Most cats show an increased appetite within 24 hours to one week of starting mirtazapine. In a survey of cat owners, about 41% noticed their cat eating more within the first 24 hours, and a full 50% saw improvement within the first week. A smaller group, around 11%, needed more than a week before appetite picked up.
What to Expect in the First Few Days
The first sign that mirtazapine is working is usually a renewed interest in food. Your cat may sniff at meals more eagerly, approach the bowl without coaxing, or eat a larger portion than they have recently. For roughly 4 in 10 cats, this happens within the first day. For most of the rest, it takes a few more days but generally less than a full week.
If you’re using the transdermal ointment (applied to the ear), the drug enters the bloodstream more gradually than an oral dose. Blood levels stay relatively steady over a 48-hour window rather than spiking and dropping the way an oral pill does. This means the appetite effect with the transdermal form can be subtler at first but more consistent throughout the day. The oral form produces a higher initial peak in the bloodstream, which some cats respond to more quickly.
Weight Gain Takes Longer
Eating more is the first milestone, but meaningful weight gain takes about two weeks. In a clinical trial of cats with chronic kidney disease, those receiving transdermal mirtazapine daily for 14 days gained an average of about 4% of their body weight, compared to less than half a percent in cats receiving a placebo. For a 10-pound cat, that’s roughly 6 ounces, which is significant for an animal that size. If your cat hasn’t gained any weight after two weeks of consistent dosing, it’s worth revisiting the treatment plan with your vet.
How Mirtazapine Stimulates Appetite
Mirtazapine works on multiple systems at once. It blocks certain serotonin receptors that normally suppress appetite, essentially removing a chemical “stop eating” signal in the brain. It also blocks histamine receptors involved in appetite regulation, which likely adds to the hunger-promoting effect. On top of that, it acts as an anti-nausea drug by blocking the same serotonin receptors responsible for triggering vomiting. This dual action is especially useful for cats with kidney disease or those undergoing chemotherapy, where nausea and poor appetite often go hand in hand.
Oral vs. Transdermal Forms
Mirtazapine comes in two forms for cats: an oral tablet (usually a fraction of a human pill) and a transdermal ointment applied to the inner ear flap. The FDA-approved transdermal version is applied as a 1.5-inch ribbon of ointment, roughly 2 mg per cat, once daily for 14 days. You alternate ears each day.
The transdermal version is often easier to administer, especially for cats that refuse pills or vomit them up. It delivers lower but steadier drug levels compared to the sharper peak of an oral dose. For many owners, this consistency matters more than a higher peak, particularly when the goal is sustained appetite over days and weeks rather than a single strong response.
Dosing Frequency for Cats With Kidney Disease
Cats with chronic kidney disease process mirtazapine more slowly. The drug’s half-life in cats with kidney disease averages about 15 hours, compared to 12 hours in healthy cats. That difference might sound small, but it means the drug accumulates more with repeated dosing. Pharmacokinetic studies support a 48-hour dosing interval for cats with kidney disease when using oral mirtazapine, rather than the daily dosing used with the transdermal ointment. Your vet will determine the right schedule based on your cat’s kidney function and the formulation being used.
Side Effects to Watch For
At the standard doses prescribed by veterinarians, mirtazapine is generally well tolerated. Side effects become much more common at higher doses. A toxicity study of 84 cats (most of whom received doses well above the therapeutic range) found that the most frequent adverse effects were vocalization (56%), agitation (31%), vomiting (26%), unsteady walking (17%), and trembling (14%). Cats receiving the lowest doses had the fewest side effects, with problems increasing in a dose-dependent pattern.
At normal prescribed doses, the most likely thing you’ll notice is mild vocalization or restlessness. These behaviors typically settle within a few hours. If your cat seems excessively agitated, is drooling heavily, or has tremors, contact your vet, as this could indicate the dose needs adjustment.
Safety Tips for Applying the Transdermal Ointment
Always wear disposable gloves when handling the ointment, since mirtazapine absorbs through human skin. After applying it to the inner ear flap (not inside the ear canal), dispose of your gloves and wash your hands with soap and water. Keep other people and pets away from the treated cat for at least two hours, as the drug can transfer through skin contact or if another animal grooms the treated ear. Before the next dose, you can wipe the ear with a dry tissue to remove any residual ointment.