Ergocalciferol
Vitamin D2, Calciferol, activated ergosterol
Also known as: Drisdol · Calciferol Drops · Vitamin D2 · Viosterol · Activated ergosterol · Irradiated ergosterol
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany | Initially 4000-6000 Units/kg PO once daily. Maintenance doses usually range from 1000-2000 Units/kg PO once daily to once weekly. | PO | once daily to once weekly | Lifelong | 🌎 NA |
- Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany: Patient should be hospitalized initially. Adjust dosage to maintain serum calcium concentrations between 8-9.5 mg/dL. Goal is to prevent hypocalcemic tetany, but not induce hypercalcemia.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany | Initially 4000-6000 Units/kg PO once daily. Maintenance doses usually range from 1000-2000 Units/kg PO once daily to once weekly. | PO | once daily to once weekly | Lifelong | 🌎 NA |
- Maintenance therapy of parathyroid failure after using parenteral calcium to control hypocalcemic tetany: Patient should be hospitalized initially. Adjust dosage to maintain serum calcium concentrations between 8-9.5 mg/dL. Goal is to prevent hypocalcemic tetany, but not induce hypercalcemia.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Ergocalciferol (Vitamin D2) is a fat-soluble vitamin and Vitamin D analog used primarily in veterinary medicine to manage hypocalcemia associated with hypoparathyroidism.
- Cost-effective: Often chosen as a less expensive alternative to calcitriol or dihydrotachysterol (DHT).
- Delayed onset: Because it requires both hepatic and renal enzymatic activation, it takes longer (5-21 days) to achieve maximal effects on serum calcium compared to active metabolites.
- Prolonged toxicity: It is highly lipophilic and stored in fat. If hypercalcemia develops, toxic effects can persist for up to 18 weeks even after discontinuation.
Clinical Pearl: Cats do not efficiently convert ergocalciferol to its 25-hydroxylated form compared to cholecalciferol (Vitamin D3). Therefore, calcitriol is generally preferred in feline patients for managing hypocalcemia.
Mechanism of action
Ergocalciferol is an inactive prodrug that requires a two-step enzymatic activation process to regulate calcium homeostasis:
- Ergocalciferol → hepatic 25-hydroxylase → 25-hydroxyvitamin D2 (calcidiol, partially active).
- 25-hydroxyvitamin D2 → renal 1-alpha-hydroxylase → 1,25-dihydroxyvitamin D2 (fully active calcitriol equivalent).
The active metabolite binds to intracellular Vitamin D Receptors (VDR), acting as a hormone alongside parathyroid hormone (PTH) and calcitonin to:
- Enhance intestinal absorption of calcium and phosphate.
- Promote renal tubular reabsorption of calcium.
- Increase the rate of accretion and osteoclastic resorption of minerals in bone, mobilizing calcium into the extracellular fluid.
Safety & warnings
Contraindications
- Hypercalcemia
- Vitamin D toxicity
- Malabsorption syndrome
- Abnormal sensitivity to the effects of vitamin D
Adverse effects
- Hypercalcemia
- Hyperphosphatemia
- Nephrocalcinosis
- Soft tissue mineralization
Precautions
Important Warnings
- Renal Impairment: Patients with kidney dysfunction may not convert ergocalciferol into the primary active metabolite. Calcitriol or DHT is preferred as they bypass renal activation.
- Hyperphosphatemia: Use with extreme caution. Elevated calcium and phosphorus simultaneously can lead to severe metastatic soft tissue mineralization (monitor calcium x phosphorus product).
- Monitoring Commitment: Chronic therapy should not be initiated unless owners are willing to commit to ongoing, rigorous patient monitoring.
- Pregnancy: Hypervitaminosis D has been implicated in causing teratogenic effects. Weigh potential benefits against risks.
- Nursing: Large doses of vitamin D are excreted into milk. Consider using milk replacer for offspring of dams receiving therapeutic dosages.
Drug interactions
Can reduce the effects of vitamin D analogs by decreasing calcium absorption and increasing renal excretion.
Patients are highly sensitive to the arrhythmogenic effects of hypercalcemia; intensified monitoring is required.
Patients are sensitive to the effects of hypercalcemia; intensified monitoring is required.
May reduce the amount of orally administered ergocalciferol absorbed from the GI tract.
May cause hypercalcemia when given in conjunction with Vitamin D analogs due to decreased renal calcium excretion.
Monitoring
- Serum calcium concentrations (weekly initially, then monthly for 6 months, then every 2-3 months)
- Serum phosphorus concentrations
- Renal function parameters (BUN, Creatinine)
- Clinical signs of hypercalcemia (PU/PD, lethargy, anorexia, vomiting)
Pharmacokinetics
Absorption
Absorbed from the small intestine in the presence of bile salts.
Distribution
Stored extensively in the liver and fat. Several days of therapy may be required until distribution steady state is achieved.
Metabolism
Hydroxylated in the liver to 25-hydroxyvitamin D, then activated in the kidneys to 1,25-dihydroxyvitamin D. Cats do not appear to convert ergocalciferol to its 25-hydroxylate form as well as cholecalciferol.
Elimination
Effects may persist for up to 18 weeks once treatment is discontinued due to slow release from fat stores.
Overdose
Acute Toxicity
- The toxic acute oral dose of ergocalciferol in dogs is reported as 4 mg/kg (160,000 Units/kg).
- Decontamination: Acute ingestions should be managed using established protocols for removal or prevention of GI absorption. Orally administered mineral oil may reduce absorption and enhance fecal elimination.
Chronic Overdosage (Hypercalcemia)
- Immediate Action: Temporarily discontinue ergocalciferol and any exogenous calcium therapy.
- Medical Management: If hypercalcemia is severe, administer 0.9% normal saline (calcium-free IV fluids), furosemide (to promote calciuresis), and corticosteroids (to decrease GI absorption and increase renal excretion of calcium). Urine acidification may also be employed.
- Prolonged Effect: Because of the long duration of action (potentially up to 18 weeks), hypercalcemia may persist for weeks to months. Restart therapy at a reduced dosage only when calcium serum levels return to the normal range.
Available products
Formulations
- Oral Liquid (Drops)
- Oral Capsules
Human-labeled
- Ergocalciferol Oral Liquid (Drops): 8,000 Units/mL (200 micrograms/mL) in 60 mL
- Ergocalciferol Oral Capsules: 50,000 Units (1.25 mg)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Ergosterol is sensitive to light, heat, and air. Store capsules or liquid at room temperature (15-30°C) and protect from light.
Client information
- Cost vs. Efficacy: While ergocalciferol is less expensive than other vitamin D medications (like calcitriol), it is usually not the first choice because it takes longer to start working and stays in the body much longer.
- Lifelong Commitment: Using vitamin D products for parathyroid issues usually requires lifelong treatment and regular veterinary visits for blood tests.
- Risk of High Calcium: While this medication treats low calcium, an overdose can cause blood calcium levels to become dangerously high. Because the drug is stored in body fat, this toxic effect can last for many weeks, even after you stop giving the medication.
- Watch for Signs: Contact your veterinarian immediately if your pet shows signs of high calcium, such as increased thirst, increased urination, vomiting, or extreme tiredness.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.