Mirtazapine for Dogs and Cats: Appetite Support, Nausea Control, and Safety

By Dr Duncan Houston

Quick Answer

Mirtazapine is a prescription medication used in dogs and cats mainly to stimulate appetite and help control nausea. It is especially useful in cats with chronic disease, kidney disease, gastrointestinal illness, or weight loss, and it is also available as Mirataz, an FDA-approved transdermal ointment for cats. It can be very effective, but dose, timing, organ function, and drug interactions all matter because side effects and serotonin syndrome are real risks. (Merck Veterinary Manual)

What Is Mirtazapine?

Mirtazapine is a tetracyclic antidepressant in human medicine, but in veterinary practice we mostly use it for a completely different reason: helping pets eat and feel less nauseous. It is commonly used in dogs and cats with poor appetite linked to kidney disease, liver disease, gastrointestinal disease, cancer, chemotherapy, and other chronic illnesses. (Merck Veterinary Manual)

In other words, this is not really a “mood medication” in day-to-day pet practice. It is much more often an appetite and nausea medication.

What Vets Actually Use Mirtazapine For

In real practice, mirtazapine is most often used for:

• poor appetite or inappetence

• nausea, especially in cats

• weight loss associated with chronic illness

• support during chemotherapy or serious disease

• selected patients where encouraging voluntary food intake may help avoid worsening decline (Merck Veterinary Manual)

That said, it is not a substitute for proper nutrition planning. In cats especially, appetite stimulants should not delay feeding tube placement when assisted nutrition is needed. (PMC)

How Mirtazapine Works

Mirtazapine has a useful combination of effects. It increases norepinephrine and serotonin activity in parts of the brain, which contributes to appetite stimulation, and it also blocks certain serotonin receptors involved in nausea. That is why it can help a pet both feel more interested in food and feel less sick about eating it. (Merck Veterinary Manual)

This dual action is why it has become such a common tool, especially in feline medicine.

Dogs Versus Cats

Mirtazapine is used in both species, but cats are where it is particularly valuable. Feline patients often develop inappetence with chronic kidney disease, GI disease, hospital stress, or nausea, and mirtazapine is well established as one of the main appetite stimulants used in those cases. (Merck Veterinary Manual)

Dogs can also be prescribed mirtazapine, but if it is not effective, other appetite stimulants may be considered depending on the case. (Merck Veterinary Manual)

Mirataz for Cats

Mirataz is an FDA-approved transdermal mirtazapine ointment for cats. It is labeled for body weight gain in cats with a history of weight loss and is applied to the inner pinna of the ear once daily for 14 days, alternating ears. The standard dose is a 1.5-inch ribbon, which delivers about 2 mg per cat. (DailyMed)

Important handling instructions include:

• wear disposable gloves

• alternate ears daily

• avoid contact between the treated cat and people or other animals for 2 hours after application (U.S. Food and Drug Administration)

That ear-gel option is a huge help for owners of cats who believe tablets are a personal insult.

Dosing and Frequency

Dose and frequency matter a lot with mirtazapine, especially in cats and especially in pets with kidney or liver disease.

Merck lists typical appetite-stimulation doses as:

• cats: 2 mg transdermally every 24 hours, or about 1.88 mg orally every 72 hours

• dogs: dose varies by body size, with daily use described in Merck dosing tables (Merck Veterinary Manual)

The 2022 ISFM guidelines note that oral mirtazapine in cats is effective, but higher dosages are more likely to cause side effects such as hyperexcitability, vocalisation, and tremors. They recommend smaller, more frequent dosing to preserve efficacy while reducing adverse effects. (PMC)

This is one of those drugs where “a little more to make it work better” is not a clever idea.

Kidney and Liver Disease

Cats with kidney or liver disease clear mirtazapine more slowly, which means the same dose can hang around longer and increase the risk of side effects. That is why lower doses or reduced frequency are often used in these patients. (Merck Veterinary Manual)

That point is clinically important because CKD cats are exactly the sort of patients who often need appetite support.

Side Effects to Watch For

Common side effects of mirtazapine include:

• drowsiness

• increased vocalisation

• restlessness or hyperactivity

• increased affection

• agitation

• gastrointestinal upset in some pets (Vca)

Mirataz labeling and feline guidance also note that behavioral changes such as vocalization and hyperactivity can occur, particularly if the dose is too high or the patient is sensitive. (DailyMed)

These effects are not always dramatic, but if a previously quiet cat suddenly turns into a loud, twitchy motivational speaker at 3 a.m., mirtazapine should be on the suspect list.

Serotonin Syndrome: The Big Safety Concern

Serotonin syndrome is the side effect owners most need to know about. It can happen when mirtazapine is combined with other serotonergic medications or when the dose is excessive. Clinical signs can include:

• tremors

• twitching

• shivering

• pacing or agitation

• dilated pupils

• elevated heart rate

• panting

• hyperthermia

• disorientation (Vca)

This is not a “wait and see for three days” issue. If serotonin syndrome is suspected, the pet needs veterinary advice promptly.

Drug Interactions That Matter

Mirtazapine should be used carefully, or avoided, with other drugs that increase serotonin or affect metabolism. Important examples include:

• SSRIs such as fluoxetine

• MAO inhibitors such as selegiline or amitraz exposure

• tramadol

• buspirone

• metoclopramide

• erythromycin

• ketoconazole

• cimetidine (Vca)

Mirataz labeling specifically warns not to use the product in cats receiving monoamine oxidase inhibitors. (dechra-us.com)

What Vets Worry About Most

From a clinical point of view, the main concerns with mirtazapine are:

• using it when the pet is nauseous but the nausea itself has not been addressed properly

• overdoing the dose in cats with kidney disease

• missing serotonin syndrome early

• assuming appetite stimulation means the underlying disease is controlled

• delaying assisted feeding when a cat still is not meeting nutritional needs (PMC)

A cat eating a little more is good. A cat slowly starving while everyone celebrates one lick of pâté is not.

Common Mistakes Owners Make

Thinking it is just an appetite drug

It also affects nausea, behavior, and serotonin pathways. It is a real medication, not a snack enhancer. (Merck Veterinary Manual)

Giving the same schedule to every cat

Cats with kidney or liver disease often need adjusted dosing frequency. (Merck Veterinary Manual)

Missing drug interactions

This is a big one, especially in complex patients already taking several medications. (Vca)

Delaying feeding support

Appetite stimulants can help, but they should not delay feeding tube discussion when nutrition is still inadequate. (PMC)

When to Contact Your Vet

Speak to your vet promptly if your pet on mirtazapine develops:

• tremors

• agitation

• severe vocalisation or hyperactivity

• sedation that seems excessive

• vomiting or diarrhea

• pacing or disorientation

• breathing changes

• poor appetite despite treatment

• worsening weight loss (Vca)

Frequently Asked Questions

What is mirtazapine used for in pets?

Mostly to stimulate appetite and help control nausea, especially in dogs and cats with chronic illness, GI disease, kidney disease, or cancer. (Merck Veterinary Manual)

Is Mirataz the same as mirtazapine?

Mirataz is a transdermal veterinary formulation of mirtazapine for cats. (DailyMed)

How often is Mirataz applied?

Once daily for 14 days, alternating ears, using a 1.5-inch ribbon. (U.S. Food and Drug Administration)

Can mirtazapine help nausea as well as appetite?

Yes. That is one of the reasons it is so useful clinically. (Merck Veterinary Manual)

Can cats with kidney disease take mirtazapine?

Yes, but dosing or frequency often needs adjustment because clearance is slower. (Merck Veterinary Manual)

What are the most common side effects?

Drowsiness, vocalisation, hyperactivity, and behavioral changes are among the more common ones. (Vca)

What drugs should not be mixed with mirtazapine?

Other serotonergic drugs or MAO inhibitors are major concerns, including fluoxetine, tramadol, selegiline, and amitraz exposure. (Vca)

Can mirtazapine cause serotonin syndrome?

Yes, especially if combined with other serotonergic medications or given inappropriately. (Vca)

Should an appetite stimulant replace feeding support?

No. If a pet still is not meeting nutritional needs, assisted feeding may be needed and should not be delayed. (PMC)

Final Thoughts

Mirtazapine is one of the most useful appetite-support medications we have in dogs and cats, especially in feline medicine. It can help a sick pet feel less nauseous, start eating again, and avoid the downward spiral that comes with prolonged inappetence.

But it works best when it is used as part of a proper plan. That means understanding why the pet is not eating, adjusting the dose to the patient, watching for side effects, and not ignoring the risk of serotonin syndrome.

If your dog or cat has poor appetite, ongoing nausea, or unexplained weight loss, ASK A VET™ can help you work through whether mirtazapine makes sense and what the next step should be.