Free tool · GLP-1 nutrition

Which magnesium is which? Compare the forms on the shelf

Oxide, citrate, glycinate, threonate: the bottles all say magnesium, and they behave differently. Pick what you want help with, compare the forms side by side, and check how much magnesium your label actually holds. Every claim is sourced at the bottom of the page.

What do you want help with?

The forms

Every form

Every supplement binds magnesium to something else. That partner changes how much magnesium each gram holds, how well it dissolves, and how likely it is to loosen stools. Pick what you want help with to narrow the list.

Forms here

Supplement limit

350 mg a day

Most tested for constipation

Oxide

Head-to-head studies

Few and small

Tick up to four forms to compare them side by side. Numbers for how much magnesium each form holds are rough, because many products use forms with water bound in.

Side by side

	MagnesiumGlycinate (bisglycinate)	MagnesiumCitrate	MagnesiumOxide
Magnesium by weight	About 14%	About 16%	60%
500 mg of the compound holds	About 70 mg	About 80 mg	About 300 mg
Loosens stools	Less often reported	Likely, it is a laxative	Likely, it is a laxative
Absorption	Few direct studies in people. In the 2019 lab model of the gut, a glycinate product was among the ones that released and absorbed magnesium best. That result has not been checked in people.	Better than oxide in most small comparisons, though one study found no difference in the magnesium that reached the urine. How the tablet is made matters too: in the 2019 lab test, two citrate tablets released their magnesium well and a third released little.	Low. In one small US study only about 4% was absorbed. In a 2019 test of 15 products, the tablets and capsules made only of oxide released and absorbed the least, and small studies have found citrate, chloride, lactate and aspartate absorbed better.
Studied in people	Sleep. In a 2025 trial of 155 adults with poor sleep, 250 mg of magnesium a day as bisglycinate for 4 weeks lowered insomnia scores by 3.9 points, against 2.3 on placebo. The authors called the effect small. Side effects were no more common than on placebo.	As a laxative, the over-the-counter liquid generally works within 1/2 to 6 hours, and its label says to use it for no more than a week unless a doctor directs. In a 30-day study of 20 postmenopausal women with osteoporosis, 290 mg of magnesium a day as citrate slowed bone turnover.	Constipation. In a 28-day trial in 34 women, 70.6% improved overall on oxide against 25% on placebo, and in a 90-person trial 68.3% improved against 11.7%. The 2023 US gastroenterology guideline suggests it for chronic constipation and rates the evidence very low certainty.
When to take it	The sleep trial had people take it 30 to 60 minutes before bed.	As a laxative, the label says to drink a full glass of liquid with each dose and to take it 2 or more hours apart from other medicines.	The constipation trials gave it in divided doses through the day, for example 0.5 g three times a day.
With food?	We found no study that tested this form with food against without it.	One small study found more magnesium reached the urine when citrate or oxide was taken with meals than on an empty stomach, so a meal is a reasonable default.	One small study found more magnesium reached the urine when citrate or oxide was taken with meals than on an empty stomach, so a meal is a reasonable default.

Tick up to four forms below. On narrow screens the table scrolls sideways.

Check your label

The Supplement Facts panel lists elemental magnesium, the magnesium itself. A number that refers to the whole compound, such as 500 mg of magnesium glycinate, is much bigger than the magnesium inside it. Enter that number here.

FormCompound, mg eachHow many a day

Magnesium in each

70 mg

Magnesium a day

70 mg

Of the 350 mg limit

20%

Glycinate (bisglycinate) is about 14% magnesium by weight, so 500 mg of it holds about 70 mg of magnesium. At 1 a day that is about 70 mg, which is within the 350 mg a day limit for magnesium from supplements.

Oxide, glycinate and L-threonate figures come from published studies. The others are worked out from each compound’s chemical formula, and products that bind water into the compound hold a little less. If the Supplement Facts panel already gives magnesium in mg, that is the number to use.

This is education, not medical advice

Magnesium is a dietary supplement, and laxative doses are a medicine. If you have kidney disease, take any of the medicines listed below, are pregnant or breastfeeding, or have constipation with belly pain, nausea or vomiting, ask your care team before you start. No product is named or ranked here, and nobody paid to be on this page.

Magnesium and a GLP-1

Both gut side effects are common

In the trials behind the Wegovy label, 24% of adults on semaglutide 2.4 mg reported constipation against 11% on placebo, and 30% reported diarrhea against 16%. Oxide, citrate and Epsom salt are laxatives. They suit the first problem and make the second one worse, so the side effect you have right now decides whether they belong on the list at all.

No trial in GLP-1 users yet

We could not find a published trial that tested a magnesium supplement in people taking a GLP-1 medication (PubMed search, October 2026). The constipation, sleep and cramp evidence on this page comes from people who were not on these medicines.

Eating less can mean less magnesium

In a 2025 study of 69 people who had taken a GLP-1 for at least a month, three-day food records showed an average of 266 mg of magnesium a day, under the recommended amount. It was small, it measured diet only, and all the authors work for a supplement retailer, so read it as a reason to look at your own food first.

Losing fluids changes the picture

The Wegovy label warns that diarrhea, nausea and vomiting can cause dehydration that may lead to kidney problems. Healthy kidneys clear extra magnesium, and the NIH notes that the risk of too much magnesium rises when kidney function drops. That is a reason to hold off on magnesium laxatives during a bout of vomiting or diarrhea and to call your prescriber.

How much, and the limit

These are the US recommended daily amounts for adults, from all sources together.

Women, 19 to 30

310 mg

Women, 31 and up

320 mg

Men, 19 to 30

400 mg

Men, 31 and up

420 mg

The 350 mg limit is for supplements only

For adults, the upper limit for magnesium from supplements and medicines is 350 mg a day. Magnesium in food does not count toward it, because healthy kidneys clear what the body does not need. That is why the limit can sit below the recommended amount: the recommendation counts food, and the limit does not. During pregnancy the recommendation is 350 to 360 mg a day for adults, and the 350 mg supplement limit stays the same.

What too much looks like

The first sign of too much from supplements is usually diarrhea, often with nausea and cramping. Very large doses, typically from laxatives and antacids giving more than 5,000 mg a day, have caused magnesium toxicity, with low blood pressure, muscle weakness, trouble breathing and an irregular heartbeat. A few cases have been fatal.

Food first

The body absorbs about 30 to 40% of the magnesium in food. Some of the richest sources are small portions, which helps when appetite is low: an ounce of roasted pumpkin seeds has 156 mg, an ounce of chia seeds 111 mg, an ounce of dry roasted almonds 80 mg, half a cup of boiled spinach 78 mg, and half a cup of cooked black beans 60 mg. Many of these foods are also fiber sources, and the fiber calculator can help you build a day around them.

How sure is the absorption ranking?

Thinner than the marketing suggests

A 2021 systematic review searched four databases and found 433 studies, of which only 14 compared magnesium forms in a way it could use. It concluded that inorganic forms such as oxide seem less well absorbed than organic forms such as citrate, that the share absorbed depends on the dose, and that any of these supplements can keep levels normal in healthy people who are not already low.

Studies measure it differently, and disagree

Absorption is usually judged by the magnesium that turns up in blood or urine after a dose. Less than 1% of the body’s magnesium is in the blood, and studies of the same two forms, citrate and oxide, have come out both ways depending on how they measured. Most of the head-to-head work is small and short.

How the pill is made matters too

In a 2019 test of 15 products, how well each one dissolved predicted absorption better than how much magnesium it held. A tablet with 196 mg of magnesium raised blood levels more than an oxide tablet with 450 mg, and two citrate tablets performed very differently from each other. The study was funded by the maker of the best-performing product.

Soaks and sprays

Epsom salt baths and magnesium sprays are sold on the claim that magnesium soaks in through the skin. A 2017 review called that claim scientifically unsupported. The best known Epsom bath study, which reported higher blood magnesium after daily baths, was published on an industry website and never in a peer-reviewed journal. A warm soak can still be pleasant. The evidence that it raises your magnesium is weak.

Medicines to space it from

Magnesium can bind to some medicines in the gut, and some medicines lower magnesium. If you take any of these regularly, the NIH advises talking with your care team about your magnesium.

Tetracycline and quinolone antibiotics

Magnesium forms insoluble complexes with antibiotics such as doxycycline, ciprofloxacin and levofloxacin. Take the antibiotic at least 2 hours before, or 4 to 6 hours after, a magnesium supplement.

Bisphosphonates for bone, such as alendronate

Magnesium-rich supplements can reduce how much of the medicine is absorbed. Keep them at least 2 hours apart.

Levothyroxine for thyroid

Its label lists antacids made with magnesium hydroxide among products that can reduce its absorption, and tells people to keep iron, calcium and antacids 4 hours away from it. The label does not name magnesium supplements, so ask your pharmacist how to time them.

Semaglutide tablets

The label says to take the tablet on an empty stomach with up to 4 ounces of water, then wait at least 30 minutes before eating, drinking or taking other oral medicines. Keep magnesium out of that window too.

Laxative doses with any medicine

The magnesium citrate and Epsom salt laxative labels both say to take them 2 or more hours before or after other drugs.

Medicines that lower magnesium

Proton pump inhibitors for reflux, such as esomeprazole and lansoprazole, can cause low magnesium when taken for more than about a year. In the cases the FDA reviewed, supplements often brought levels back up, and in 25% they did not and the medicine had to be stopped. Loop and thiazide diuretics, such as furosemide and hydrochlorothiazide, increase the magnesium lost in urine. Potassium-sparing diuretics such as spironolactone do the opposite and reduce it.

Talk to your care team first if this is you

Kidney disease or reduced kidney function

This matters most. The kidneys are what remove extra magnesium, and the risk of magnesium toxicity rises when they work less well. Both the citrate and Epsom salt laxative labels say to ask a doctor before use if you have kidney disease.

Constipation with belly pain, nausea or vomiting

The laxative labels say to ask a doctor first in that case. The Wegovy label lists severe constipation, including fecal impaction, and intestinal blockage among reports since approval, so that combination is a call to your prescriber.

Type 2 diabetes

The NIH lists type 2 diabetes among the conditions linked to extra magnesium loss in urine, and the American Diabetes Association finds too little evidence to use magnesium to improve blood sugar control.

Pregnant or breastfeeding

The laxative labels say to ask a health professional before use.

Doses above 350 mg a day

Higher doses, such as those used for migraine prevention, go above the supplement limit, and the NIH says they should be used only under the direction of a health care provider.

Sources

23 references

NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Updated January 6, 2026. ods.od.nih.gov. Source of the recommended amounts by sex, age and pregnancy, the 350 mg adult limit for supplements and medicines and why food does not count, the 30 to 40% absorbed from food, the food values, the Supplement Facts panel listing elemental magnesium, aspartate, citrate, lactate and chloride absorbing better than oxide and sulfate, the forms most often reported to cause diarrhea and the osmotic laxative effect, laxative doses above the limit, toxicity and its link to kidney function, serum magnesium as a poor measure of body stores, deficiency signs including numbness, tingling and cramps, type 2 diabetes and urinary loss, the chloride, aspartate and citrate trials, the American Diabetes Association position, migraine doses needing supervision, and the antibiotic, bisphosphonate, diuretic and proton pump inhibitor interactions.
Pardo MR, Garicano Vilar E, San Mauro Martín I, Camina Martín MA. Bioavailability of magnesium food supplements: a systematic review. Nutrition. 2021;89:111294. doi:10.1016/j.nut.2021.111294. Source of the 433 studies found and 14 included, inorganic forms seeming less bioavailable than organic, dose-dependent absorption, and any supplement keeping levels normal in healthy people without a deficit.
Blancquaert L, Vervaet C, Derave W. Predicting and testing bioavailability of magnesium supplements. Nutrients. 2019;11(7):1663. doi:10.3390/nu11071663. Source of the 15-product lab test, oxide-only products performing worst, the citrate and glycinate product results, the 196 mg versus 450 mg comparison, solubility predicting absorption better than magnesium content, oxide at 60% magnesium, earlier urine-based studies of citrate and oxide disagreeing, less than 1% of body magnesium in blood, and the funding by the maker of the best-performing product.
Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnesium Research. 2001;14(4):257–262. PubMed 11794633. Source of the 4% fractional absorption of oxide and the higher, similar absorption of chloride, lactate and aspartate.
Lindberg J, Harvey J, Pak CY. Effect of magnesium citrate and magnesium oxide on the crystallization of calcium salts in urine: changes produced by food-magnesium interaction. The Journal of Urology. 1990;143(2):248–251. doi:10.1016/s0022-5347(17)39924-x. Source of the finding that more magnesium reached the urine when citrate or oxide was taken with meals than on an empty stomach.
Chang L, Chey WD, Imdad A, et al. American Gastroenterological Association-American College of Gastroenterology clinical practice guideline: pharmacological management of chronic idiopathic constipation. Gastroenterology. 2023;164(7):1086–1106. doi:10.1053/j.gastro.2023.03.214. Source of the conditional recommendation for magnesium oxide in chronic constipation, with very low certainty of evidence.
Mori S, Tomita T, Fujimura K, et al. A randomized double-blind placebo-controlled trial on the effect of magnesium oxide in patients with chronic constipation. Journal of Neurogastroenterology and Motility. 2019;25(4):563–575. doi:10.5056/jnm18194. Source of the 34-woman, 28-day trial, the 0.5 g three times a day dose, and the 70.6% versus 25% overall improvement.
Morishita D, Tomita T, Mori S, et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. The American Journal of Gastroenterology. 2021;116(1):152–161. doi:10.14309/ajg.0000000000000942. Source of the 90-person trial, the 1.5 g a day dose, and the 68.3% versus 11.7% response.
Schuster J, Cycelskij I, Lopresti A, Hahn A. Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. Nature and Science of Sleep. 2025;17:2027–2040. doi:10.2147/NSS.S524348. Source of the 155-adult, 4-week trial, the 250 mg dose, 893 mg capsules holding 125 mg of magnesium, taking it 30 to 60 minutes before bed, the 3.9 versus 2.3 point change and small effect size, the possible larger benefit with low dietary intake, and side effects no more common than placebo.
Lopresti AL, Smith SJ. The effects of magnesium L-threonate […] on cognitive performance and sleep quality in adults: a randomised, double-blind, placebo-controlled trial. Frontiers in Nutrition. 2026;12:1729164. doi:10.3389/fnut.2025.1729164. Source of the 100-adult, 6-week trial, 2 g giving 145 mg of magnesium, the morning and evening dosing with or without food, the cognition and sleep findings, the animal studies behind the brain claim, and the funding by the ingredient’s maker.
Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review & meta-analysis. BMC Complementary Medicine and Therapies. 2021;21(1):125. doi:10.1186/s12906-021-03297-z. A correction was published in 2024. Source of the three trials in 151 older adults, the 17-minute shorter time to fall asleep, and the low to very low quality rating.
Garrison SR, Korownyk CS, Kolber MR, Allan GM, Musini VM, Sekhon RK, Dugré N. Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews. 2020;9:CD009402. doi:10.1002/14651858.CD009402.pub3. Source of the 11 trials and 735 people, the moderate-certainty finding for older adults, the unclear result in pregnancy, no trials in exercise cramps, and the common gut side effects.
Boulis M, Boulis M, Clauw D. Magnesium and fibromyalgia: a literature review. Journal of Primary Care & Community Health. 2021;12:21501327211038433. doi:10.1177/21501327211038433. Source of the malic acid and magnesium trials, the 24-person blinded trial with no improvement over placebo, and the absence of a blinded placebo-controlled trial of magnesium alone.
Dereli S. Effects of magnesium taurate-potassium citrate on blood pressure and vascular stiffness in low-risk adults with elevated blood pressure. Magnesium Research. 2026;38(4):118–125. doi:10.1684/mrh.2026.0552. Source of the single-arm, 121-adult, 3-month taurate study and its lack of a comparison group.
Stepura OB, Martynow AI. Magnesium orotate in severe congestive heart failure (MACH). International Journal of Cardiology. 2009;134(1):145–147. doi:10.1016/j.ijcard.2009.01.047. Source of the 79-patient orotate trial and the 75.7% versus 51.6% survival figures.
Gröber U, Werner T, Vormann J, Kisters K. Myth or reality: transdermal magnesium? Nutrients. 2017;9(8):813. doi:10.3390/nu9080813. Source of the conclusion that transdermal magnesium is scientifically unsupported, the skin barrier, the 9-person spray study measured in hair, the 25-person cream pilot, and the Epsom bath study published only on an industry website.
Johnson B, Milstead M, Thomas O, et al. Investigating nutrient intake during use of glucagon-like peptide-1 receptor agonist: a cross-sectional study. Frontiers in Nutrition. 2025;12:1566498. doi:10.3389/fnut.2025.1566498. Source of the 69 GLP-1 users, the three-day food records, the 266 mg a day average, and the authors’ employment by a supplement retailer.
Wegovy (semaglutide) injection Prescribing Information. Novo Nordisk. Current label at DailyMed, checked October 4, 2026. Source of the 24% versus 11% constipation and 30% versus 16% diarrhea figures, the dehydration and kidney warning, and the postmarketing reports of severe constipation and intestinal blockage.
Semaglutide tablets Prescribing Information. Novo Nordisk. Current label at DailyMed, checked October 4, 2026. Source of the empty stomach, 4 ounces of water and 30-minute wait before other oral medicines.
Levothyroxine sodium tablets Prescribing Information. AbbVie. Current label at DailyMed, checked October 4, 2026. Source of magnesium hydroxide antacids listed among products that reduce absorption, and the 4-hour spacing from iron, calcium and antacids.
Magnesium citrate oral solution, saline laxative. Over-the-counter Drug Facts label at DailyMed, checked October 4, 2026. Source of 290 mg of magnesium per fluid ounce, the adult dose, the 1/2 to 6 hour onset, the one-week limit, the 2-hour spacing from other drugs, and the advice to ask a doctor first with kidney disease, belly pain, nausea or vomiting, or during pregnancy or breastfeeding.
Epsom salt (magnesium sulfate), saline laxative. Over-the-counter Drug Facts label at DailyMed, checked October 4, 2026. Source of 495 mg of magnesium per teaspoon, the 2 to 6 teaspoon adult dose, the 1/2 to 6 hour onset, the 2-hour spacing from other drugs, the kidney disease warning, and the caution on hot soaks for people with diabetes.
National Center for Biotechnology Information. PubChem. pubchem.ncbi.nlm.nih.gov, checked October 4, 2026. Molecular formulas and weights used to work out the share of magnesium in citrate (trimagnesium dicitrate), malate, chloride (hexahydrate and dry), taurate, lactate, aspartate (tetrahydrate), orotate and sulfate (heptahydrate).

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