Vetoryl (Trilostane) is a prescription medication used to treat pituitary-dependent and adrenal-dependent hyperadrenocorticism (Cushing's disease) in dogs. It works by inhibiting the production of cortisol by the adrenal glands.

Starting Dose Guidelines

The recommended starting dose of Trilostane is 2–5 mg/kg once daily with food. A conservative starting dose of 2.2–2.7 mg/kg is commonly used. Your vet will adjust based on ACTH stimulation testing after 10 days and at 4 and 12 weeks.

Available Capsule Sizes

Vetoryl capsules come in 10 mg, 30 mg, 60 mg, and 120 mg. Because the capsules cannot be split, the dose is rounded to the nearest available capsule size. Often a combination of capsule sizes is used.

Critical Monitoring Requirements

An ACTH stimulation test must be performed 4–6 hours post-pill at 10 days, 4 weeks, 12 weeks, and then every 3 months. Target post-ACTH cortisol is 40–150 nmol/L. Never adjust the dose without vet guidance. Signs of Addisonian crisis (over-suppression) — lethargy, vomiting, collapse — require immediate emergency treatment.

🧬 Adrenal Steroidogenesis Inhibitor

Vetoryl Dog Dosage Calculator

Precise trilostane (Vetoryl) starting and adjusted dosing for dogs with hyperadrenocorticism (Cushing’s disease) — covering PDH, adrenal-dependent and iatrogenic forms, capsule selection, and ACTH stimulation test monitoring targets.

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Never use without confirmed Cushing’s diagnosis and regular ACTH stimulation test monitoring. Trilostane inhibits adrenal steroidogenesis and can cause life-threatening hypoadrenocorticism (Addison’s crisis) if the dose is too high. Mandatory ACTH stimulation test at 10–14 days, 4 weeks, 12 weeks, then every 3 months. Emergency glucocorticoid supplementation protocol must be established before starting.

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About Trilostane (Vetoryl)

Drug Class

3β-Hydroxysteroid Dehydrogenase Inhibitor — Adrenal Cortisol Synthesis Blocker

Mechanism of Action

Trilostane reversibly inhibits the enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD) in the adrenal cortex, blocking the conversion of pregnenolone to progesterone, which is a key early step in cortisol, aldosterone, and sex steroid biosynthesis. This selectively reduces cortisol hypersecretion in Cushing’s disease.

Primary Uses in Dogs

Pituitary-dependent hyperadrenocorticism (PDH — the most common form, ~85% of cases), and adrenal-dependent HAC (ADH) in dogs not suitable for surgery. It is the only FDA-approved medical treatment for canine Cushing’s disease.

Dosing Quick Reference

Indication	Dose	Frequency
Starting dose (PDH)	2–5 mg/kg	q24h PO with food
Starting dose (ADH)	2–10 mg/kg	q24h PO with food
Dose adjustment	Target post-ACTH cortisol	50–150 nmol/L (1.4–5.5 mcg/dL)

Common Side Effects

Anorexia and lethargy (early, often mild)
Vomiting and diarrhoea
Adrenal insufficiency (Addison-like crisis — serious, requires emergency treatment)
Adrenal necrosis/haemorrhage (rare but severe)
Electrolyte disturbances (Na:K ratio changes)

Monitoring

ACTH stimulation test (post-ACTH cortisol target 50–150 nmol/L) at Days 10–14, 1 month, 3 months, then every 3 months. Electrolytes (Na, K) at each recheck. Physical signs, water intake, and urination frequency tracking.

⚠️ Adrenal insufficiency is the most serious risk — signs include profound lethargy, weakness, vomiting, and collapse. Owners must have emergency access to veterinary care. ACTH stimulation tests are required at Days 10–14, 4 weeks, 12 weeks, then every 3 months — these are non-optional for safe monitoring.

What Is Trilostane (Vetoryl)?

Vetoryl (trilostane) is the FDA- and EMA-approved treatment for both pituitary-dependent hyperadrenocorticism (PDH) and adrenal-dependent hyperadrenocorticism (ADH) in dogs. It competitively inhibits 3β-hydroxysteroid dehydrogenase (3β-HSD), a key enzyme in the adrenal steroidogenesis cascade, reducing synthesis of cortisol, aldosterone, and other steroids from cholesterol precursors in the adrenal cortex. Unlike mitotane (o,p’-DDD), trilostane is a reversible inhibitor — stopping the drug allows adrenal function to recover.

Trilostane is available as 10 mg, 30 mg, 60 mg, and 120 mg hard capsules (Vetoryl). The drug must be given with food to ensure consistent absorption — fat-containing meals significantly improve bioavailability. Capsules must not be opened or crushed (risk of topical exposure).

Key Pharmacokinetics

Oral bioavailability: ~30–40%; highly variable — ALWAYS give with food
Onset of cortisol suppression: Within 1–2 hours of dosing
Duration of effect: ~12–18 hours per dose (important for timing monitoring tests)
Half-life: ~1.2 hours (but active metabolite ketotrilostane is longer-acting)
Metabolism: Hepatic; ketotrilostane is the primary active metabolite

Dosing Overview

PDH starting dose: 2–3 mg/kg q24h with food (most dogs start here)
ADH starting dose: 2–3 mg/kg q24h (may need higher doses for adrenal tumours)
Maximum starting dose: Typically capped at 6–7 mg/kg q24h initially
q12h dosing: If q24h control is inadequate — split total daily dose into two equal doses
Dose range in practice: 1–20 mg/kg/day (highly individual — always ACTH-guided)
Capsule rounding: Always round to nearest available capsule combination

How to Use This Calculator

Enter your dog’s body weight in kg or lbs — use lean body weight if obese
Select the type of Cushing’s disease (PDH is most common — ~85% of cases)
Select whether this is the initial starting dose or a dose adjustment based on ACTH stimulation results
If adjusting, enter the post-ACTH cortisol result to guide the new dose direction
Select your available capsule strengths — the calculator will suggest an optimal capsule combination
Click Calculate to see starting dose, capsule recommendation, and the monitoring schedule

🧮 Trilostane Dose Calculator

Dog’s Weight

Weight Unit

Cushing’s Type

Dosing Phase

Post-ACTH Cortisol (if adjusting)Target range: 40–200 nmol/L (1.5–7.5 mcg/dL). Leave blank if starting dose.

Cortisol Unit

Dosing Frequency

Preferred Capsule Size

Trilostane Dosing Result

📅 Mandatory Monitoring Schedule

ACTH Stimulation Test Interpretation & Dose Adjustments

Post-ACTH Cortisol	Interpretation	Clinical Signs	Action
<40 nmol/L (<1.5 mcg/dL)	Adrenal insufficiency	Lethargy, vomiting, collapse	STOP trilostane; supportive glucocorticoids; emergency vet
40–200 nmol/L (1.5–7.5 mcg/dL)	Target range ✓	Clinical signs controlled	Maintain current dose; recheck in 3 months
200–300 nmol/L (7.5–10.9 mcg/dL)	Partial control	Some signs remain	Increase dose ~25%; recheck ACTH in 4 weeks
>300 nmol/L (>10.9 mcg/dL)	Inadequate control	PU/PD, pot belly persist	Increase dose 25–50% or switch to q12h; recheck in 4 weeks

ACTH stimulation test timing: perform 4–6 hours AFTER the morning trilostane dose (not before), using 5 mcg/kg synthetic ACTH (tetracosactide) IV. This captures the nadir of cortisol suppression to detect over-suppression, and the plateau to assess overall control.

Trilostane Capsule Size Quick Reference

Dog Weight	Starting Dose (2.5 mg/kg)	Suggested Capsule(s)
5 kg	12.5 mg	10 mg × 1 (monitor closely)
8 kg	20 mg	10 mg × 2
12 kg	30 mg	30 mg × 1
16 kg	40 mg	30 mg + 10 mg
24 kg	60 mg	60 mg × 1
32 kg	80 mg	60 mg + 10 mg × 2
40 kg	100 mg	60 mg + 30 mg + 10 mg
48 kg	120 mg	120 mg × 1

Frequently Asked Questions

Trilostane absorption is highly variable (30–40% bioavailability) and is significantly increased by fat-containing food. Studies have shown that feeding a consistent meal with the capsule at the same time each day reduces variability in plasma drug levels and improves clinical control. Giving trilostane without food can result in subtherapeutic blood levels and poor Cushing’s control on that day. Always feed the dog first, then administer the capsule in a piece of food or immediately after the meal, every day at the same time.

Signs of hypoadrenocorticism (Addison’s crisis) caused by over-suppression with trilostane include: profound lethargy, weakness, collapse, vomiting and/or diarrhoea (sometimes with blood), loss of appetite, shaking or muscle tremors, and in severe cases cardiovascular collapse. These signs can occur at any time — not just when starting trilostane — and are a veterinary emergency. Every owner of a Vetoryl patient must be counselled to stop medication and seek emergency care immediately if these signs appear. Carry dexamethasone at home if your vet advises.

Unlike most ACTH stimulation tests (which are performed before morning medication), the Vetoryl monitoring ACTH stimulation test is performed 4–6 hours AFTER the morning dose. This is because trilostane reaches peak effect approximately 4–6 hours post-administration, and the test is designed to capture the maximum degree of cortisol suppression. Performing the test before the dose would miss over-suppression. The dog should receive its normal breakfast with the trilostane dose, then arrive at the clinic 4–6 hours later for the ACTH test.

No — Vetoryl capsules must never be opened, cut, or crushed. The powder inside can cause skin irritation and mucous membrane irritation on contact. Intact capsules must be swallowed whole. For small dogs requiring very low doses (below 10 mg — the smallest capsule size), close veterinary monitoring is needed when using a 10 mg capsule to ensure the dog is not over-dosed. Some veterinary compounding pharmacies can prepare lower-strength trilostane formulations for very small dogs.

No — trilostane controls but does not cure Cushing’s disease. It must be given for life (unless the underlying cause is surgically addressed). Stopping trilostane allows cortisol production to resume within days, and Cushing’s clinical signs return. Some dogs have been maintained on trilostane therapy for many years with excellent quality of life. The goal of therapy is to achieve adrenal suppression into the target cortisol range (40–200 nmol/L post-ACTH) while preventing the dangerous opposite (adrenal insufficiency).

Both are used for Cushing’s disease in dogs. Trilostane (Vetoryl) is a reversible, competitive enzyme inhibitor — stopping it allows adrenal recovery. Mitotane (Lysodren/o,p’-DDD) is an adrenocorticolytic agent that irreversibly destroys adrenal cortex cells. Mitotane has a narrower therapeutic window and a higher risk of permanent adrenal destruction, but is still used in some countries where trilostane is not available or for refractory cases. In the UK, USA, and most of Europe, trilostane is the preferred first-line treatment due to its reversibility and predictable pharmacology.

References

Vetoryl (trilostane) Prescribing Information. Dechra Veterinary Products; 2023.
Plumb DC. Plumb’s Veterinary Drug Handbook, 9th ed. Wiley-Blackwell; 2018.
Feldman EC, et al. Evaluation of twice-daily, low-dose trilostane treatment administered orally in dogs with naturally occurring hyperadrenocorticism. J Am Vet Med Assoc. 2004;224(7):1132-1138.
Ramsey IK. Trilostane in dogs. Vet Clin North Am Small Anim Pract. 2010;40(2):269-283.
Behrend EN, et al. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM consensus statement. J Vet Intern Med. 2013;27(6):1292-1304.
Lemetayer J, Blois S. Update on the use of trilostane in dogs. Can Vet J. 2018;59(4):397-407.