Drug Monograph
Full clinical overview, indications, dosage references & safety notes.
Available forms3 forms · 9 strengths documentedShow all ↓
Capsule 0.25 µg Rocaltrol, calcitriolCapsule 0.5 µg Rocaltrol, calcitriolCapsule 0.25 µg alfacalcidolCapsule 0.5 µg alfacalcidolCapsule 1 µg alfacalcidol
Oral solution 1 µg/mL Rocaltrol, calcitriolOral solution drops 2 µg/mL One-alpha, alfacalcidol
1 µg/mL Calcijex, calcitriol, 1 mL amp2 µg/mL Calcijex, calcitriol, 1 mL amp
Overview
Vitamin D refers to a group of hormones that regulate calcium and phosphorus homeostasis in the body. In veterinary medicine, the most commonly used forms are calcitriol (1,25-dihydroxycholecalciferol), the active form of vitamin D3, and alfacalcidol, a precursor that is rapidly converted to an active metabolite. These agents are used primarily for the long-term management of disorders associated with impaired calcium regulation.
In dogs and cats, vitamin D therapy is most commonly used to manage hypocalcaemia associated with hypoparathyroidism and, in selected patients, renal secondary hyperparathyroidism. Calcitriol and alfacalcidol are preferred because of their rapid onset (1–2 days;) and short duration of effect half-life <1 day , allowing easier dose adjustment and monitoring.
Vitamin D therapy requires careful monitoring because these drugs have a narrow therapeutic index and excessive supplementation can rapidly result in clinically significant toxicity. Regular assessment of serum calcium concentrations, preferably ionized calcium, is essential throughout treatment.
Mechanism of Action (MOA): Vitamin D acts in conjunction with parathyroid hormone and calcitonin to regulate calcium and phosphorus balance. It promotes intestinal calcium absorption, supports normal bone mineral metabolism, and helps maintain adequate serum calcium concentrations through multiple regulatory mechanisms.
Indications
Vitamin D preparations are used in dogs and cats to manage disorders associated with impaired calcium regulation and parathyroid hormone dysfunction.
- Hypocalcaemia associated with hypoparathyroidism: Long-term management of chronic hypocalcaemia caused by immune-mediated or iatrogenic hypoparathyroidism.
- Renal secondary hyperparathyroidism: Calcitriol may be used to reduce excessive parathyroid hormone concentrations in selected patients with chronic kidney disease.
- Vitamin D deficiency: Used to correct clinically significant deficiency states associated with inadequate vitamin D activity.
Dosage (Reference)
Dog
Calcitriol and alfacalcidol are the preferred vitamin D preparations in dogs because of their relatively rapid onset of action and ease of dose adjustment. Dosage should be individualized based on clinical response and calcium monitoring.
| Clinical use | Route | Dose | Frequency | Notes |
|---|---|---|---|---|
| Hypocalcaemia / vitamin D deficiency (Calcitriol) | PO | 10–15 ng/kg q12h for 3–4 days, then 2.5–7.5 ng/kg q12h | q12h | Initial loading phase followed by long-term maintenance dosing. |
| Hypocalcaemia (Alfacalcidol) | PO | 0.01–0.03 µg/kg q24h | q24h | Adjust dose according to clinical response and calcium concentrations. |
| Renal secondary hyperparathyroidism (Calcitriol) | PO | 1.5–3.5 ng/kg q24h | q24h | Some authors recommend up to 6 ng/kg/day in refractory cases with appropriate monitoring. |
- Maintenance doses should be adjusted according to serum calcium concentrations and clinical response.
- Refractory renal secondary hyperparathyroidism may require doses up to 6 ng/kg/day of calcitriol when close monitoring is available.
- In patients with renal secondary hyperparathyroidism, serum calcium and phosphate concentrations should be assessed serially during treatment.
- Maintain the total calcium × phosphate product below 4.2 (calcium and phosphate in mmol/L). do not use if this cannot be kept below threshold.
- Empty-stomach / evening dosing: Give on an empty stomach, preferably in the evening, to minimise meal-driven calcium absorption and the risk of hypercalcaemia.
- Phosphate control first: Normalise serum phosphate before starting: keep phosphorus ≤6 mg/dL and calcium/phosphorus in the low-normal range . A phosphate binder is often needed first; using calcitriol with high phosphate risks tissue mineralization.
- Dog CKD evidence dose: Dog CKD evidence: strong evidence supports use in IRIS CKD Stages 3–4. Do not exceed 5 ng/kg/day; if hypercalcaemia develops, give double the daily dose every other day to reduce GI calcium absorption.
Cat
Vitamin D therapy in cats is used primarily for hypocalcaemia and selected cases of renal secondary hyperparathyroidism. Dose adjustments should be based on treatment response and calcium monitoring.
| Clinical use | Route | Dose | Frequency | Notes |
|---|---|---|---|---|
| Hypocalcaemia (Calcitriol) | PO | 10–15 ng/kg/day q12h for 3–4 days, then 2.5–7.5 ng/kg q12h | q12h | Initial loading phase followed by maintenance therapy. |
| Hypocalcaemia (Alfacalcidol) | PO | 0.01–0.03 µg/kg q24h | q24h | Titrate according to clinical response and calcium concentrations. |
| Renal secondary hyperparathyroidism (Calcitriol) | PO | 2.5–3.5 ng/kg/day | q24h | Used to suppress excessive parathyroid hormone activity in selected patients. |
- Long-term dosing should be individualized according to calcium monitoring results.
- Calcitriol and alfacalcidol are preferred because of their rapid onset and shorter duration of action.
- Regular assessment of calcium status is essential throughout therapy.
- Dose adjustments should be made cautiously because of the narrow therapeutic index of vitamin D preparations.
- Cat CKD evidence is weak: Reducing renal secondary PTH in cats with calcitriol is controversial and not supported by strong evidence; supplementation may slow progression in some cats but others show no benefit.
- Empty-stomach / evening dosing: In cats, do not administer simultaneously with a meal — this increases calcium absorption. Best given in the evening before a meal.
Warnings & Precautions
Vitamin D preparations require careful use in dogs and cats because they have a narrow therapeutic index and excessive dosing can rapidly result in clinically significant toxicity.
- Narrow therapeutic index: The difference between therapeutic and toxic doses is small, making accurate dosing and regular reassessment critical.
- Monitor calcium concentrations closely: Serum calcium, preferably ionized calcium, should be monitored frequently during therapy to allow timely dose adjustments.
- Renal failure patients: Only calcitriol should be used in patients with renal failure because it is already in the active form and does not require renal activation.
- Hyperphosphataemia: Do not use in patients with hyperphosphataemia. a calcium × phosphorus product >70 is considered a contraindication — using calcitriol with high phosphate increases tissue mineralization and additional renal tissue damage.
- Malabsorption syndromes: Contraindicated in patients with malabsorption disorders because therapeutic response may be unreliable.
- Pregnancy: Do not use in pregnant animals. calcitriol is teratogenic in rabbits at high doses — reduced fetal weight, skeletal abnormalities, neonatal hypercalcaemia, and reduced pup survival have been observed. It is also passed into milk.
- Product selection: Avoid highly concentrated vitamin A, D3, and E preparations intended for farm animals, as they may lead to dosing errors and toxicity in dogs and cats.
- Long-term therapy: Regular monitoring and dose adjustments are necessary throughout treatment to maintain safe and effective calcium regulation.
- calciTRIOL vs calciTONIN: Do NOT confuse calciTRIOL with calciTONIN — they have opposite calcium effects. writing orders and prescriptions with tall-man lettering: calciTONIN and calciTRIOL.
- Calcium-oxalate urolith caution: Use with caution in animals susceptible to calcium-oxalate uroliths, because calcitriol can promote hypercalciuria.
- Biliary / hepatic disease: Absorption is diminished in patients with steatorrhea or hepatic or biliary disease, because bile is required for adequate GI absorption of vitamin D analogues.
- Early hypercalcaemia signs: Watch for early signs of hypercalcaemia — polydipsia, polyuria, and anorexia — and contact the clinic promptly; these can appear before serum changes are confirmed.
- Human capsule overdose risk: Human calcitriol capsules can contain doses far too high for dogs and cats; reformulation or compounding (e.g. dilution in MCT/olive oil) is often required for accurate small-animal dosing.
Drug Interactions
Several medications can alter the effectiveness of vitamin D therapy or increase the risk of adverse effects related to calcium and phosphorus metabolism. Patients receiving concurrent treatment should be monitored carefully.
- Corticosteroids: May antagonize the effects of vitamin D preparations and reduce their therapeutic efficacy.
- Sucralfate: Decreases gastrointestinal absorption of vitamin D and may reduce treatment response. Conversely calcitriol may increase aluminum absorption from sucralfate — a bidirectional interaction.
- Hepatic enzyme inducers (e.g., barbiturates): Increase vitamin D metabolism, potentially lowering circulating concentrations and reducing clinical effectiveness.
- Calcium-containing antacids: Concurrent use may increase the risk of hypercalcaemia and requires careful monitoring.
- Magnesium-containing antacids: May increase the risk of hypermagnesaemia when administered with vitamin D preparations.
- Thiazide diuretics: May increase the likelihood of hypercalcaemia when used concurrently with vitamin D.
- Digoxin: Hypercalcaemia associated with vitamin D therapy may potentiate digoxin toxicity; close monitoring is recommended.
- Verapamil: The toxic effects of verapamil may be enhanced in patients that develop hypercalcaemia during vitamin D therapy.
- Aluminum phosphate binders: Aluminum-containing phosphate binders (e.g. aluminum hydroxide): calcitriol may increase aluminum absorption, raising the risk of aluminum accumulation.
- Cholestyramine / Sevelamer: Cholestyramine and sevelamer may reduce calcitriol absorption — separate administration by several hours.
Side Effects & Overdose
Side Effects
Adverse effects of vitamin D therapy are primarily related to excessive calcium and phosphorus concentrations. Because vitamin D has a narrow therapeutic index, careful monitoring is required throughout treatment.
- Hypercalcaemia: The most common and clinically important adverse effect, particularly with excessive dosing or inadequate monitoring. Signs include polydipsia, polyuria, and anorexia . At low CKD doses hypercalcaemia is infrequent unless combined with a calcium-containing phosphorus binder (especially calcium carbonate).
- Hyperphosphataemia: May develop during treatment and can contribute to calcium-phosphorus imbalance and tissue mineralization.
- Soft-tissue calcification: Soft-tissue / tissue calcification can occur in dogs after chronic (up to 26-week) dosing even at marginally elevated calcitriol concentrations.
Overdose
Vitamin D overdose is a medical emergency due to its potent effects on calcium and phosphorus metabolism. Toxicity can occur relatively easily because of the drug’s narrow therapeutic index.
- Severe hypercalcaemia: The principal consequence of overdose and the major cause of clinical toxicity.
- Marked hyperphosphataemia: May occur concurrently and increase the risk of soft tissue and organ mineralization.
- Calcium-phosphorus imbalance: Excessive elevations can result in widespread tissue damage and organ dysfunction.
- Management: WITHHOLD calcitriol (do not merely reduce the dose) and treat. For acute ingestion, oral cholestyramine or mineral oil may reduce absorption. For severe hypercalcaemia: a bisphosphonate (e.g. pamidronate), furosemide, calcium-free IV fluids (normal saline), urine acidification, and corticosteroids may be used. Hypercalcaemia may take several days to a week to resolve; consult a 24-hour veterinary poison centre.
- Monitoring: Serial assessment of serum calcium, ionized calcium, and phosphorus concentrations is essential until values stabilize.
Key Notes
Practical clinical points that can help optimize the use of vitamin D preparations in dogs and cats:
- Calcitriol is often preferred for dose adjustments: Its rapid onset and shorter duration allow clinicians to achieve and fine-tune calcium control more predictably than older vitamin D preparations.
- Hypoparathyroidism usually requires lifelong therapy: Many patients require long-term or permanent vitamin D supplementation to maintain normal calcium concentrations.
- Vitamin D therapy is often combined with calcium supplementation: Successful management of hypocalcaemia frequently requires concurrent calcium administration, especially during initial stabilization. combine with oral calcium to reduce the vitamin D dose requirement; oral calcium can usually be tapered and discontinued within a week of starting calcitriol.
- Clinical improvement may precede laboratory stabilization: Patients can appear clinically normal before calcium regulation is fully stabilized, making continued follow-up important.
- Individual dose requirements vary considerably: Maintenance doses differ widely among patients, and long-term requirements may change over time.
- Renal secondary hyperparathyroidism treatment aims to suppress PTH: The goal is not simply calcium supplementation, but reduction of excessive parathyroid hormone activity associated with chronic kidney disease.
- Transition periods require particular attention: The highest risk of treatment failure occurs during initiation and major dose adjustments, when calcium balance is still being established.
- Empty-stomach / evening dosing: Give on an empty stomach, preferably in the evening, to minimise meal-driven calcium absorption and the risk of hypercalcaemia.
- Normal calcium ranges + stop-threshold: Normal total calcium: dogs 9–11.5 mg/dL, cats 8–10.5 mg/dL. If ionized calcium rises above the reference range (above 4.5–5.5 mg/dL), STOP treatment and reintroduce at a lower dose.
- Vitamin D unit conversion: 1 unit of vitamin D = 0.025 µg of cholecalciferol or ergocalciferol (so 400 units = 10 µg). Useful when reconciling doses across different vitamin D products.
