Watch for the kind of vitamin D your elderly patients are taking...The limitations of vitamin D therapy

Cholecalciferol is converted to active form, calcitriol (1,25-dihydroxyvitamin D3 ) in the kidneys. 

The active form of vitamin D3, 1,25-dihydroxyvitamin D3, is the major regulator of bone and calcium metabolism in the body.

What happens when kidney function has declined? Production of active form declines.

Calcitriol is available as tablets, capsules and liquid formulations

Calcitriol is 1,25-dihydroxycholecalciferol. It is the active metabolite of vitamin D (cholecalciferol). Increases the levels of calcium in the blood by improving its absorption.

Calcitriol is used in patients with kidney disease.

• Contraindicated in patients with hypercalcemia, vitamin D toxicity; Concurrent use of magnesium-containing antacids or other vitamin D supplements;
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• Side effects include weakness, headache, constipation, vomiting, arrthythmia, hypertension, hypotonia, polydipsia, polyuria, myalgia, metastatic calcification, etc. Allergic reactions, including anaphylaxis, have been reported. 
• Interactions Drug-Drug: Cholestyramine, colestipol, or mineral oil, vitamin D analogues. Use with thiazide diuretics like hydrochlorothiazide may result in hypercalcemia. Corticosteroid decrease the effectiveness of vitamin D analogues. Use with digoxin increases risk of arrhythmias. Concurrent use of magnesium-containing drugs may lead to hypermagnesemia. Calcium-containing drugs may increase risk of hypercalcemia. Concurrent use of other Vitamin D supplements(increases risk of hypercalcemia).
• Route/Dosage: Hypocalcemia During Dialysis PO (Adults): 0.25 mcg/day or every other day; if needed, may increase by 0.25 mcg/day at 4– 8 wk intervals (typical dosage 0.5– 1 mcg/day).
• May increase serum creatinine levels in predialysis patients.

package insert

Ref: https://davisplus.fadavis.com/3976/meddeck/pdf/calcitriol.pdf