Taking Calcium and Magnesium Supplements Together

Yes, you can take calcium and magnesium together. In a controlled trial in 26 girls, adding 1,000 mg of calcium a day did not change how much magnesium they absorbed (55% vs 50%)¹. The limit that matters more is calcium itself: the body absorbs supplements best in doses of 500 mg or less².

So split a larger calcium dose across two meals; taking magnesium at a different meal is an optional extra step. For what each mineral does on its own, see our evidence pages on calcium and magnesium.

Yes. Controlled absorption studies in people find that extra calcium does not reduce magnesium absorption^{1, 3, 4}. Magnesium's effect on calcium is mixed: one study found none, while two found it lowered calcium absorption, by 23.5% in one of them^{5, 6, 4}. All of these studies are small.

The key studies, in order of how directly they test the question:

• Calcium on magnesium, 26 girls: 1,000 mg/day of calcium citrate malate or placebo for 14 days; magnesium absorption was 55% vs 50% and magnesium balance did not differ¹.
• Calcium on magnesium, 5 girls: a randomized crossover of 800 vs 1,800 mg/day calcium; magnesium absorption was 44% vs 39%, with no significant difference in any measure³.
• Magnesium on calcium, 5 men: magnesium oxide raised total magnesium intake to about 800 mg/day with no change in calcium absorption at either 241 or 812 mg/day of calcium⁵.
• Magnesium on calcium, 20 women with low red-cell magnesium: after 4 weeks on a control preparation, 4 weeks of 250 mg/day magnesium citrate malate cut fractional calcium absorption by 23.5%, with no change in markers of bone breakdown⁶.
• Gut perfusion study: calcium had little or no influence on magnesium absorption in the human small intestine, while magnesium lowered calcium absorption to a modest extent⁴.

Should you take them separately?

Most people don't need to. To give calcium every advantage, take magnesium at a different meal^{6, 4}.

In three-in-one products, check the zinc dose. In one study, a very high zinc intake of 142 mg/day lowered magnesium absorption and balance in adult men^{7, 8}. Our guide to what not to take with magnesium covers the other pairings.

They depend on each other. Magnesium helps move calcium and potassium across cell membranes, which nerve signals, muscle contraction and a normal heart rhythm rely on⁸. It also affects parathyroid hormone and the active form of vitamin D, the two main regulators of bone⁸.

That link shows up in blood tests. Low blood calcium is usually caused by a lack of vitamin D or magnesium, among other causes². So if a magnesium deficiency is behind low calcium, extra calcium alone may not fix it.

Magnesium is also a cofactor in making, transporting and activating vitamin D, which controls how much calcium the gut absorbs^{9, 2}.

No trial has tested one. The most cited figure is a proposed range of 1.70 to 2.60 parts calcium to 1 part magnesium by weight, based on observational studies¹⁰. The familiar 2:1 product sits inside that range.

A 2021 review of US supplement labels and diet surveys found¹⁰:

• Diet alone: US adults have eaten a ratio above 3.00 since 2000, meaning more calcium relative to magnesium.
• Supplements: the mean ratio across calcium- and magnesium-containing products was 2.90, from 0.10 in liquids to 48.5 in powders.
• Spread: 31% of products fell below the range, 40.5% within it and 28.3% above it.
• Typical doses: users of magnesium supplements took a mean 146 mg/day; users of calcium supplements took a mean 479 mg/day.

Another review suggested ratios below 1.7 or above 2.8 can be harmful and that about 2.0 may be best⁹. Its example: US studies, where diets already ran above 3.0, linked extra magnesium to less cardiovascular disease, while Chinese studies with ratios below 1.7 linked it to more⁹. These are associations, so the ratio is a reasonable rule of thumb, not a proven target.

The pair has not been shown to do more than each mineral alone. Trials that tested calcium and magnesium for blood pressure found no effect, and the main bone trials used calcium with vitamin D, not magnesium^{11, 12, 13}.

• Blood pressure: in 698 adults with high-normal blood pressure, 1 g/day of calcium or 360 mg/day of magnesium did not change blood pressure at 3 or 6 months¹². A Cochrane review of 3 trials in 277 adults found no reliable evidence that any calcium, magnesium or potassium combination lowers blood pressure; the one calcium-plus-magnesium arm showed a non-significant rise¹¹.
• Bones: in 36,282 postmenopausal women, 1,000 mg/day calcium plus 400 IU vitamin D for 7 years left hip bone density 1.06% higher than placebo (measured at 3 centers) but did not significantly cut hip fractures (hazard ratio 0.88, 95% CI 0.72 to 1.08)¹³. Whether magnesium supplements help prevent osteoporosis still needs research⁸. See calcium for bone health.
• PMS: calcium alone has trial support. In 466 women, 1,200 mg/day of calcium, given as calcium carbonate, cut premenstrual symptom scores by 48% by the third cycle, against 30% on placebo¹⁴. The evidence sits on our calcium for PMS page.
• Muscle cramps: a Cochrane review of 11 trials in 735 people found magnesium unlikely to prevent cramps in older adults, with conflicting results in pregnancy¹⁵. See magnesium for muscle cramps.

Sleep claims for combined products rest on magnesium alone; see magnesium for sleep quality.

Related health goals

• Bone Health Support
• PMS Symptom Relief
• Muscle Relaxation & Cramp Relief
• Improved Sleep Quality

Yes, as long as the total stays within the daily limits. Food counts toward the calcium limit, but only supplements and medicines count toward the magnesium one^{2, 8}.

• Calcium RDA: 1,000 mg for adults 19 to 50 and men 51 to 70; 1,200 mg for women over 50 and everyone over 70².
• Calcium upper limit: 2,500 mg/day at ages 19 to 50 and 2,000 mg/day from 51, from food and supplements combined².
• Magnesium RDA: 400 to 420 mg for men and 310 to 320 mg for women⁸.
• Magnesium upper limit: 350 mg/day from supplements and medicines for adults; above that, diarrhea, nausea and cramping become likely⁸.
• Children: the supplemental magnesium limit is 65 mg/day at ages 1 to 3 and 110 mg/day at 4 to 8, so one adult tablet can exceed it⁸. Don't give adult calcium-magnesium products to children without a clinician's advice.

Dose size changes how much calcium you keep. The body absorbs about 36% of a 300 mg dose but only 28% of a 1,000 mg dose².

The form matters too. Calcium carbonate needs stomach acid, so take it with a meal; calcium citrate can be taken without food². For magnesium, the citrate, aspartate, lactate and chloride forms are absorbed better than oxide and sulfate⁸. Our comparisons of magnesium glycinate vs citrate and magnesium oxide vs glycinate cover glycinate.

Is it better to take them in the morning or at night?

No study we found tested timing for the pair, so take them when you will remember. What matters is taking calcium carbonate with food and splitting larger calcium doses². Our guides on when to take magnesium and when to take vitamins cover timing for each.

It still is, for people who fall short. What changed is routine supplements for healthy adults. In 2018 the US Preventive Services Task Force advised against daily supplements of 1,000 mg or less of calcium plus 400 IU or less of vitamin D to prevent fractures in postmenopausal women living at home. It judged they had no net benefit: no fewer fractures, and more kidney stones¹⁶.

The Task Force found the evidence insufficient for men, premenopausal women and higher doses, and an update to the recommendation is in progress¹⁶. The advice does not apply to people with osteoporosis, a past osteoporotic fracture or vitamin D deficiency¹⁶.

Two safety questions also cooled enthusiasm:

• Heart attacks: a 2010 meta-analysis linked calcium supplements without vitamin D to more heart attacks (143 vs 111 events; hazard ratio 1.31)¹⁷. A 2023 meta-analysis of 11 trials found no significant rise in heart attack, stroke or death, with calcium alone or with vitamin D¹⁸.
• Kidney stones: in the Women's Health Initiative, calcium plus vitamin D raised urinary tract stones by 17% (449 vs 381 women) over 7 years¹⁹. That is about 1 extra stone for every 273 women treated¹⁶.

Food is the safer place to start. See our lists of calcium-rich foods and magnesium-rich foods.

Only to fill a gap in your diet. Breastfeeding does not raise the requirement: the calcium RDA stays at 1,000 mg (1,300 mg under age 19), and the magnesium RDA is 310 to 320 mg for women 19 to 50^{2, 8}.

Extra calcium does not stop the bone loss that comes with breastfeeding. In a trial of 1 g/day, lactating women lost 4.2% of spine bone density on calcium and 4.9% on placebo, and calcium did not change breast milk calcium²⁰. A meta-analysis of 5 trials in 567 lactating women found no significant effect on spine or forearm density²¹.

Its authors still advise meeting the RDA, with a supplement if your diet is low²¹. The adult upper limits apply while breastfeeding too^{2, 8}.

People on certain medicines and people with kidney disease. Both minerals bind some drugs in the gut, so spacing matters^{2, 8}.

• Levothyroxine: take it at least 4 hours apart from calcium carbonate². See hypothyroidism.
• Dolutegravir (an HIV medicine): calcium can lower its blood levels substantially; take it 2 hours before or 6 hours after calcium².
• Quinolone antibiotics (ciprofloxacin, levofloxacin): take the antibiotic 2 hours before or after calcium², and 2 hours before or 4 to 6 hours after magnesium⁸.
• Tetracyclines (doxycycline): take them 2 hours before or 4 to 6 hours after magnesium⁸.
• Bisphosphonates (alendronate): keep magnesium at least 2 hours away⁸.
• Lithium: long-term use can raise blood calcium, and calcium supplements may add to that risk².
• Diuretics and acid reducers: loop and thiazide diuretics, and proton pump inhibitors taken for more than a year, can lower magnesium⁸.
• Thiazide diuretics (hydrochlorothiazide): they reduce calcium loss in urine, so calcium supplements can raise blood calcium; the label advises monitoring blood calcium²².
• Nitrofurantoin (Macrobid): the label warns against antacids containing magnesium trisilicate, which reduce the antibiotic's absorption²³.

The risk of magnesium toxicity rises with reduced kidney function, because the kidneys can't clear the excess⁸. If you have chronic kidney disease or a history of kidney stones, ask your clinician before taking either mineral. Our guides on antibiotics and supplements and blood pressure medicines and supplements go further.

What does too much calcium or magnesium look like?

High blood calcium, above 10.5 mg/dL, is rare in healthy people and usually comes from cancer or overactive parathyroid glands². Its signs include²:

• poor muscle tone and fatigue;
• constipation, nausea and weight loss;
• passing a lot of urine;
• kidney problems;
• an irregular heartbeat.

Too much magnesium shows first as diarrhea, nausea and stomach cramps. Very high doses, usually over 5,000 mg/day from laxatives or antacids, can cause low blood pressure, vomiting, muscle weakness, trouble breathing, an irregular heartbeat and cardiac arrest; deaths have been reported, including a young child⁸.

Work out what your diet already supplies, then supplement only the gap^{2, 8}.

1. Count food first. If you eat dairy, fortified foods, greens, nuts and beans every day, you may already meet the RDAs above^{2, 8}.
2. Split calcium doses. Take no more than 500 mg at a time, for example half with breakfast and half with dinner².
3. Add magnesium where it suits you. Taking it with calcium is fine; a different meal gives calcium a slight edge⁶.
4. Stay under the limits in the daily-limits list above, including the lower limits for children^{2, 8}.
5. Space your medicines. Keep 4 hours between calcium carbonate and levothyroxine². Take quinolone antibiotics 2 hours before or after calcium², and 2 hours before or 4 to 6 hours after magnesium⁸. Keep magnesium at least 2 hours from bisphosphonates such as alendronate⁸.

If you take vitamin D as well, our guide to taking vitamin D and vitamin K together covers the rest of a bone-health routine.

Supplements in this guide

• Calcium
• Magnesium
• Vitamin D
• Zinc

We searched PubMed through Firecrawl's research index for human studies of calcium and magnesium absorption, the calcium-to-magnesium ratio, combined supplementation, cardiovascular and kidney stone safety, and lactation. We also read the NIH Office of Dietary Supplements fact sheets on calcium and magnesium, the 2018 US Preventive Services Task Force recommendation and two drug labels on DailyMed (nitrofurantoin and a hydrochlorothiazide combination).

We read 18 study abstracts and cited all 18, along with 5 official sources. An independent reviewer then checked every number against its source before publication. We gave most weight to meta-analyses, Cochrane reviews and randomized trials in people. The absorption studies are small (5 to 26 people) and short, and no trial has tested whether a calcium-to-magnesium ratio changes health outcomes. Researched and written in October 2026.

1. Andon MB, et al. Magnesium balance in adolescent females consuming a low- or high-calcium diet. Am J Clin Nutr. 1996;63(6):950-3. doi:10.1093/ajcn/63.6.950
2. National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/
3. Sojka J, et al. Magnesium kinetics in adolescent girls determined using stable isotopes: effects of high and low calcium intake. Am J Physiol. 1997;273(2 Pt 2):R710-5. doi:10.1152/ajpregu.1997.273.2.R710
4. Brannan PG, et al. Magnesium absorption in the human small intestine. Results in normal subjects, patients with chronic renal disease, and patients with absorptive hypercalciuria. J Clin Invest. 1976;57(6):1412-8. doi:10.1172/JCI108410
5. Spencer H, et al. Effect of magnesium on the intestinal absorption of calcium in man. J Am Coll Nutr. 1994;13(5):485-92. doi:10.1080/07315724.1994.10718439
6. Basso LE, et al. Effect of magnesium supplementation on the fractional intestinal absorption of 45CaCl2 in women with a low erythrocyte magnesium concentration. Metabolism. 2000;49(8):1092-6. doi:10.1053/meta.2000.7711
7. Spencer H, et al. Inhibitory effects of zinc on magnesium balance and magnesium absorption in man. J Am Coll Nutr. 1994;13(5):479-84. doi:10.1080/07315724.1994.10718438
8. National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
9. Rosanoff A, et al. Essential Nutrient Interactions: Does Low or Suboptimal Magnesium Status Interact with Vitamin D and/or Calcium Status?. Adv Nutr. 2016;7(1):25-43. doi:10.3945/an.115.008631
10. Costello RB, et al. Perspective: Characterization of Dietary Supplements Containing Calcium and Magnesium and Their Respective Ratio-Is a Rising Ratio a Cause for Concern?. Adv Nutr. 2021;12(2):291-297. doi:10.1093/advances/nmaa160
11. Beyer FR, et al. Combined calcium, magnesium and potassium supplementation for the management of primary hypertension in adults. Cochrane Database Syst Rev. 2006;2006(3):CD004805. doi:10.1002/14651858.CD004805.pub2
12. Yamamoto ME, et al. Lack of blood pressure effect with calcium and magnesium supplementation in adults with high-normal blood pressure. Results from Phase I of the Trials of Hypertension Prevention (TOHP). Trials of Hypertension Prevention (TOHP) Collaborative Research Group. Ann Epidemiol. 1995;5(2):96-107. doi:10.1016/1047-2797(94)00054-w
13. Jackson RD, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006;354(7):669-83. doi:10.1056/NEJMoa055218
14. Thys-Jacobs S, et al. Calcium carbonate and the premenstrual syndrome: effects on premenstrual and menstrual symptoms. Premenstrual Syndrome Study Group. Am J Obstet Gynecol. 1998;179(2):444-52. doi:10.1016/s0002-9378(98)70377-1 Our study summary.
15. Garrison SR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020;9(9):CD009402. doi:10.1002/14651858.CD009402.pub3 Our study summary.
16. US Preventive Services Task Force. Vitamin D, Calcium, or Combined Supplementation for the Primary Prevention of Fractures in Community-Dwelling Adults: Preventive Medication. Final Recommendation Statement, April 17, 2018. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/vitamin-d-calcium-or-combined-supplementation-for-the-primary-prevention-of-fractures-in-adults-preventive-medication
17. Bolland MJ, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010;341:c3691. doi:10.1136/bmj.c3691 Our study summary.
18. Huo X, et al. Calcium Supplements and Risk of CVD: A Meta-Analysis of Randomized Trials. Curr Dev Nutr. 2023;7(3):100046. doi:10.1016/j.cdnut.2023.100046
19. Wallace RB, et al. Urinary tract stone occurrence in the Women's Health Initiative (WHI) randomized clinical trial of calcium and vitamin D supplements. Am J Clin Nutr. 2011;94(1):270-7. doi:10.3945/ajcn.110.003350
20. Kalkwarf HJ, et al. The effect of calcium supplementation on bone density during lactation and after weaning. N Engl J Med. 1997;337(8):523-8. doi:10.1056/NEJM199708213370803
21. Cai G, et al. Calcium supplementation for improving bone density in lactating women: a systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2020;112(1):48-56. doi:10.1093/ajcn/nqaa103
22. DailyMed (US National Library of Medicine). Captopril and hydrochlorothiazide tablet: prescribing information (drug interactions: calcium salts). https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f0266d1a-2af2-415b-9433-69b5a690f9bb
23. DailyMed (US National Library of Medicine). Macrobid (nitrofurantoin monohydrate and nitrofurantoin, macrocrystalline) capsule: prescribing information. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=1971e893-5fdb-41e3-a1e9-5e52deed03d1