By Rania Gollakner, BS, DVM, MPH

What is mirtazapine?

Mirtazapine (brand name: Remeron®, Mirataz®) is an anti-depressant often used to treat weight loss and anorexia (loss of appetite), mainly in cats. It also has antiemetic (anti-vomiting) and anti-nausea effects, and has been used to treat side effects of chemotherapy. Rarely, it may be used to treat behavioral problems.

Its use in cats and dogs to treat vomiting or decreased appetite is ‘off-label’. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their direction may be significantly different from those on the label.

How do I give mirtazapine to my pet?

Mirtazapine comes in pill form and is given orally (by mouth). It may be given with or without food; however, if vomiting occurs when dosed on an empty stomach, give future doses with food. In some cases, your veterinarian may have the pills compounded into liquid form to make oral dosing easier, especially for cats. It may also be compounded into a transdermal gel to be used on the skin; when administering this form, gloves should be worn. (See handout “Transdermal Medications: How They Work and How to Apply Them” for more information.)

This medication should take effect within one to two days, and improvements in clinical signs should follow. Do not abruptly stop this medication without consulting your veterinarian.

What if I miss giving my pet the medication?

Give the missed dose as soon as you remember, and then wait the recommended amount of time between doses. Never give your pet two doses at once or give extra doses.

Are there any potential side effects from mirtazapine?

Mirtazapine is generally tolerated well but side effects are possible. The most common side effects are vocalization, agitation, vomiting, incoordination while walking, restlessness, muscle tremors, increased salivation, and sleepiness. These signs may indicate serotonin syndrome and a need for dose reduction, so please contact your veterinarian if these signs are noted. Transdermal mirtazapine can cause skin reactions such as redness or scabbing. Rarely, this drug can cause abnormalities in the bone marrow.

This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Pets that are allergic or hypersensitive to mirtazapine should not take it. Do not use in pets that have taken monoamine oxidase inhibitors (MAOIs) in the past 14 days. Mirtazapine should be used cautiously in patients with liver, kidney, or heart disease, and a lower dose may be recommended for these cases. Pets with a history of leukemia, diabetes, glaucoma, difficulty urinating, low platelets, or any blood disease should use mirtazapine cautiously, and blood test monitoring may be recommended. Use cautiously in pregnant or nursing animals.

Are there any drug interactions I should be aware of?

MAOIs such as amitraz (Mitaban®) or selegiline (Anipryl®, Eldepryl®, Carbex®), tricyclic antidepressants such as clomipramine (Clomicalm®, Anafranil®), and serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac®) may interact with mirtazapine and can cause many of the side effects listed above. Other drugs that can interact with mirtazapine include tramadol (Ultram®) and other pain medications, diazepam (Valium®)), and cimetidine (Tagamet®), and should be used with caution.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

Monitor your pet for side effects and for efficacy. As with any medication, your pet should be monitored for signs of an allergic reaction, such as breathing difficulties, hives, or facial swelling. Pets with a history or concern of blood disease should have blood work monitored regularly. Your veterinarian may also want to monitor liver enzymes.

How do I store mirtazapine?

This medication should be stored at room temperature between 20°C and 25°C or 68°F to 77°F and protected from light and moisture.

What should I do in case of an emergency?

If you suspect an overdose or a negative reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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