A 1.5-inch ribbon of ointment, roughly 2 mg, on the inner surface of one ear once a day for 14 days is the entire licensed protocol. In the field trial behind that label, 83 cats gained 3.9 percent of body weight in two weeks while 94 cats on a placebo ointment gained 0.4 percent. The gap and its limits are what this page is about.
What the 2018 approval actually covers
Mirataz was approved by the FDA on 4 May 2018 under NADA 141-481, and the indication is narrow: body weight gain in cats with a history of weight loss. It is not licensed as a general appetite stimulant for a fussy cat. The tube holds 20 mg of mirtazapine per gram, so 5 g contains 100 mg, 50 doses at 2 mg. The label also says to discard the tube 30 days after first use, which cuts the practical budget to 30 doses and makes storage a cost issue.
Why the inner ear absorbs it at all
Mirtazapine fits the profile of a drug that crosses skin: 265 daltons, a logP of 2.9 to 3.2, no hydrogen-bond donors, three acceptors. The pinna is thin, sparsely haired and well supplied with blood, which is why the label names the inner surface rather than the flank. Absorption is slow. In an eight-cat crossover the mean half-life was 26.8 hours after topical dosing against 10.1 hours after oral dosing, and a 14-day repeat-dose study reported 20.7 hours. A long half-life explains two things owners notice: a missed dose does not crash the effect, and a side effect does not vanish in an hour.
The trial numbers, and what 3.9 percent means
| Study | Cats | Dose | Result |
|---|---|---|---|
| Poole 2019, field trial | 83 treated, 94 placebo | 2 mg daily, 14 days | +3.9% body weight vs +0.4% (P<0.0001) |
| Quimby 2020, CKD, higher gel | 9 | 3.75 mg every other day, 3 weeks | Median +0.22 kg (0.04 to 0.44) |
| Quimby 2020, CKD, lower gel | 10 | 1.88 mg every other day, 3 weeks | Median +0.26 kg (-0.25 to 0.5) |
| Ferguson 2015, poison centre | 84 exposures | 1.88 to 30 mg | Vocalization in 56%, resolved in 12 to 48 hours |
Three point nine percent is easy to dismiss until it is converted: for a 4 kg cat, 156 g in 14 days. The field study enrolled cats aged 2.8 to 24.6 years and 2.1 to 9.2 kg, all with at least 5 percent documented loss, and 55.7 percent of the treated group had kidney disease. It looks like a normal senior appointment, not a clean laboratory sample. Weight is also a crude endpoint: part of the gain is fat, part refilled gut and water, and none of it treats why the cat stopped eating.
Applying it so the dose lands
- Wear disposable gloves, then throw them away. The label asks for this and owners skip it most often.
- Alternate ears daily. Two weeks on the same pinna leaves the skin red and absorption unpredictable.
- Inside the pinna only, never into the ear canal. The target is skin, not the ear.
- Wipe old residue with a dry tissue before the next dose. No scrubbing, no alcohol wipes.
- Apply when the cat is calm, not straight after a meal, so it does not wash the ear immediately.
- If a dose is missed, apply the next one as scheduled. Do not double up.
- Store below 25 C and write the opening date on the carton.
Side effects, ranked by how often they happen
The field study is the honest source because it was blinded, with 115 cats per arm. Vocalization, meaning crying or mewing, occurred in 11.3 percent of treated cats against 1.7 percent of controls. Hyperactivity or restlessness occurred in 7.0 percent against 0.9 percent; disorientation or ataxia in 3.5 percent against 1.7 percent. Vomiting was 11.3 percent treated and 13.0 percent control, a reminder that many of these cats were already vomiting. Erythema at the site ran 10.4 percent against 17.4 percent on the vehicle ointment, so the base itself irritates. Signs usually follow a dose by 15 minutes to 3 hours.
Overdose: the 15 mg tablet is the real hazard
Eighty-four cat exposures reported to the ASPCA poison centre between 2006 and 2011 show where risk sits. Seventy point two percent were accidental, and 40 of the 84 involved a whole 15 mg human tablet, more than seven times the label dose. Vocalization appeared at a mean dose of 2.56 mg per kg, roughly five times what a 4 kg cat gets from 2 mg. Onset ran from 15 minutes to 3 hours; signs resolved in 12 to 48 hours. If a cat eats a tablet, call the clinic rather than waiting for symptoms, and do not induce vomiting. Report every serotonergic drug on board, including tramadol and selegiline.
What mirtazapine cannot do
It does not find the cause. Dental pain, kidney disease, hyperthyroidism, lymphoma and inflammatory bowel disease all produce the same dropped intake, and mirtazapine will push a cat to eat through any of them, which can delay a diagnosis by weeks. The label asks for caution in liver disease because the drug can raise liver enzymes, and in kidney disease because reduced clearance means higher exposure. It has not been evaluated in cats under 2 kg or under 6 months, and safety in pregnancy is unknown. After the course stops, intake can fall again: call if food intake drops more than 75 percent for several days, or if the cat stops eating for over 48 hours.
Cost, and the alternative
A 5 g tube listed at 53.06 US dollars at Chewy in November 2026, with discount pricing from about 47.38 and a two-pack at 106.12. A 14-day course uses 28 mg, about 1.4 g, so a tube reused inside the 30-day window costs roughly 15 dollars per course. Compounded gels are cheaper but vary: the CKD study found significant inconsistency between batches, which is why an approved ointment exists. Capromorelin is the other licensed option and works through a different receptor; compare it in the capromorelin trial data.
Where this fits in a feeding plan
Drugs buy days, not habits. Warm the wet food, offer 15 to 25 g four to six times a day, raise the bowl 5 to 10 cm and feed away from other cats; the sequence is in the five-step appetite ladder and the waiting limits in the 24-hour and 48-hour lines. Weigh weekly on the same scale: 100 g held for three weeks beats one good appetite.
Disclaimer
This article is educational and does not replace examination, diagnosis or a prescription from a veterinarian. Mirtazapine is a prescription drug with real serotonergic effects; the dose and duration belong to the vet who has seen the cat.
Terms mentioned in this article
Frequently asked
Can my cat stay on mirtazapine longer than 14 days?
The label covers 14 days, and that is the length of the controlled field study, so anything longer is a clinical judgement rather than a tested protocol. Vets do prescribe repeat courses in cats with chronic kidney disease or cancer, usually with monthly weight and bloodwork checks, and often at the compounded 1.88 mg dose used in the CKD crossover trial. If weight keeps falling on treatment, the drug is not the answer; the underlying disease needs another look.
My cat weighs 1.8 kg. Can I still use the 2 mg ribbon?
The label has not evaluated cats under 2 kg, and at 1.8 kg a 2 mg dose is about 1.1 mg per kg, more than double what a 4 kg cat receives. Ask the vet about a compounded 1.88 mg gel, an every-other-day schedule, or oral dosing measured to the cat, rather than trimming the ribbon by eye.
Is the ointment dangerous to me or to my other cats?
Wear disposable gloves and wash your hands, because the drug is absorbed through human skin too. Keep the treated cat away from other cats for the first hour so the ointment is not groomed off, and store the tube where nobody can chew it. Pregnant or breastfeeding owners should let someone else apply it, since mirtazapine is an antidepressant with systemic effects.