Calcium carbonate is one of the most common and affordable forms of calcium used in supplements and antacids. It is a naturally occurring compound found in limestone, marble, chalk, and the shells of marine animals like oysters. When taken as a supplement, it provides elemental calcium that your body needs for strong bones, muscle function, and nerve signaling.
What Is Calcium Carbonate?
Calcium carbonate is a chemical compound with the formula CaCO₃. It is the same calcium found in eggshells, coral, and some antacids. Your body does not use calcium carbonate directly — it breaks it down in the stomach to release calcium ions, which are then absorbed in the small intestine.
Supplements labeled as calcium carbonate typically contain about 40 percent elemental calcium by weight. That means a 500 mg tablet provides 200 mg of actual calcium. Many antacids also use calcium carbonate as the active ingredient to neutralize stomach acid.
How Does Calcium Carbonate Compare to Other Calcium Supplements?
Calcium supplements come in several forms. The two most common are calcium carbonate and calcium citrate. Here is how they compare:
| Characteristic | Calcium Carbonate | Calcium Citrate |
|---|---|---|
| Elemental calcium per dose | 40% | 21% |
| Cost | Low | Higher |
| Absorption | Best with food (needs stomach acid) | Well absorbed with or without food |
| Tablet size | Smaller (fewer pills for same calcium) | Larger (more pills needed) |
| Best for | Most healthy adults, especially those with normal stomach acid | Older adults, people on acid-reducing medications, those with absorption issues |
Some evidence suggests calcium citrate is less likely to cause constipation, but the data is not strong. For most people, calcium carbonate is a safe and effective choice when taken correctly.
What Are the Main Uses of Calcium Carbonate?
Calcium carbonate has three common uses in health and medicine:
- Dietary supplement: To help meet daily calcium needs for bone health, especially for people who do not get enough calcium from food.
- Antacid: To relieve heartburn, indigestion, and sour stomach by neutralizing stomach acid. Products like Tums® and generic store brands contain calcium carbonate.
- Food additive: Used as a calcium fortifier in some plant milks, orange juice, cereals, and tofu, and as a firming agent in canned vegetables.
It is also used in some toothpastes as a mild abrasive and in calcium supplements for livestock.
How Much Calcium Carbonate Should You Take?
The amount depends on your age, sex, and whether you are pregnant or breastfeeding. National guidelines recommend the following total daily calcium intake from all sources (food plus supplements):
- Adults 19–50 years: 1,000 mg
- Women 51 and older: 1,200 mg
- Men 71 and older: 1,200 mg
- Pregnant and breastfeeding women: 1,000–1,300 mg
To avoid side effects and health risks, do not exceed 2,000–2,500 mg of total calcium per day from all sources. This includes calcium from food, drinks, and supplements. Most people can meet their needs from food alone. If you need a supplement, calcium carbonate is often taken in doses of 500–600 mg one to three times daily with meals.
What Are the Side Effects and Risks?
Calcium carbonate is generally safe when taken as directed, but it can cause side effects. The most common are constipation, bloating, and gas. Taking it with food helps reduce these issues.
High doses may increase the risk of kidney stones in people prone to them. Some research suggests that calcium from supplements may contribute to stone formation more than calcium from food, but the evidence is not conclusive. If you have had kidney stones, ask your doctor before taking calcium supplements.
Calcium carbonate can also interfere with how your body absorbs other medications, especially iron supplements, thyroid hormone (levothyroxine), and some antibiotics. To avoid problems, take calcium supplements at least two hours apart from these medications.
What Should You Look for in a Calcium Carbonate Supplement?
Not all supplements are created equal. Look for products that have been tested for quality by an independent third party, such as USP, NSF, or ConsumerLab. Avoid supplements that contain unrefined calcium sources like oyster shell or coral, as these may contain lead or other heavy metals.
Tablets and chewables are the most common forms. Check the label for the amount of elemental calcium per serving — that is the actual calcium your body uses. For example, a 1,250 mg tablet of calcium carbonate usually provides 500 mg of elemental calcium.
Vitamin D is often added to calcium supplements because it helps your body absorb calcium. Many multivitamins also contain calcium carbonate.
Frequently Asked Questions
Is calcium carbonate better than calcium citrate?
Neither is inherently better — it depends on your situation. Calcium carbonate is cheaper and requires fewer pills, but you must take it with food for best absorption. Calcium citrate is better if you take acid-reducing medications or have a sensitive stomach.
Can I take calcium carbonate on an empty stomach?
No, it is not recommended. Calcium carbonate needs stomach acid to be absorbed properly, and an empty stomach produces less acid. Taking it without food may reduce absorption and increase the chance of side effects like bloating.
Does calcium carbonate cause kidney stones?
There is mixed evidence. Some studies show that high calcium intake from supplements may slightly increase kidney stone risk in susceptible people, while calcium from food appears protective. If you have a history of kidney stones, talk to your doctor before taking any calcium supplement.
How much calcium carbonate is in a Tums?
It varies by product. A regular strength Tums tablet (750 mg) contains 300 mg of elemental calcium. Extra strength Tums (1,000 mg) provides 400 mg of elemental calcium. Always check the label for the exact amount.
