Calcium carbonate

Calcium carbonate, or CaCO3, is used as a dietary supplement when one's diet doesn't comprise of enough calcium.

It comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years.

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• Why is this medication prescribed?