Calcium carbonate is a chemical compound with formula CaCO3. Calcium carbonate is an inorganic salt used as an antacid. It is a basic compound that acts by neutralizing hydrochloric acid in gastric secretions. Increases in pH may inhibit the action of pepsin. It is a common substance found in rocks as the minerals calcite and aragonite  and is the main component of pearls and the shells of marine organisms, snails, and eggs. It is used as a calcium supplement and as an antacid, but excessive consumption can be hazardous and cause poor digestion.Calcium carbonate is used to prevent or to treat a calcium deficiency.There are number of brands and forms of calcium carbonate available.
Calcium Carbonate is a medication used to prevent or treat low blood calcium levels in people who do not get enough calcium from the diets. It may be used to treat conditions caused by low calcium levels such as bone loss caleed as osteoporosis, weak bones (osteomalacia/rickets), decreased activity of the parathyroid gland known as hypoparathyroidism, and a certain muscle disease (latent tetany). It may also be used in certain patients to make sure that they are getting enough calcium like the womens who are pregnant, nursing or postmenopausal.Calcium carbonate may also be used as a nutritional supplement or to treat hypocalcemia.

Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and multiple other biological effects. In humans, the most important compounds in this group are vitamin D3 which is also known as cholecalciferol.
The major natural source of the vitamin is synthesis of cholecalciferol in the lower layers of skin epidermis through a chemical reaction that is dependent on sun . Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D.  Mushrooms exposed to ultraviolet light contribute useful amounts of vitamin D.Dietary recommendations typically assume that all of a person's vitamin D is taken by mouth, as sun exposure in the population is variable and recommendations about the amount of sun exposure that is safe are uncertain in view of the skin cancer risk. Vitamin D from the diet, or from skin synthesis, is biologically inactive. A protein enzyme must hydroxylate it to convert it to the active form.

About Composition:

●It is a dietary supplement used to treat the deficiency of calcium.Calcium is needed by the body for healthy bones, muscles, nervous system, and heart.
●It is also used as an antacid to relieve heartburn, acid indigestion, upset stomach ●to treat hypocalcemia



Pharmacology
Mechanism of action Calcium carbonate is an inorganic salt used as an antacid. It is a basic compound that acts by neutralizing hydrochloric acid in gastric secretions. An increase in bicarbonate ions and prostaglandins may also confer cytoprotective effects. Calcium carbonate is a basic inorganic salt that acts by neutralizing hydrochloric acid in gastric secretions. It also inhibits the action of pepsin, it increases the pH. Cytoprotective effects may occur through increases in bicarbonate ion (HCO3-) and prostaglandins. Neutralization of hydrochloric acid results in the formation of calcium chloride, carbon dioxide and water. 90% of calcium chloride is converted to insoluble calcium salts.
Vitamin D3: The active form of vitamin D binds to intracellular receptors that then function as transcription factors to modulate gene expression. Like the receptors for other steroid hormones and thyroid hormones, the vitamin D receptor has hormone-binding and DNA-binding domains.

Pharmacodynamics: Calcium Carbonate: Gastric-peptic disease occurs as a result of an imbalance between protective factors, such as mucus, bicarbonate, and prostaglandin secretion, and aggressive factors, such as hydrochloric acid, pepsin, and Helicobacter pylori . The acid-neutralizing capacity of calcium carbonate is 58 mEq/15 ml. When used as a nutritional supplement, calcium carbonate acts by directly increasing calcium stores within the body.
Vitamin D3: The in vivo synthesis of the predominant two biologically active metabolites of vitamin D occurs in two steps. The first hydroxylation of vitamin D3 cholecalciferol (or D2) occurs in the liver to yield 25-hydroxyvitamin D while the second hydroxylation happens in the kidneys to give 1, 25-dihydroxyvitamin D . These vitamin D metabolites subsequently facilitate the active absorption of calcium and phosphorus in the small intestine, serving to increase serum calcium and phosphate levels sufficiently to allow bone mineralization . There exists a period of 10 to 24 hours between the administration of cholecalciferol and the initiation of its action in the body due to the necessity of synthesis of the active vitamin D metabolites in the liver and kidneys . It is parathyroid hormone that is responsible for the regulation of such metabolism at the level of the kidneys.

Pharmacokinetics: Calcium Carbonate:
Absorption: Maximal absorption occurs at doses of 500 mg or less taken with food. Oral bioavailability depends on intestinal pH, the presence of food inside and dosage.
Distribution: Calcium is rapidly distributed taken up by skeletal tissues following absorption and distribution into extracellular fluids. Bone contains 99% of the body's calcium and the remaining 1% is approximately, equally distributed between intracellular and extracellular fluids. Calcium acts as a co-factor to numerous enzymes.
Elimination: Excreted mainly in the feces. The majority of renally filtered calcium is reabsorbed in the ascending limb of the loop of Henle and the proximal and distal convoluted tubules. Secreted by sweat glands also.

Vitamin D3:
Absorption: Cholecalciferol is readily absorbed from the small intestine if fat absorption is normal . Moreover, bile is necessary for absorption as well.
In particular, recent studies have determined aspects about the absorption of vitamin D, like the fact that a) the 25-hydroxyvitamin D metabolite of cholecalciferol is absorbed to a greater extent than the nonhydroxy form of cholecalciferol, b) the quantity of fat with which cholecalciferol is ingested does not appear to largely affect its bioavailability, and c) age does not apparently effect vitamin D cholecalciferol
Protein binding:The protein binding documented for cholecalciferol is 50 to 80% 8. Specifically, in the plasma, vitamin D3 (from either diet or the skin) is bound to vitamin D-binding protein produced in the liver, for transport to the liver. Ultimately, the form of vitamin D3 reaching the liver is 25-hydroxylated, and such 25-hydroxycholecalciferol is bound to DBP (α2-globulin) whilst circulating in the plasma
Metabolism:Within the liver, cholecalciferol is hydroxylated to calcifediol (25-hydroxycholecalciferol) by the enzyme vitamin D-25-hydroxylase . At the kidney, calcifediol subsequently serves as a substrate for 1-alpha-hydroxylase, yielding calcitriol (1,25-dihydroxycholecalciferol), the biologically active form of vitamin D3
Elimination: It has been observed that administered cholecalciferol and its metabolites are excreted primarily in the bile and feces

Side Effects

● upset stomach
● vomiting
● belching,constipation
● dry mouth
● increased urination
● loss of appetite
● metallic taste,nausea

Contraindication
● Hypersensitivity
● High calcium levels in the urine
● Kidney stones (renal calculi)
● Low phosphate levels
● High calcium levels
● Suspected digoxin toxicity

Special warnings and precautions
Before taking calcium carbonate
●tell your doctor if you are allergic to calcium carbonate or any other drugs.
●tell your doctor what prescription and nonprescription medications you are taking, especially digoxin (Lanoxin), etidronate (Didronel), phenytoin (Dilantin), tetracycline (Sumycin) .
●tell your doctor if you are pregnant, plan to become pregnant, or are breast-feeding.

Pregnancy and Lactation:
●Use calcium carbonate during pregnancy with caution if benefits outweigh risks.
●Calcium carbonate crosses the placenta.Calcium carbonate use when breastfeeding is safe; it appears in breast milk.

Storage
Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).