Oral mirtazapine is a widely used appetite stimulant for cats experiencing weight loss and poor appetite due to chronic kidney disease, cancer, and other conditions. While Mirataz (transdermal mirtazapine) is the FDA-approved topical formulation for feline weight loss management, oral mirtazapine has been used off-label in cats for many years and remains a common and cost-effective option, particularly when transdermal application is not practical.
Overview
Mirtazapine is a tetracyclic antidepressant originally developed for human depression. In cats, it is used primarily for its potent appetite-stimulating and anti-nausea properties. Mirtazapine works through alpha-2 adrenergic antagonism (increasing norepinephrine and serotonin release), 5-HT3 serotonin receptor antagonism (reducing nausea), and H1 histamine receptor antagonism (promoting appetite).
A critical consideration in feline medicine is that cats metabolize mirtazapine significantly more slowly than humans or dogs. The elimination half-life in healthy cats is approximately 15 hours, but in cats with hepatic or renal disease — the very patients most likely to need appetite support — the half-life can extend to 30 hours or longer. This slower metabolism means that dosing intervals appropriate for humans (daily) will cause drug accumulation and toxicity in cats. Feline-specific dosing (every 48 to 72 hours) is essential.
Uses
- Appetite stimulation: First-line oral appetite stimulant for cats with reduced food intake.
- Weight loss management: Helps cats regain weight lost due to chronic illness.
- Nausea reduction: The anti-5-HT3 properties provide antiemetic effects, addressing a common cause of poor appetite.
- Chronic kidney disease (CKD): One of the most common scenarios for mirtazapine use, as CKD frequently causes nausea and anorexia.
- Cancer-associated anorexia: Supportive care for cats with neoplastic disease.
- Post-surgical or post-illness appetite support: Short-term use to help cats resume eating.
Dosage
| Parameter | Detail |
|---|---|
| Dose (cats) | 1.88 mg (one quarter of a 7.5 mg tablet) per cat |
| Frequency | Every 48 hours (every other day) for healthy cats; every 72 hours for cats with hepatic or renal disease |
| Route | Oral |
| Formulation | 7.5 mg, 15 mg, 30 mg tablets (human); compounded veterinary formulations |
| Onset of appetite effect | 1–4 hours |
Important dosing notes:
- The standard feline dose is 1.88 mg (one quarter of a 7.5 mg tablet) every 48 hours. Do not dose daily.
- Cats with CKD or liver disease should receive mirtazapine every 72 hours due to prolonged drug clearance.
- Compounded formulations are available in smaller doses and flavored liquids for easier administration.
- If a dose is missed, give it when remembered and resume the regular every-other-day schedule.
- Do not increase the dose or frequency without veterinary approval.
Side Effects
| Common Side Effects | Serious Side Effects (Contact Your Vet Immediately) |
|---|---|
| Increased vocalization | Serotonin syndrome (tremors, agitation, rapid heart rate, high temperature, disorientation) |
| Increased affection or attention-seeking | Persistent vomiting or diarrhea |
| Mild restlessness or hyperactivity | Severe agitation or aggression |
| Mild sedation (less common in cats) | Seizures |
| Profound lethargy or unresponsiveness |
Serotonin syndrome is the most serious risk, particularly if mirtazapine is combined with other serotonergic drugs or given too frequently. Signs include tremors, twitching, agitation, dilated pupils, rapid heart rate, and hyperthermia. This is a medical emergency.
Drug Interactions
- Serotonergic drugs (fluoxetine, tramadol, ondansetron, amitriptyline, trazodone): Risk of serotonin syndrome when combined with mirtazapine. Avoid concurrent use or use only under close veterinary supervision.
- MAO inhibitors (selegiline/Anipryl): Contraindicated due to severe serotonin syndrome risk.
- Cyproheptadine: May antagonize the appetite-stimulating effects of mirtazapine.
- Other CNS depressants: Additive sedation.
- Transdermal mirtazapine (Mirataz): Do not use oral and transdermal mirtazapine simultaneously — this will result in overdose.
Contraindications
- Do not use in cats with known hypersensitivity to mirtazapine.
- Do not use concurrently with MAO inhibitors or within 14 days of MAO inhibitor discontinuation.
- Do not use simultaneously with transdermal mirtazapine (Mirataz).
- Use with extreme caution in cats with severe hepatic disease (significantly prolonged metabolism).
- Use with caution in cats with severe renal disease; extend the dosing interval to every 72 hours.
- Safety has not been evaluated in pregnant or lactating cats.
Storage
- Store tablets at room temperature (68–77 degrees F / 20–25 degrees C).
- Keep in a tightly closed container, protected from moisture and light.
- Compounded formulations may have different storage requirements; follow your pharmacy’s instructions.
- Keep out of reach of children and pets.
FAQ
Q: What is the difference between oral mirtazapine and Mirataz? A: Both contain the same active ingredient (mirtazapine). Mirataz is an FDA-approved transdermal ointment applied to the inner ear pinna, while oral mirtazapine is given by mouth as a tablet or compounded liquid. The transdermal form avoids the need for oral dosing in nauseous cats and has been specifically studied and approved for feline use. Oral mirtazapine is used off-label but has a longer track record. Never use both simultaneously.
Q: Why can’t I give mirtazapine to my cat every day? A: Cats metabolize mirtazapine much more slowly than humans or dogs. Daily dosing leads to drug accumulation, which increases the risk of side effects including serotonin syndrome. The recommended every-48-hour (or every-72-hour for CKD/liver disease cats) schedule accounts for this slower feline metabolism.
Q: How quickly will my cat’s appetite improve? A: Most cats show increased appetite within 1 to 4 hours of receiving oral mirtazapine. You may notice your cat becoming more vocal, seeking food, or eating with more enthusiasm. If no improvement is seen after 2 to 3 doses, contact your veterinarian.
Q: Is it safe to use mirtazapine long-term in my cat? A: Mirtazapine can be used for extended periods under veterinary supervision, particularly for cats with chronic conditions like CKD. Your veterinarian will periodically reassess your cat’s need for appetite support and monitor for any adverse effects.
This content is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian before starting, changing, or stopping any medication for your cat.