Does calcium cause constipation?

Calcium supplements cause gut trouble a little more often than placebo, and constipation is part of it. Pooling 7 placebo-controlled trials, mostly in older women, self-reported gut complaints came to 14.1% on calcium against 10.0% on placebo, a relative risk of 1.43 (95% CI 1.28 to 1.59). That pool lumps constipation together with bloating, cramping and other gut symptoms, so nobody can say from it how much of the extra is constipation. Calcium from food is a different matter, and the only data on it point the other way.

Unsettled. No trial we found was designed to answer this question. The firm part of the answer comes from side effects counted in trials about bones, where constipation was one complaint among several. Two trials that did measure bowel habit found no change at all, one of them small and short. So calcium supplements probably constipate some people, and nobody has measured how many.

What the trials found

Side effects in trials of supplements

The pooled result above is the strongest evidence, and it is a count of complaints, not a test of bowel function. In one of those trials, older women on calcium were admitted to hospital for functional gut problems in 6.8% of cases against 3.6% on placebo, and that category was not limited to constipation. The authors of a BMJ systematic review of calcium and fractures write that 10 to 20% of people get gut side effects such as constipation and that a considerable number stop taking the supplements because of them. That figure is their reading of the trials they cite, not a pooled estimate of the review.

In a 5-year trial of 1,471 healthy postmenopausal women around 74 years old, 1 g of calcium citrate a day slowed bone loss, and constipation was more common on calcium. The abstract gives no rates, and only 55 to 58% of women kept taking their tablets. A meta-analysis of 19 trials in 69,234 older people taking calcium with vitamin D found fewer fractures, and its authors advise paying attention to gut side effects without giving a number in the abstract.

Trials that measured the bowel

Here the answer was no change. In a double-blind crossover trial of 27 healthy women, 500 mg of calcium a day as carbonate or as phosphate for 2 weeks each did not change stool frequency or form. Stools held at 1.3 a day in every period, and hard stools ran at 34% at baseline, 34% on carbonate and 37% on phosphate. That was a small pilot at a moderate dose. In a 2-year double-blind trial of 323 healthy men aged 40 and over, 600 or 1200 mg of calcium a day left constipation unaffected compared with placebo. Constipation was a secondary outcome there, and the abstract gives no rates.

Pregnancy points the same way. The 2024 Cochrane review of 19 trials in 17,370 women, 17 of them using more than 1000 mg a day, found no evidence that calcium changed gut symptoms. Those symptoms were a secondary outcome and sparsely reported.

Calcium from food

Among 4,838 US adults with metabolic syndrome, those eating the most calcium from food, above 1114 mg a day, had about a quarter of the odds of constipation of those eating under 502 mg. The odds ratio was 0.282 (95% CI 0.115 to 0.691). That is a cross-sectional survey from dietary recall. People who eat more calcium-rich food may also eat more fiber and move more, and the study says nothing about tablets. It is still the only evidence here on food, and it does not support the idea that dairy or other calcium foods bind you up.

Who should be careful

Not for everyone. The form of calcium matters. NIH ODS says calcium carbonate appears to cause more gas, bloating and constipation than calcium citrate, especially in older adults who have less stomach acid. A double-blind crossover trial in 24 adults with hypoparathyroidism, who take high doses, found less constipation on citrate than on carbonate over a month of each.

Citrate is not free of it. In the 5-year trial above, it was citrate that came with more constipation. In a one-year study of 268 older adults taking 500 mg of calcium citrate a day, nine people, 3.9%, reported side effects, mostly in the gut and none serious. That study had no comparison group.

Constipation can also be a sign of too much calcium in the blood. NIH ODS sets the adult upper limit at 2,000 to 2,500 mg a day and lists constipation, nausea and fatigue among the effects of high blood calcium. It also notes that high blood calcium is rare in healthy people and usually comes from cancer, primary hyperparathyroidism or other conditions. New constipation with nausea or tiredness in someone on high-dose calcium is a reason to get checked rather than to change brands.

What expert bodies say

NIH ODS says some people who take calcium supplements get gas, bloating, constipation or a mix of these, and that carbonate seems to cause more of them than citrate. That is an agency summary without an analysis of its own. We did not find a guideline that addresses calcium and constipation directly.

If a calcium supplement seems to be constipating you, the form is the first thing worth discussing with whoever recommended it. If you are not taking a supplement, the evidence above gives no reason to blame the calcium in your food.

How we searched

What would change this answer

• A placebo-controlled trial with constipation as its main outcome, measured by stool diary and standard criteria, at the doses people are usually told to take, and comparing carbonate with citrate. We found none.
• A meta-analysis that counts constipation separately from bloating, cramping and other gut complaints. None exists in what we searched.
• The constipation rates per arm from the large trials already done, which sit in full texts we could not read.
• A trial of calcium from food rather than tablets, which would test what the survey above can only suggest.

Sources

1. ev-caco-01 · Meta-analysis or systematic review · pooled analysis of adverse events in randomized controlled trials · n = 7 trials
   In seven RCTs summary self-reported gastrointestinal (GI) adverse event rates were more common in participants receiving calcium. These were described as constipation, excessive abdominal cramping, bloating, upper GI events, GI disease, GI symptoms, and severe diarrhoea or abdominal pain (calcium 14.1%, placebo 10.0%), relative risk (RR) 1.43 95% confidence interval (CI) 1.28 to 1.59, p < 0.001.
   
   Who: participants of seven randomized placebo-controlled trials of calcium supplementation, mostly older women
   Effect: GI adverse events 14.1% vs 10.0%, RR 1.43 (95% CI 1.28 to 1.59); constipation pooled with cramping, bloating, upper GI events and diarrhea
   Certainty: constipation not separated from other GI complaints; self-reported events
   Journal of Bone and Mineral Research, 2012, 27(3):719-722 (Lewis, Zhu, Prince) · checked 2026-10-06 · we read the abstract
2. ev-caco-02 · Meta-analysis or systematic review · pooled analysis of adverse events in randomized controlled trials · n = 1 trial
   Adjudicated functional GI hospitalizations in one study were calcium 6.8%, placebo 3.6% (RR 1.92, 95% CI 1.21-3.05, p = 0.006).
   
   Who: older women in a randomized placebo-controlled trial of calcium supplementation, adjudicated hospital records
   Effect: functional GI hospitalizations 6.8% vs 3.6%, RR 1.92 (95% CI 1.21 to 3.05)
   Certainty: one trial; hospitalization category not limited to constipation
   Journal of Bone and Mineral Research, 2012, 27(3):719-722 (Lewis, Zhu, Prince) · checked 2026-10-06 · we read the abstract
3. ev-caco-03 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs and cohort studies · n = —
   In addition, 10-20% of people experience gastrointestinal side effects such as constipation, which cause a considerable number to stop taking the supplements.
   
   Who: older adults taking calcium supplements in randomized trials of fracture prevention
   Effect: 10-20% with GI side effects such as constipation; leads to discontinuation
   Certainty: statement in the discussion, drawn from cited trials, not a pooled estimate of this review
   BMJ, 2015 · checked 2026-10-06 · we read the fulltext
4. ev-caco-04 · Meta-analysis or systematic review · Cochrane systematic review of RCTs · n = 17,370
   There was no evidence that calcium supplementation had any effect on maternal weight gain during pregnancy; increasing bone mineral density in pregnant women; rate of intrauterine growth restriction; perinatal mortality; stillbirth or fetal death rate; increase birth length or fetal head circumference; and adverse effects such as postpartum haemorrhage, gall stones, gastrointestinal symptoms, headache, urinary stones, urinary tract infection or impaired renal function.
   
   Who: pregnant women in randomized trials of calcium supplementation vs placebo or no treatment; 17 trials with >1000 mg/day
   Effect: no evidence of an effect on gastrointestinal symptoms, among other adverse effects
   Certainty: GI symptoms a secondary outcome, sparsely reported; no GRADE for this outcome
   Cochrane Database Syst Rev, 2024 · checked 2026-10-06 · we read the abstract
5. ev-caco-05 · Meta-analysis or systematic review · meta-analysis of RCTs · n = 69,234
   The combination of vitamin D and calcium has no difference in mortality rate, and it can prevent fractures in the elderly, and enhance bone density and serum 25-hydroxyvitamin D concentration, but still need to pay attention to adverse reactions in the gastrointestinal tract.
   
   Who: older people in randomized trials of calcium combined with vitamin D for osteoporotic fractures
   Effect: fractures prevented, no mortality difference; gastrointestinal adverse reactions flagged, no number in abstract
   Certainty: calcium given with vitamin D; GI events not quantified in the abstract, full text not retrievable
   Asia Pac J Clin Nutr, 2024 · checked 2026-10-06 · we read the abstract
6. ev-caco-06 · Randomized controlled trial(s) · randomized double-blind crossover pilot trial · n = 27
   This study suggests that neither calcium carbonate nor calcium phosphate, providing 500 mg/d of calcium, affects stool frequency or form.
   
   Who: healthy females aged 43.0 +/- 10.6 years; split dose 500 mg/d elemental calcium, 2 weeks per form
   Effect: 1.3 stools/day in every period; hard stools (BSFS 1-2) 34% baseline, 34% carbonate, 37% phosphate, 29% washout
   Certainty: small pilot, 2-week periods, moderate dose
   Can J Diet Pract Res, 2016 · checked 2026-10-06 · we read the abstract
7. ev-caco-07 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial · n = 323
   Tooth loss, constipation, and cramps were unaffected by calcium supplementation, falls tended to be less frequent in the group receiving calcium, 1200 mg/d, but vascular events tended to be more common in the groups receiving calcium vs the group receiving placebo.
   
   Who: healthy men at least 40 years old, mean age 57; calcium 600 mg/d, 1200 mg/d or placebo for 2 years
   Effect: constipation unaffected by calcium supplementation
   Certainty: constipation a secondary outcome, no rates in abstract
   Arch Intern Med, 2008 · checked 2026-10-06 · we read the abstract
8. ev-caco-08 · Randomized controlled trial(s) · randomized placebo-controlled trial · n = 1,471
   Serum alkaline phosphatase and procollagen type-I N-terminal propeptide were lower in the calcium group at 5 years, but constipation was more common.
   
   Who: healthy postmenopausal women aged 74 +/- 4 years; calcium 1 g/day as citrate vs placebo for 5 years
   Effect: constipation more common with calcium (rates not in abstract); compliance only 55-58%
   Certainty: constipation rates not given in abstract; full text not in PMC
   Am J Med, 2006 · checked 2026-10-06 · we read the abstract
9. ev-caco-09 · Randomized controlled trial(s) · randomized double-blind crossover trial · n = 24
   Ca-Cit was associated with less constipation (p = 0.047).
   
   Who: 24 adults with postsurgical chronic hypoparathyroidism, Rome; 1 month on each salt
   Effect: less constipation with citrate than carbonate, p = 0.047
   Certainty: small, special population on high calcium doses
   J Bone Miner Res, 2022 · checked 2026-10-06 · we read the abstract
10. ev-caco-10 · Observational data · cross-sectional analysis of NHANES · n = 4,838
    In the adjusted Model 2, the association in Q4 persisted (OR: 0.282, 95% CI: 0.115–0.691, p = 0.008)
    
    Who: adults with metabolic syndrome, NHANES 2005-2010; dietary calcium quartiles Q1 <502 mg, Q4 >1114 mg
    Effect: Q4 vs Q1 OR 0.282 (95% CI 0.115 to 0.691) adjusted; per SD of dietary calcium OR 0.562 (0.379 to 0.835)
    Certainty: cross-sectional, dietary recall; food calcium tracks fiber and activity; does not test supplements
    Frontiers in Nutrition, 2024, 11:1422564 (NHANES 2005-2010, dietary calcium and constipation in metabolic syndrome) · checked 2026-10-06 · we read the fulltext
11. ev-caco-11 · Observational data · non-interventional prospective multicenter study · n = 268
    ARs were reported by nine (3.9%) subjects, primarily gastrointestinal disorders, with no serious ARs.
    
    Who: 268 Caucasian subjects, 91.4% female, mean age 70; 500 mg calcium citrate daily up to one year
    Effect: adverse reactions in 9 (3.9%), primarily gastrointestinal; more adverse events in those with adherence <80%
    Certainty: no control group; manufacturer-linked setting
    Aging Clin Exp Res, 2024 · checked 2026-10-06 · we read the abstract
12. ev-caco-12 · Position of an expert body · agency fact sheet · n = —
    Some individuals who take calcium supplements might experience gastrointestinal side effects, including gas, bloating, constipation, or a combination of these symptoms. Calcium carbonate appears to cause more of these side effects than calcium citrate, especially in older adults who have lower levels of stomach acid [1].
    
    Who: people taking calcium supplements, especially older adults with low stomach acid
    Effect: no effect size; carbonate more side effects than citrate
    Certainty: agency position without own analysis
    NIH Office of Dietary Supplements, Calcium Fact Sheet for Health Professionals · checked 2026-10-06 · we read the section
13. ev-caco-13 · Position of an expert body · agency fact sheet · n = —
    The tolerable upper intake level for calcium ranges from 2,000 mg to 2,500 mg for adults and from 1,000 mg to 3,000 mg for infants, children, and adolescents, depending on age. Hypercalcemia (serum levels >10.5 mg/dL [2.63 mmol/L]) and hypercalciuria (urinary calcium levels >250 mg/day in women and 275 mg/day in men) are rare in healthy people and usually result from cancer, primary hyperparathyroidism, and other conditions [1,4]. Hypercalcemia and hypercalciuria can cause poor muscle tone, renal insufficiency, hypophosphatemia, constipation, nausea, weight loss, fatigue, polyuria, heart arrhythmias, and a higher risk of CVD mortality [1,4,49].
    
    Who: adults; hypercalcemia and hypercalciuria
    Effect: UL 2,000-2,500 mg/day for adults; hypercalcemia can cause constipation
    Certainty: hypercalcemia is rare in healthy people
    NIH Office of Dietary Supplements, Calcium Fact Sheet for Health Professionals · checked 2026-10-06 · we read the section