Trilostane
4-alpha, 5-alpha-Epoxy-17-beta-hydroxy-3-oxoandrostane-2-alpha-carbonitrile
Also known as: Vetoryl · Modrastane · Desopan · Modrenal · WIN 24540 · Trilostanum
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 |
|---|---|---|---|---|---|
| Hyperadrenocorticism (HAC) - Label Dose | 2.2-6.7 mg/kg PO once a day with food | PO | q24h | — | 🌎 NA |
| Hyperadrenocorticism (HAC) - Alternative Protocol | 2-10 mg/kg PO once daily | PO | q24h | — | 🌎 NA |
| Hyperadrenocorticism (HAC) - Low Dose Protocol | 1 mg/kg PO once daily | PO | q24h | — | 🌎 NA |
| Alopecia X (Alaskan Malamutes) | 3-3.6 mg/kg PO twice a day | PO | q12h | 4-6 months | 🌎 NA |
| Alopecia X (Miniature poodles and Pomeranians) | 10.85 mg/kg per day given either once a day or divided twice a day | PO | q24h or q12h | 4-8 weeks | 🌎 NA |
| Hyperadrenocorticism | 2 mg/kg p.o. q24h or 1 mg/kg p.o. q12h | PO | q12h or q24h | Lifelong | 🌍 EU |
- Hyperadrenocorticism (HAC) - Label Dose: Adjust dose based on ACTH stimulation test 10-14 days post-initiation. May require twice daily dosing if clinical signs are not controlled for the full day.
- Hyperadrenocorticism (HAC) - Alternative Protocol: Doses of up to 50 mg/kg/day divided twice daily have been given. Adjust based on ACTH stim test 4-6 hours post-dose.
- Hyperadrenocorticism (HAC) - Low Dose Protocol: Recheck in 1 week. Adjust based on clinical response, UCCR, and ACTH stimulation test.
- Alopecia X (Miniature poodles and Pomeranians): Average dose reported.
- Hyperadrenocorticism: Give with food. Adjust dose based on monitoring.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hyperadrenocorticism | 7 mg/kg/day divided and given twice daily | PO | q12h | — | 🌎 NA |
| Hyperadrenocorticism | 15 mg (total dose) PO once daily to 60 mg (total dose) PO q12h | PO | q24h to q12h | — | 🌎 NA |
| Hyperadrenocorticism | 10-30 mg/cat p.o. q12-24h | PO | q12h to q24h | Lifelong | 🌍 EU |
- Hyperadrenocorticism: Doses of up to 60 mg per cat per day have been used.
- Hyperadrenocorticism: Titrate dose with ACTH stimulation tests. Cats typically remain diabetic despite clinical improvement.
- Hyperadrenocorticism: Give with food.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Equine Cushing's syndrome | 0.4-1 mg/kg (total dose 120-240 mg) PO once daily | PO | q24h | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Trilostane is a synthetic steroid analog and currently the standard of care for managing pituitary-dependent and adrenal-dependent hyperadrenocorticism (Cushing's syndrome) in dogs.
Unlike older therapies such as mitotane, which cause targeted necrosis of the adrenal cortex, trilostane works by reversibly inhibiting steroidogenesis. This makes it generally safer and more predictable, though careful monitoring is still required to prevent iatrogenic hypoadrenocorticism (Addison's disease).
Key Clinical Uses:
- Dogs: Pituitary-dependent and adrenal-dependent hyperadrenocorticism; Alopecia X (especially in Pomeranians and Alaskan Malamutes).
- Cats: Feline pituitary-dependent hyperadrenocorticism (off-label).
- Horses: Equine hyperadrenocorticism / Pituitary Pars Intermedia Dysfunction (PPID) (off-label, though pergolide is more commonly the first-line treatment for PPID).
Clinical Pearl: When used in dogs with adrenal tumors, trilostane will control the clinical signs of Cushing's but will not shrink the tumor. In fact, the adrenal glands may undergo compensatory hypertrophy and increase in size during treatment.
Mechanism of action
Trilostane acts as a competitive, reversible, and dose-dependent inhibitor of the enzyme 3-beta hydroxysteroid dehydrogenase (3β-HSD).
- Pathway Blockade: Pregnenolone → [3β-HSD Blocked] → Progesterone
- By blocking this crucial early step in the adrenal steroidogenesis pathway, trilostane effectively reduces the downstream synthesis of cortisol, aldosterone, and adrenal androgens.
- Because the inhibition is competitive and reversible, the suppressive effects on cortisol production wane within 10 to 20 hours, necessitating daily or twice-daily dosing.
Safety & warnings
Contraindications
- Hypersensitivity to trilostane
- Pregnancy (reduces progesterone synthesis)
- Use with caution in patients with renal impairment
- Use with caution in patients with hepatic impairment
- Renal insufficiency
- Hepatic insufficiency
Adverse effects
- Lethargy
- Inappetence
- Vomiting
- Diarrhea
- Mild electrolyte abnormalities (hyponatremia, hyperkalemia)
- Iatrogenic hypoadrenocorticism (Addisonian crisis)
- Adrenal necrosis (rare but potentially fatal)
- Mild gastrointestinal signs
- Mild increases in serum potassium, bilirubin, and calcium
- Clinical hypoadrenocorticism (Addisonian crisis)
- Adrenal hyperplasia (with prolonged treatment)
- Prolonged adrenal suppression after drug withdrawal
Precautions
Trilostane is contraindicated in pregnant dogs and should be used with caution in lactating animals. It should be used cautiously in patients with pre-existing renal or hepatic impairment.
Important: Trilostane does not shrink adrenal tumors; adrenal glands may actually increase in size during therapy due to loss of negative feedback.
Human Safety Warning: Trilostane is an FDA Category X drug (contraindicated in pregnancy). Pregnant women or women trying to conceive must not handle the capsules.
Drug interactions
Could increase risk for hyperkalemia
May potentiate the effects of trilostane and lead to hypoadrenocorticism
May potentiate the effects of trilostane and lead to hypoadrenocorticism
May potentiate the effects of trilostane and lead to hypoadrenocorticism
Could increase risk for hyperkalemia
Could increase risk for hyperkalemia
Concurrent suppression of adrenal function
Monitoring
- Clinical signs (water intake, urination, appetite, energy level)
- Adverse effects (vomiting, diarrhea, lethargy)
- Serum electrolytes (Sodium and Potassium)
- Urinalysis (Specific gravity, glucose, urine cortisol:creatinine ratio [UCCR])
- ACTH stimulation tests (conducted 4-6 hours post-pill)
- Clinical signs (water intake, urination, appetite, hair coat)
- ACTH stimulation test (3 hours post-pill)
- Pre-pill baseline cortisol
- Serum electrolytes (potassium, calcium)
- Liver parameters (bilirubin)
Pharmacokinetics
Absorption
In dogs, orally administered trilostane is rapidly but erratically absorbed. Peak levels occur between 1.5-2 hours post-dose. It is unknown if food significantly alters absorption, but administration with food is recommended.
Metabolism
Metabolized in the liver to several metabolites, including the active metabolite ketotrilostane.
Elimination
Drug levels return to baseline after 18 hours. Effects on cortisol production wane within 10-20 hours of dosing.
Overdose
Acute overdoses are unlikely to be life-threatening, and severe clinical signs are not typically expected immediately.
- Monitoring: Assess blood pressure, hydration status, and electrolyte balance (Na/K).
- Treatment: If the animal is stressed or showing signs of hypoadrenocorticism, consider administering exogenous corticosteroids short-term.
- Because the drug's effects are relatively short-lived, monitoring of uncomplicated patients is usually only required for a few days post-ingestion.
Available products
Formulations
- Oral capsules: 10 mg, 30 mg, 60 mg, 120 mg
- Oral capsules: 5 mg (named patient basis)
- Liquid formulation (named patient basis)
Veterinary
- Vetoryl Oral Capsules: 10 mg, 30 mg, & 60 mg (packaged in aluminum foil blister cards)
- Vetoryl 10 mg, 30 mg, 60 mg, 120 mg capsules
Human-labeled
- Modrastane (Withdrawn from the US market in 1994)
Regulatory status
POM-V
POM-V
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store commercially available trilostane capsules at room temperature in tight, light-resistant containers.
Client information
Trilostane is a highly effective medication for managing Cushing's disease, but it requires careful handling and monitoring.
Handling Precautions:
- Keep out of reach of children and pets.
- Pregnancy Warning: Women who are pregnant or trying to conceive should NOT handle these capsules, as the drug can affect human pregnancies.
- Wash your hands thoroughly after handling the medication.
- Do not open, empty, or attempt to divide the capsules.
Administration:
- Give the medication exactly as prescribed, ideally with food to enhance absorption.
- Understand that this medication is a treatment, not a cure. Your pet will likely need this medication for the rest of their life.
What to Watch For:
- During the first few days, mild lethargy or a decrease in appetite can occur as your pet's body adjusts to lower steroid levels.
- STOP the medication and contact your veterinarian immediately if your pet experiences severe lethargy, vomiting, diarrhea, collapse, or completely stops eating. These can be signs that the cortisol levels have dropped too low (an Addisonian crisis).
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.