Mirtazapine is widely used by veterinarians as an appetite stimulant for cats that are not eating due to illness, stress, treatment side effects, or chronic disease. It can be helpful, but it affects brain chemistry and has a range of expected reactions and, rarely, serious adverse effects. Real experience from the Feline Diabetes Message Board (FDMB) reinforces the types of responses caregivers should be aware of. At appropriate doses, severe or fatal reactions are extremely rare, but misunderstanding normal side effects often leads caregivers to fear the worst.
Mirtazapine stimulates appetite and may reduce nausea in cats. It’s often prescribed for cats with chronic kidney disease, diabetes, cancer, or inflammatory bowel disease when poor appetite threatens weight loss and overall health. It does not treat the underlying problem but it does help cats feel more motivated to eat. For our diabetic cats, it’s important for them to eat when receiving insulin therapy to help keep their blood glucose in a safe range.
Appetite Stimulants and Nausea
Mirtazapine increases appetite drive, but it does not reliably treat nausea on its own. Giving an appetite stimulant to a cat that feels nauseated can backfire. The cat may feel a strong urge to eat while still feeling sick, leading to pacing, agitation, drooling, lip-smacking, gagging, or distress without meaningful food intake.
Caregivers sometimes interpret this reaction as a dangerous side effect of mirtazapine, when in reality the medication is amplifying appetite in a cat that still feels unwell. This mismatch can make cats appear frantic, uncomfortable, or “wired,” especially in the hours after dosing.
In many cases, addressing nausea first with an appropriate anti-nausea medication allows appetite stimulants like mirtazapine to work more smoothly and with fewer behavioral side effects. If a cat repeatedly shows agitation or refuses food after mirtazapine, untreated nausea should be strongly suspected. For some cats, repeatedly pairing nausea with food or forced appetite stimulation can lead to food aversion, where the cat begins to avoid foods that were offered while they felt sick. This can complicate feeding long after the nausea itself has resolved.
An appetite stimulant should not be used to push food into a nauseated cat. If nausea is present, it needs to be managed first.
Expected Behavioral Effects
Caregivers on the FDMB commonly report significant vocalization and restlessness after mirtazapine doses. Cats can become unusually loud or “meowy” shortly after dosing, with many caregivers describing the effect as intense but ultimately transient. In one forum thread, excessive vocalizing occurred after a 3.75 mg dose that was likely higher than needed; reducing the dose to about half produced fewer side effects while still supporting appetite.
In another thread, cats showed a pattern of eating small bites, wandering, and frequent vocalization for hours after the medication. Experienced caregivers called this reaction non-dangerous and said it typically wears off within a day. Some compared it to a “stoned cat with munchies,” noting that restlessness and calling were not life-threatening.
These reactions reflect the brain-stimulating effects of mirtazapine and are more likely with higher doses or more frequent dosing than necessary.
Common Physical Side Effects
Typical physical reactions may include slight drooling, mild agitation, pacing, and behavior changes. These generally resolve as the dose wears off. Vocalization and agitation are often the most noticeable signs for caregivers, frequently peaking within hours of administration.
When Side Effects Are a Concern
Behavioral changes that go beyond increased vocalization or excitement can indicate intolerance. Per caregiver reports and clinical understanding, the following should prompt a veterinary reassessment:
- Severe agitation that does not lessen within 12-24 hours
- Disorientation, confusion, or uncoordinated movements
- Tremors or muscle twitching
- Marked lethargy or collapse
- Signs of significant distress or inability to eat despite stimulation
While typical “weird” behavior described by caregivers (restlessness, vocalizing, pacing) is alarming, it is usually self-limiting and benign. It typically resolves within a day and should not be interpreted as a sign of fatal toxicity.
Rare but Serious Reactions
Serotonin syndrome, though uncommon at therapeutic doses, is the most serious potential complication. It can occur when mirtazapine’s influence on serotonin accumulates or interacts with other medications that affect serotonin. Signs include tremors, elevated body temperature, rapid heart rate, dilated pupils, hyperactivity beyond normal, and neurological instability. Severe serotonin excess is a medical emergency.
Forum contributors referenced informational sources cautioning about serotonin syndrome and its symptoms, though typical dosing alone rarely produces this syndrome.
Seizures and extreme agitation are rare but have been cited in veterinary literature as possible in overdose scenarios or when risk factors like kidney disease slow drug clearance. This reaction is rare and typically associated with excessive dosing, drug interactions, or impaired drug clearance.
Dosing and Risk Factors
Clinical guidance and caregiver experience align: mirtazapine dosing for cats is low, and overdosing dramatically increases side effect risk. Veterinarians and experienced caregivers find that very small doses (often around 1.88 mg or less) are sufficient…
Cats with kidney disease, liver impairment, or on medications that influence neurotransmitter systems are at higher risk of exaggerated responses. Smaller cats may also need lower doses to avoid excessive stimulation.
“Mirtazapine Killed My Cat”
On the Feline Diabetes Message Board (FDMB), we have not seen mirtazapine directly causing sudden death at appropriate doses. Claims suggesting fatal outcomes often stem from misinterpreting expected behavioral side effects as toxic or life-threatening. Caregivers emphasized that even accidental extra doses are unlikely to be fatal, though they can increase restlessness, vocalization, and tremors, and merit veterinary oversight. This does not mean every reaction is benign, but it does mean behavior alone is not always evidence of lethal toxicity.
Severe outcomes attributed to mirtazapine on the forum often involve complex underlying illness or concurrent conditions, not the medication alone.
What to Do When Side Effects Occur
If a cat shows mild to moderate vocalization or restlessness after mirtazapine, monitor and note the timing of symptoms relative to dosing. Many owners find that reducing dose or extending the dosing interval decreases these reactions.
If concerning symptoms appear (severe tremors, disorientation, collapse, sustained distress, or signs suggestive of serotonin syndrome), stop further dosing and contact a veterinarian immediately. Cyproheptadine is cited as a potential antagonist for serotonin excess in severe cases, but this should be used only under veterinary advice. If a reaction makes you feel something is “not right,” it is always appropriate to stop the medication and reassess with your veterinarian before giving another dose.
