Zycortal (DOCP) for Dogs with Addison’s Disease: Complete Guide – Dosing Protocol, Na+/K+ Monitoring, Stress Dosing & Zycortal vs Fludrocortisone

Zycortal is the mineralocorticoid replacement injection for dogs diagnosed with primary Addison’s disease (hypoadrenocorticism). If your dog has just been diagnosed, or if you’re comparing Zycortal against fludrocortisone, this guide covers everything: what Addison’s disease is, why two separate medications are needed, the full Zycortal dosing and monitoring protocol, how to interpret the Na+/K+ ratio, stress dosing, and a practical comparison between the two mineralocorticoid options.

What Is Addison’s Disease in Dogs?

Primary hypoadrenocorticism (Addison’s disease) occurs when the adrenal glands are destroyed — most commonly by immune-mediated attack — and fail to produce the hormones they normally make. The adrenal cortex produces two essential hormone types:

• Mineralocorticoids (primarily aldosterone) — regulate sodium and potassium in the kidneys, and therefore blood volume and blood pressure
• Glucocorticoids (primarily cortisol) — regulate metabolism, immune function, the inflammatory response, and the body’s ability to cope with stress

In Addison’s disease, both are lost. The mineralocorticoid deficiency is particularly dangerous: without aldosterone, the kidneys excrete sodium (and water follows) while retaining potassium. The result is dehydration, low blood pressure, and rising potassium levels that can cause fatal cardiac arrhythmias. Left untreated, this progresses to an Addisonian crisis — collapse, cardiovascular shock, and death.

The condition is called “the great pretender” because its signs (lethargy, vomiting, reduced appetite, weakness) are vague and mimic dozens of other conditions. Diagnosis is confirmed by an ACTH stimulation test showing flat cortisol response, combined with low Na+/K+ ratio on biochemistry.

Why Two Medications Are Always Required

This is the most commonly misunderstood aspect of Addison’s management. Zycortal (DOCP) replaces mineralocorticoid function only. It has virtually no glucocorticoid activity. A dog started on Zycortal alone will still lack cortisol — and will remain chronically unwell, intolerant of stress, prone to vomiting, and unable to mount a normal response to illness.

Complete Addison’s management requires:

1. Zycortal (injected every 25–40 days) — replaces aldosterone/mineralocorticoid function
2. Prednisolone (oral daily tablets) — replaces cortisol/glucocorticoid function at physiological replacement doses (typically 0.1–0.2mg/kg/day)

Exception: some dogs with early or atypical Addison’s may initially have preserved glucocorticoid function (“atypical Addison’s”) and only require mineralocorticoid replacement — but this should be assessed by your vet.

What Is Zycortal and How Does It Work?

Zycortal contains desoxycortone pivalate (DOCP) 25mg/ml in a prolonged-release suspension. The pivalate ester creates a depot at the injection site: after subcutaneous injection, the drug is slowly absorbed over several weeks rather than being absorbed immediately. This gives Zycortal its key practical advantage — single-injection dosing every 25–40 days rather than daily tablets.

Pharmacokinetics: Peak blood concentration (Tmax) occurs approximately 10 days after injection. Plasma half-life is approximately 17 days. This is why the monitoring protocol checks electrolytes at Day 10 (peak effect) and Day 25 (pre-dose trough) — these two points reveal both the maximum effect of the current dose and how long it is lasting.

At the kidney, DOCP binds mineralocorticoid receptors in the renal tubules and promotes sodium (and chloride) retention while driving potassium and hydrogen ion excretion — exactly replacing what aldosterone would normally do. Correcting this balance restores blood volume, blood pressure, and cardiovascular stability.

The Zycortal Dosing Protocol: Step by Step

Before Starting: Confirm the Diagnosis

Do not start Zycortal without a confirmed diagnosis. ACTH stimulation testing is required to confirm hypoadrenocorticism. If the dog is in Addisonian crisis at the time of diagnosis, emergency stabilisation with IV saline and injectable glucocorticoids must come first — Zycortal is not given to a haemodynamically unstable dog.

Step 1: Initial Dose (Day 1)

Starting dose: 2.2mg/kg body weight by subcutaneous injection.

• Shake the vial gently to resuspend before drawing up
• Use a graduated syringe for accuracy — especially important for small dogs
• Allow the vial to reach room temperature before injection (stored refrigerated at 2–8°C)
• Start or continue prednisolone at replacement doses simultaneously

Step 2: Day 10 Check

Around 10 days after the injection (peak DOCP blood level), take a blood sample to measure serum electrolytes and calculate the Na+/K+ ratio. Target is 27–32. Use this result to plan the dose adjustment for Dose 2:

Step 3: Day 25 Check and Second Dose

Around Day 25, reassess the dog’s clinical status and electrolytes again. By this point, the first dose effect is waning and you’re approaching the pre-dose trough:

• If the dog is clinically normal and Na+/K+ ratio is 27–32 → give the adjusted second dose
• If clinically normal and Na+/K+ ratio > 32 → either reduce the dose or extend the interval slightly
• If signs are returning or ratio is < 27 → the dog needs mineralocorticoid sooner; consider shortening the interval

Step 4: Long-Term Stability

Once the dog has stable clinical signs and a normal Na+/K+ ratio on a consistent dose and interval, ongoing monitoring moves to every 3–6 months. The average stabilised dog in the Zycortal clinical trial received:

• Final dose: 1.9 mg/kg (range 1.2–2.5 mg/kg)
• Dosing interval: 38.7 days on average (range 20–99 days)
• Most dogs: 25–46 day interval

Interpreting the Na+/K+ Ratio

The serum Na+/K+ ratio is the primary tool for monitoring Zycortal adequacy. Normal for dogs is approximately 27–40; Addison’s dogs with mineralocorticoid deficiency typically have ratios well below 27 at diagnosis (sometimes below 20). The Zycortal target is 27–32 — a slightly conservative range to ensure adequate mineralocorticoid replacement without over-correction.

• Ratio < 27: Under-mineralised — sodium too low, potassium too high. Risk of Addisonian deterioration. Shorten interval or increase dose.
• Ratio 27–32: Target range — well controlled.
• Ratio > 32: Over-mineralised — potassium too low, sodium elevated. Risk of hypertension and hypokalaemia. Reduce dose or extend interval.

Stress Dosing — The Extra Step Many Owners Miss

One of the critical concepts in Addison’s management that is not covered by Zycortal alone: stress dosing. Dogs with Addison’s cannot increase their cortisol production in response to illness, surgery, trauma, or significant stress. Without additional glucocorticoid coverage during these events, they risk an acute Addisonian crisis even if their mineralocorticoid replacement is perfect.

Stress dosing protocol:

• Minor stress (e.g., vet visit, travel, mild illness): double the prednisolone dose for 1–2 days
• Moderate stress (e.g., injury, elective surgery, significant illness): triple the prednisolone dose and continue for several days until recovered
• Emergency (vomiting, unable to take oral medication): injectable dexamethasone or prednisolone is required — discuss with your vet about keeping an emergency syringe at home

Zycortal dosing adjustments are generally not needed for short-term stressors as the mineralocorticoid component is covered by the depot. Focus stress dosing adjustments on the glucocorticoid (prednisolone) component.

Addisonian Crisis — Emergency Recognition and Response

An Addisonian crisis is a life-threatening emergency. Recognise these signs in your dog:

• Sudden severe weakness or inability to stand
• Collapse or profound depression
• Profuse vomiting or diarrhoea
• Shaking or trembling
• Pale or grey gums
• Rapid or weak pulse

If you see these signs: this is a veterinary emergency. Do not give Zycortal at home. Take the dog to an emergency vet immediately.

Emergency treatment is IV isotonic saline (to restore sodium and blood volume), injectable glucocorticoids (dexamethasone IV), and cardiac monitoring for hyperkalaemia-induced arrhythmias. Zycortal is only started once the dog is haemodynamically stable.

Zycortal vs Fludrocortisone: Which Is Better?

Both are effective mineralocorticoid replacement options for Addison’s disease. The choice depends on owner preference, the dog’s specific profile, and your vet’s experience.

Zycortal’s main practical advantage is eliminating daily medication — a significant compliance benefit for many owners, particularly those with dogs resistant to tablets. The monthly vet visit also ensures regular professional monitoring.

Practical Vial Guide: How Many Doses from the 4ml Vial?

Each 4ml vial contains 100mg desoxycortone pivalate (25mg/ml).

For smaller dogs, your vet may draw up from a broached vial across multiple doses if stored appropriately. Always discuss multi-dose vial use with your veterinary practice.

Where to Buy Zycortal

You can order Zycortal 25mg/ml (DOCP) 4ml Injection for Dogs from PetShopBoss.com with free worldwide shipping.

Related: Prednisolone 5mg Tablets – Glucocorticoid replacement for Addison’s disease co-management