Calcium
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Calcium (carbonate, citrate, gluconate, chloride salts)
Tums, Caltrate, Os-Cal, Citracal; many generics
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Summary
Common uses
Hypocalcemia treatment and prevention0, Osteoporosis (with vitamin D)0, Dyspepsia/heartburn (carbonate antacid)0, Membrane stabilization in life-threatening hyperkalemia (IV gluconate or chloride)0, Phosphate binding in CKD-mineral bone disorder (carbonate or acetate)0
Pharmacy
Starting dose
Oral: 1000-1500 mg elemental calcium/day in divided doses for supplementation; IV gluconate 1 g (4.65 mEq) over 5-10 min for hyperkalemia or symptomatic hypocalcemia
Preparations
Carbonate 200, 400, 500, 600 mg elemental tablets and chewables; citrate 200, 250, 315 mg elemental; gluconate 1 g (94 mg elemental, 4.65 mEq) IV; chloride 1 g (270 mg elemental, 13.6 mEq) IV
US FDA Max
~2500 mg elemental/d combined diet + supplements (chronic; UL)
Pharmacology
Routes
Oral, IV (gluconate or chloride for acute use)
Onset
Acute: minutes (IV); chronic: bone effect over months
Duration
Variable
Half-life
Not meaningfully described (electrolyte)
Bioavailability
Carbonate ~30-40% (best with food and acid); citrate ~24% (absorbable without acid; preferred in achlorhydria, PPI use, post-bariatric)
Pregnancy
Routinely supplemented in pregnancy; needs higher in pregnancy and lactation.[citation needed]
Legal status
OTC (oral supplements) and Rx-only (IV) in US
Purported mechanism
Oral calcium repletes the serum and skeletal calcium pool; IV calcium (gluconate or chloride) stabilizes the cardiac myocyte resting potential in hyperkalemia by raising the action-potential threshold relative to the membrane potential, restoring electrochemical gradient without affecting serum potassium itself.0 Calcium carbonate is also an effective antacid via direct gastric acid neutralization. Co-administration with iron and tetracyclines reduces absorption of both; separate dosing windows. Excess oral calcium may increase nephrolithiasis and cardiovascular events in some studies, fueling preference for dietary over supplement-driven repletion.