Calcium

Food first. A small tablet only to fill the gap your food leaves, and never to keep the arteries clear.

At a glance

Our verdict	Food first. A supplement only for a measured shortfall, 500 mg or less. Never sold "for the arteries".
Best use	Bones, when food falls short
How sure the evidence is	Low for fractures
Cost a month	About $3 to $5

Written for adults in Canada. It explains the evidence; it is not advice for you personally. Last reviewed 20 October 2026.

Supplement Check

Is it right for you?

Whether you need calcium depends on how much you already eat, your kidneys, any kidney stones, and the bone or heart medicines you take. We estimate the calcium in your food and add up everything you already take before suggesting a tablet. Our Supplement Check looks at:

High blood calcium, an overactive parathyroid gland, or sarcoidosis
Kidney stones
Kidney function below normal (eGFR under 60), or not known
Cancer that has spread to bone, or high calcium from cancer
Taking a bisphosphonate, lithium, digoxin or an HIV integrase inhibitor
Every medicine you take
The total across everything you already take, against the safe upper limits

Coming soon. The Supplement Check and the Online Health Assessment open once our safety testing is complete.

$59 when it opens, paid once. Adults 18 and over, anywhere in Canada. While the service is new, Dr. French reads every report himself, and your report arrives within 3 working days. More about the Supplement Check

Tell me when it opens

What it is

Calcium is the main mineral in bone. Supplements come as calcium carbonate (40% calcium; it needs food and stomach acid to be absorbed) or calcium citrate (21% calcium; it can be taken at any time).

We have not named a calcium product yet. Our rule is refined calcium carbonate or citrate with published heavy-metal results. Never dolomite, bone meal or unrefined oyster shell: lead is a known problem in some natural sources of calcium.

Structure of Calcium — Structure: PubChem (public domain).

How it acts

Calcium is the raw material for bone mineral. When intake is low, the body raises parathyroid hormone and takes calcium out of bone; enough calcium turns that signal down. A tablet causes a brief rise in blood calcium that food does not, and that is the proposed reason for the heart signal described below. It is a mechanism, not proof.

Outline of a person showing where Calcium acts in the body

What it is used for, and what the evidence says

Calcium is used for a dietary shortfall, alongside osteoporosis treatment, and with long-term steroid tablets. Dr. French asked one more question: does calcium, with or without magnesium, change calcium in the arteries?

Use	What the trials found	How sure we are
Fractures	26 trials: 11% fewer fractures overall (relative risk 0.89) and fewer spine fractures, but no clear effect on hip fractures. In the 4 trials least open to bias (44,505 people) there was no effect at any site. Calcium from food was not linked with fractures. [1]	Low
Bone density	Calcium from food or from tablets raised bone density by 0.7 to 1.8% over 1 to 2 years, with no further gain after that. [2]	Moderate
Heart attacks	Calcium without vitamin D: 31% more heart attacks in a pooled analysis (relative risk 1.31). [3] A reanalysis of the Women's Health Initiative with a meta-analysis: about 21% more heart attacks. [4] A 2016 guideline found no good evidence of harm up to 2,000 to 2,500 mg a day in total. [5] Heart attacks were secondary outcomes, partly self-reported, and the analyses disagree.	Low
Stroke, calcium with vitamin D	An umbrella review of 277 trials found calcium taken with vitamin D raised stroke by about 17% (relative risk 1.17). [6]	Moderate
Calcium in the heart arteries	5,448 adults followed for 10 years: those with the highest calcium intake, mostly from food, were 27% less likely to develop new calcium in the heart arteries; those taking calcium tablets were 22% more likely (relative risk 1.22). An observational study, not a trial. [7]	Low
Magnesium to keep the arteries clear	Higher magnesium intake went with less calcium in the heart arteries and aorta in one cross-sectional study of 2,695 people. [8] No trial has tested a magnesium supplement for this.	No evidence (no trial)
Vitamin K2 to keep the arteries clear	In men with calcium on the aortic valve, vitamin K2 (720 micrograms) with vitamin D for 2 years did not slow valve calcification. [9] In kidney transplant patients it did not change the blood's tendency to calcify or the stiffness of the arteries. [10] The trials are few and small.	Low

How sure we are: High means further research is very unlikely to change the answer; Moderate, it might; Low, it may well change; Very low, we are very unsure. These follow the GRADE method that guideline panels use.

Benefits

Bone density about 1 to 2% higher (moderate certainty), the same gain as from food calcium.
Fewer fractures only weakly and inconsistently; the benefit is clearer only in frail older women in care who are short of both calcium and vitamin D.
Cheap.
No benefit for the arteries. No trial shows calcium, magnesium or vitamin K2 keeps arteries clear.

Risks and side effects

Constipation and bloating.
Kidney stones. In the Women's Health Initiative, about 17% more with calcium plus vitamin D.
A possible heart-attack signal with tablets, not with food (low certainty). [3, 4]
Too much calcium in the blood with high doses, especially with vitamin D or thiazide water pills.
Lead in some natural-source calcium (dolomite, bone meal, oyster shell), hence our product rule.

Interactions with medicines

Calcium does not change how the liver clears medicines. Its interactions come from binding medicines in the gut, and from raising blood calcium.

Medicine	What can happen	What to do
Levothyroxine (thyroid hormone)	Calcium binds it and less is absorbed	Take calcium at least 4 hours apart
Quinolone and tetracycline antibiotics; iron	Calcium binds them and less is absorbed	Space the doses
Bone medicines (bisphosphonates)	Calcium is part of the bone treatment, and the amount is set by your doctor	No calcium from the Supplement Check
HIV integrase inhibitors	Calcium binds them and less is absorbed	No calcium from the Supplement Check
Digoxin, lithium	High blood calcium raises digoxin's toxicity; lithium with calcium is one of our listed stops	No calcium from the Supplement Check
Thiazide water pills	Raise blood calcium	Food first; no high doses

Who should not take it

High blood calcium, an overactive parathyroid gland, or sarcoidosis.
Kidney stones: held for a doctor.
Kidney function below normal (eGFR under 60), or not known: held until a kidney blood test.
Cancer that has spread to bone, or high calcium from cancer: stop.
Taking a bisphosphonate, lithium, digoxin or an HIV integrase inhibitor.
Pregnancy and breastfeeding: 1,000 mg a day in total, from food; a tablet only for a shortfall.
Older adults: food first; a tablet only for a shortfall.
Under 18: outside the Supplement Check.

Dose and timing

Aim for 1,000 to 1,200 mg a day in total, from food first. Our check estimates the calcium in your food.
A tablet only if your food supplies under about 700 mg a day: 500 mg once a day. Never more than 500 mg at a time.
Carbonate with meals; citrate at any time; 4 hours away from levothyroxine.
Upper limit, food and tablets together: 2,500 mg a day at 19 to 50, and 2,000 mg from 51. [11] Calcium in a multivitamin counts (an age-50+ multivitamin has 220 mg).
How to tell it is working: your food-plus-tablet total reaches the target. There is no blood test for it; bone density is checked at the usual interval if you have osteoporosis.
Store at room temperature.

What we know about Canadians

Most Canadian adults do not get enough calcium from food. In the 2015 national nutrition survey, more than 40% of men and 60% of women were below the average requirement from food, rising to 84% of men and 88% of women over 70. [12] Calcium from food fell between 2004 and 2015 as people drank less milk, and fewer people took calcium supplements (27.5% down to 22.0%); counting supplements, about 32% of Canadians were below the requirement in 2015. [13] In households with severe food insecurity, 79% were below it. [14]

Most at risk: women, everyone over 70, people who avoid dairy and fortified plant drinks, and food-insecure households.

How sure are we: moderate. National survey, but self-reported food intake and 11 years old.

An honest comparison: fractures

The same outcome, with the cost a month in Canadian dollars.

Option	Effect	How sure we are	Cost a month (CAD)
Calcium tablet	11% fewer fractures overall; none in the trials least open to bias [1]	Low	$3 to $5
Exercise (balance and resistance training)	Fewer falls; small gains in bone density (not reviewed in detail)	Moderate	$0 to $60
Calcium from food	Not linked with fractures; the same bone-density gain as tablets [1, 2]	Low to Moderate	Part of your food
Alendronate (for osteoporosis)	Large reductions in spine and hip fractures in women with osteoporosis (we did not re-check the trial figures)	High	$10 to $15

For the arteries there is nothing to compare: calcium tablets went with more new artery calcium, magnesium has no trial, vitamin K2 had no effect, and no medicine reverses artery calcium. [7, 9]

Our verdict

Food first. If your food supplies under about 700 mg a day, a 500 mg tablet once a day covers the gap. More from tablets is not advised.

We never suggest calcium, magnesium or vitamin K2 to keep the arteries clear: no trial shows a benefit.

Dr. French asked us to decide calcium on the evidence, and especially its effect on the arteries. Tablets add a little bone density, the same as food does, and the fracture benefit disappears in the best trials. Some analyses link tablets, not food, to heart attacks and new artery calcium; that evidence is low certainty and contested, but it is enough to keep tablets to the gap food leaves. Calcium tablets are cheap: the reason to limit them is safety, not cost.