[7] Excessive amounts or intake can result in weakness, headache, nausea, constipation, urinary tract infections, and abdominal pain.

[7] Calcitriol was identified as the active form of vitamin D in 1971 and the drug was approved for medical use in the United States in 1978.

[21] The main adverse drug reaction associated with calcitriol therapy is hypercalcaemia – early symptoms include: nausea, vomiting, constipation, anorexia, apathy, headache, thirst, pruritus, sweating, and/or polyuria.

Compared to other vitamin D compounds in clinical use (cholecalciferol, ergocalciferol), calcitriol has a higher risk of inducing hypercalcemia.

Consumption of these glycosides by grazing animals leads to vitamin D toxicity, resulting in calcinosis, the deposition of excessive calcium in soft tissues.

However, the main effect of PTH is to increase the rate at which the kidneys excrete inorganic phosphate (Pi), the counterion of Ca2+.

[26] For instance, the unbound inactive form of the calcitriol receptor in intestinal epithelial cells resides in the cytoplasm.

[27] The maintenance of electroneutrality requires that the transport of Ca2+ ions catalyzed by the intestinal epithelial cells be accompanied by counterions, primarily inorganic phosphate.

[25] Calcitriol is produced in the cells of the proximal tubule of the nephron in the kidneys by the action of 25-hydroxyvitamin D3 1-alpha-hydroxylase, a mitochondrial oxygenase and an enzyme which catalyzes the hydroxylation of 25-hydroxycholecalciferol (calcifediol) in the 1-alpha position.

[25] Additional effects on the production of calcitriol include an increase by prolactin, a hormone which stimulates lactogenesis (the formation of milk in mammary glands), a process which requires large amounts of calcium.

[29] Activity is also decreased by high levels of serum phosphate and by an increase in the production of the hormone FGF23 by osteocyte cells in bone.

[32] Calcitriol is marketed as a pharmaceutical for medical use under various brand names including Rocaltrol (Roche), Calcijex (Abbott), Decostriol (Mibe, Jesalis), Vectical (Galderma), and Rolsical (Sun Pharma).