Most appetite stimulants begin working in cats within 1 to 3 days, though some cats show interest in food within hours of the first dose. The exact timeline depends on which medication your cat was prescribed, how it’s delivered, and your cat’s underlying condition. Here’s what to expect from each option.
Mirtazapine: The Most Commonly Prescribed Option
Mirtazapine is the most widely used appetite stimulant for cats, available as both an oral tablet and a transdermal ointment applied to the inner ear. The transdermal version (Mirataz) is FDA-approved for cats and is applied as a small ribbon of ointment once daily for 14 days, alternating between the left and right ears.
In a survey of cat owners using the transdermal ointment, about 41% noticed their cat eating more within 24 hours. Half saw improvement within the first week, and roughly 11% didn’t see results until after a full week of treatment. So while some cats respond quickly, it’s not unusual for the medication to take several days to fully kick in.
In terms of weight gain, a controlled clinical study found that cats on transdermal mirtazapine gained an average of 3.9% of their body weight over 14 days, compared to just 0.4% in cats receiving a placebo. That’s a meaningful difference, especially for a cat that’s been steadily losing weight. But the gain is gradual, so don’t expect dramatic changes overnight.
Side Effects to Watch For
Mirtazapine side effects, when they occur, tend to show up fast. In cases of accidental overdose or sensitivity, cats have developed signs like vocalization, restlessness, or hyperactivity within 15 minutes to 3 hours of receiving the drug. These effects typically resolve within 12 to 48 hours. At the correct dose, side effects are less common, but if your cat seems unusually agitated or vocal shortly after a dose, that’s worth reporting to your vet.
Capromorelin: The Oral Liquid
Capromorelin is a liquid medication given by mouth that works by mimicking a hunger hormone. It was originally approved for dogs and has been increasingly used in cats as well. The drug is typically given once daily.
Most cats should show some appetite response within the first 24 to 48 hours. If your cat hasn’t shown any interest in food after two days, your vet may consider adjusting the dose upward rather than continuing to wait. The liquid form can be easier to give than a pill, though some cats dislike the taste and may drool or resist the syringe.
Cyproheptadine: A Slower Start
Cyproheptadine is an antihistamine that happens to stimulate appetite as a side effect. It’s been used in cats for years and is available as an oral tablet or compounded liquid. Unlike mirtazapine, which often produces a noticeable response within a day, cyproheptadine can take a few days to build up to effective levels in your cat’s system. This slower onset makes it a less popular first choice when a cat needs to start eating urgently, but it remains an option when other medications aren’t suitable.
What Affects How Quickly Your Cat Responds
The medication itself is only one piece of the puzzle. Several factors influence how fast you’ll see your cat eating again.
- The underlying illness: A cat with mild nausea from a medication change may perk up quickly, while a cat with advanced kidney disease or cancer may take longer to respond, or may need additional treatments alongside the appetite stimulant.
- How long your cat has been off food: Cats that have gone several days without eating sometimes develop an aversion to food itself. The stimulant increases the urge to eat, but your cat may still need coaxing with warmed, strong-smelling foods.
- Nausea: Appetite stimulants increase hunger signals, but they don’t always address nausea. If your cat approaches the food bowl and then walks away, nausea may be a separate problem your vet needs to treat.
When to Expect Meaningful Results
For most cats on mirtazapine or capromorelin, you should see at least some increased interest in food within the first 1 to 2 days. That might look like your cat approaching the bowl more often, eating a few bites where they previously ignored food entirely, or showing interest when you open a can. A full return to normal eating habits often takes closer to a week.
If you’ve given the medication consistently for 48 hours with zero change in your cat’s behavior around food, contact your vet. They may switch to a different stimulant, adjust the dose, or investigate whether something else (like nausea, pain, or disease progression) is overriding the medication’s effect. Cats that stop eating for more than 2 to 3 days are at risk for a serious liver condition called hepatic lipidosis, so a medication that isn’t producing any response shouldn’t be given more time without veterinary input.
Keep track of what and how much your cat eats each day once treatment starts. “Eating more” is subjective, and having actual numbers (even rough ones, like “ate half a can” versus “ate two bites”) gives your vet much better information to work with if adjustments are needed.