Home Ingredients Calcium Citrate
Bone HealthKidney/Urinary Tract

Calcium Citrate

Evidence Level
Strong
3 Clinical Trials
3 Documented Benefits
4/5 Evidence Score

Calcium citrate is calcium bound to citric acid. Its main practical difference from calcium carbonate is that it does not depend on stomach acid: in fasting people with no stomach acid, calcium from citrate was absorbed about ten times better than from carbonate, although carbonate was absorbed normally when taken with breakfast. That makes citrate a reasonable choice for people with low stomach acid or who take calcium between meals. In a 2-year placebo-controlled trial in women soon after menopause, 800 mg of calcium a day as citrate kept spine and forearm bone density stable while it fell on placebo. A short trial in healthy postmenopausal women found it raised urinary calcium and citrate without raising calcium oxalate saturation. By weight it contains less calcium than carbonate (about 21%), so more or larger tablets are needed.

Studied Dose Bone trial: 800 mg calcium/day as citrate (400 mg twice daily) for up to 2 years. Urine chemistry trial: same dose for 2 weeks. Absorption studies: single 250 to 500 mg calcium doses.
Active Compound Calcium citrate

Benefits

Absorption without stomach acid

In 11 fasting people with no stomach acid (achlorhydria), about 45% of the calcium from citrate was absorbed versus about 4% from carbonate. In 9 people with normal stomach acid the two forms were absorbed equally, and carbonate was absorbed normally by the achlorhydric patients when taken with breakfast. The advantage of citrate is clearest when calcium is taken without food in people with low stomach acid.

Supported by Trial 1

Effect on urine stone-risk markers

In a small trial, 18 healthy postmenopausal women took 400 mg of calcium twice daily as citrate for 2 weeks. Urinary calcium and citrate rose and urinary oxalate fell, while calcium oxalate, brushite and uric acid saturation did not change compared with placebo. This was a short check in women without stones, not a test of stone prevention.

Supported by Trial 3, Trial 2

Bone density support

In a placebo-controlled trial in women within about 10 years of menopause, 800 mg of calcium a day as citrate (400 mg twice daily) kept spine and forearm bone density stable over 2 years, while spine density fell by 2.4% and forearm density by 3.0% on placebo. Hip (femoral neck) density did not change in either group. Fractures were not measured.

Supported by Trial 2

Mechanism of action

1

Absorption that does not depend on stomach acid

Calcium carbonate needs acid to dissolve, while calcium citrate does not. This is why, in fasting people without stomach acid, calcium from citrate was absorbed far better than from carbonate. With a meal, carbonate was absorbed normally.

2

Citrate in urine

Citrate in urine binds calcium, which lowers the amount free to form calcium oxalate crystals. In a 2-week trial, calcium citrate raised urinary citrate in healthy postmenopausal women but also raised urinary calcium, so overall calcium oxalate saturation did not change.

3

Standard Calcium Absorption

Once dissolved, calcium absorbs via vitamin D-dependent active transcellular transport and passive paracellular transport — same as calcium from any source.

Clinical trials

1
Calcium citrate versus carbonate in people without stomach acid
PubMed

Comparison of fractional calcium absorption from calcium carbonate and a pH-adjusted calcium citrate, 0.25 g calcium, measured by a double-isotope method in fasting subjects (Recker 1985, N Engl J Med)

11 fasting patients with achlorhydria and 9 fasting healthy subjects.

In achlorhydria, absorption was 45% from citrate versus 4% from carbonate (p<0.0001). In healthy subjects, absorption was similar (24% versus 23%). Carbonate taken with breakfast was absorbed normally by the achlorhydric patients.

2
Calcium citrate and bone density after menopause
PubMed

Randomized, placebo-controlled trial of 800 mg/day calcium as citrate (400 mg twice daily) for 1 to 2 years (Ruml et al. 1999, Am J Ther)

63 women in the early or mid postmenopausal period; bone density data for 25 on calcium citrate and 31 on placebo.

Spine (L2 to L4) bone density did not change on calcium citrate (+1.0% at 2 years) but fell 2.4% on placebo; radial shaft fell 3.0% on placebo and did not change on citrate. Femoral neck did not change in either group. Bone turnover markers fell during some treatment periods.

3
Calcium citrate and urine stone-risk markers
PubMed

Randomized 4-phase trial of placebo, calcium citrate (400 mg calcium twice daily), potassium citrate, or both, each for 2 weeks (Sakhaee et al. 2004, J Urol)

18 healthy postmenopausal women without kidney stones.

Compared with placebo, calcium citrate raised urinary calcium and citrate and lowered urinary oxalate and phosphate. Urinary saturation of calcium oxalate, brushite and uric acid did not change. Adding potassium citrate raised urinary citrate further.

Side effects and drug interactions

Common Potential side effects

Generally well-tolerated.
Mild GI distress at high doses.
Larger pills due to lower elemental calcium content.
Kidney stones — even calcium citrate, at very high supplemental doses, can theoretically increase stone risk; consult urology if stone-former.
Hypercalcemia at chronic very high doses. Cardiovascular consideration: some analyses have linked calcium taken as supplements — not calcium obtained from food — with a modest increase in cardiovascular events. The evidence is inconsistent and the question remains unresolved, but the sensible approach is to cover as much of the 1,000–1,200 mg daily target from diet as possible and use a supplement only to fill the remaining gap.

Important Drug interactions

Same general calcium interactions as carbonate.
Tetracycline/quinolone antibiotics — chelation; separate by 2 hours.
Bisphosphonates — separate by 30 min-2 hours.
Levothyroxine — separate by 4 hours.
Iron — competes; separate.
Thiazide diuretics — hypercalcemia risk.
Aluminum-containing drugs — citrate increases aluminum absorption — caution in CKD/dialysis.

More Bone Health Ingredients

Bone Health
Vitamin D
Vitamin D is a fat-soluble vitamin the body turns into a hormone that controls calcium abs…
Bone Health
Calcium
Calcium is the most abundant mineral in the body: more than 99% sits in bones and teeth, a…
Bone Health
Copper
Copper is an essential trace mineral that a number of enzymes need in order to work, inclu…
Bone Health
Calcium Carbonate
Calcium carbonate is one of the two most common forms of calcium in supplements and is abo…

Related articles from our blog

📝 Article
The 7 Best Calcium Supplements of 2026
Calcium looks like the simplest supplement on the shelf, but the form on the label decides how well it works and how it…
July 4, 2026
📝 Article
The 7 Best Menopause Supplements of 2026
Menopause is one of the most heavily marketed corners of the supplement world, and one of the most oversold, full of “ho…
September 16, 2026
📝 Article
Probiotics for Bone Density: What a New 2026 Meta-Analysis Found
Of all the things the gut microbiome has been blamed for and credited with, your skeleton is one of the least obvious. Y…
July 30, 2026

Frequently asked questions about Calcium Citrate

What is calcium citrate?

Calcium citrate is a well-absorbed calcium form that does not require stomach acid, so it can be taken with or without food. It contains less elemental calcium per pill than carbonate (about 21%), so you may need more pills.

Is calcium citrate better than calcium carbonate?

Citrate is absorbed well even without stomach acid, so it suits people with low stomach acid or who take calcium between meals. Taken with food, carbonate is absorbed about as well and contains more calcium per tablet.

How much calcium citrate should I take?

Use a supplement only to fill the gap your diet leaves, and ask your doctor about the right daily total for you. The bone trial on this form used 400 mg of calcium twice a day. Check the label for elemental calcium, since citrate contains less per tablet.

When should I take calcium citrate?

Any time, with or without food, since it does not need stomach acid. Avoid taking it at the same time as iron or thyroid medication, which calcium can block.

What is Calcium Citrate used for?

Calcium Citrate is researched primarily for Bone Health and Kidney/Urinary Tract. In 11 fasting people with no stomach acid (achlorhydria), about 45% of the calcium from citrate was absorbed versus about 4% from carbonate.

What is the recommended dosage of Calcium Citrate?

The clinically studied dose is Bone trial: 800 mg calcium/day as citrate (400 mg twice daily) for up to 2 years. Urine chemistry trial: same dose for 2 weeks. Absorption studies: single 250 to 500 mg calcium doses. Always follow the product label and check with a healthcare provider for personal advice.

Is Calcium Citrate safe, and does it have side effects?

For most healthy adults, Calcium Citrate is well tolerated at studied doses. Reported effects can include: Generally well-tolerated. Mild GI distress at high doses. It may also interact with some medications. Calcium Citrate is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Calcium Citrate interact with any medications?

Possible interactions include: Same general calcium interactions as carbonate. Tetracycline/quinolone antibiotics — chelation; separate by 2 hours. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Calcium Citrate?

NutraSmarts rates the evidence for Calcium Citrate as Strong (4 out of 5). It is backed by 3 clinical trials and 5 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(5 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Pak CY, Harvey JA, Hsu MC. Enhanced calcium bioavailability from a solubilized form of calcium citrate. J Clin Endocrinol Metab. 1987;65(4):801-5. doi: 10.1210/jcem-65-4-801.PubMedUsed to support: In normal subjects, a solubilized mixture of calcium hydroxide and citric acid gave higher calcium absorption than standard tricalcium dicitrate, both by the rise in urinary calcium (15 subjects) and by fractional absorption (11 subjects, 0.32 versus 0.17). Shows that formulation affects how well calcium citrate is absorbed.
  2. Sakhaee K, Poindexter JR, Griffith CS, Pak CY. Stone forming risk of calcium citrate supplementation in healthy postmenopausal women. J Urol. 2004;172(3):958-61. doi: 10.1097/01.ju.0000136400.14728.cd.PubMedUsed to support: Randomized 4-phase trial in 18 healthy postmenopausal women: 2 weeks of calcium citrate (400 mg calcium twice daily) raised urinary calcium and citrate and lowered urinary oxalate, without changing calcium oxalate, brushite or uric acid saturation versus placebo.
  3. Ruml LA, Sakhaee K, Peterson R, Adams-Huet B, Pak CY. The effect of calcium citrate on bone density in the early and mid-postmenopausal period: a randomized placebo-controlled study. Am J Ther. 1999;6(6):303-11. doi: 10.1097/00045391-199911000-00004.PubMedUsed to support: Randomized placebo-controlled trial in early and mid-postmenopausal women (bone data for 56): 800 mg/day calcium as citrate kept spine and radial shaft bone density stable over 2 years while both fell on placebo; femoral neck did not change in either group.
  4. Sakhaee K, Griffith C, Pak CY. Biochemical control of bone loss and stone-forming propensity by potassium-calcium citrate after bariatric surgery. Surg Obes Relat Dis. 2012;8(1):67-72. doi: 10.1016/j.soard.2011.05.001.PubMedUsed to support: Randomized 2-phase crossover in 24 gastric bypass patients: 2 weeks of effervescent potassium calcium citrate lowered bone resorption markers and urinary uric acid saturation versus placebo, while calcium oxalate and brushite saturation did not change.
  5. Recker RR. Calcium absorption and achlorhydria. N Engl J Med. 1985;313(2):70-3. doi: 10.1056/NEJM198507113130202.PubMedUsed to support: In fasting patients with achlorhydria, calcium absorption was 45% from citrate versus 4% from carbonate; in healthy fasting subjects the forms were similar, and carbonate taken with breakfast was absorbed normally by achlorhydric patients.