified, 125 mg each; no constipation trial |
How much magnesium to take for constipation
Start with the label dose: one 250 mg oxide tablet, or about 300 mg of citrate. That keeps you under 350 mg a day, the limit NIH sets for magnesium from supplements and medicines. Take it with a full glass of water. If one tablet is not enough, talk to your doctor before you take more. The 2023 guideline says to start low and raise the dose only as needed. The two trials behind it were small: 94 people in all, mostly women, in Japan, for 4 weeks. Amounts by age: how much magnesium per day.
Oxide or citrate for constipation?
Both work the same way. Magnesium that your gut does not absorb pulls water into the bowel and softens stool (NIH). Oxide is mostly not absorbed, which makes it a good laxative and a poor way to raise your magnesium. Citrate is absorbed better, so less of each dose stays in the gut. The trials and the guideline back oxide; for citrate pills and powders, the guideline calls the effect unknown. Magnesium is not the only choice, either. The same guideline backs fiber about as much as magnesium oxide, and gives its strongest backing to polyethylene glycol, a drugstore laxative powder. Absorption and cost side by side: magnesium citrate vs oxide. Stools too loose? See magnesium side effects.
Questions people ask
Does magnesium citrate work for constipation?
As a drink, yes: magnesium citrate oral solution is sold for occasional constipation (MedlinePlus). As a daily pill or powder, no trial has tested it. Citrate can still loosen stools at higher doses, so it may help some people.
How long does magnesium take to work for constipation?
The citrate drink: 30 minutes to 6 hours, by its label. Oxide tablets: the trials measured them over 4 weeks, and people had more bowel movements each week than on a placebo. No study gives a time for one supplement dose.
Can I take magnesium every day for constipation?
One 250 mg tablet a day is a normal supplement dose. More than that every day should be your doctor’s call: the guideline notes there is no long-term safety data. Do not use the citrate drink for more than a week unless a doctor says so.
Is magnesium glycinate good for constipation?
Not for this. People choose glycinate because it is gentle on the stomach, and the guideline lists its effect on constipation as unknown. To help you go, choose oxide or citrate.
How we score
Five marks: does a serving hold a studied dose; is the product on a public testing list; what 300 mg of magnesium costs at the brand’s own price against the cheapest of its form; does the label state the amount; and the brand’s FDA recall record. Dose and testing are worth up to 3 points each, cost up to 2, label and recalls up to 1 each, for 10 in all. More on our method.
Sources
- Chang L, et al. “American Gastroenterological Association-American College of Gastroenterology Clinical Practice Guideline: Pharmacological Management of Chronic Idiopathic Constipation.” Gastroenterology. 2023. PubMed 37211380
- Mori S, et al. “A Randomized Double-blind Placebo-controlled Trial on the Effect of Magnesium Oxide in Patients With Chronic Constipation.” J Neurogastroenterol Motil. 2019. PubMed 31587548
- Morishita D, et al. “Senna Versus Magnesium Oxide for the Treatment of Chronic Constipation: A Randomized, Placebo-Controlled Trial.” Am J Gastroenterol. 2021. PubMed 32969946
- van der Schoot A, et al. “The effect of food, vitamin, or mineral supplements on chronic constipation in adults: A systematic review and meta-analysis of randomized controlled trials.” Neurogastroenterol Motil. 2023. PubMed 37243443
- Wakai E, et al. “Risk factors for the development of hypermagnesemia in patients prescribed magnesium oxide: a retrospective cohort study.” J Pharm Health Care Sci. 2019. PubMed 30805197
- Mori H, Tack J, Suzuki H. “Magnesium Oxide in Constipation.” Nutrients. 2021. PubMed 33525523
- Attinger MC, et al. “Single Center, Open-Label, Randomized Crossover Trial on Drug-Drug Interactions of Levothyroxine/Magnesium-Citrate and Levothyroxine/Magnesium-Aspartate in Healthy Subjects-The ThyroMag Trial.” Clin Transl Sci. 2025. PubMed 41221788
- MedlinePlus. Magnesium citrate (saline laxative).
- Magnesium citrate oral solution, Drug Facts label. DailyMed.
- NIH Office of Dietary Supplements. Magnesium fact sheet for health professionals (updated January 6, 2026).
- Labels and prices: Nature Made and BulkSupplements pages, checked October 14, 2026 and October 19, 2026. Testing lists: NSF/ANSI 173 and USP Verified, read October 19, 2026.