Mirtazapine
Mirtazapine is a centrally acting drug used principally to improve appetite and support weight gain in cats. An authorized transdermal product has a feline weight-loss indication, while oral preparations are also used in veterinary practice. Antinausea effects contribute to some clinical applications, particularly when poor food intake accompanies chronic disease.
Group
Appetite stimulant and antiemetic
Mechanism of Action
Mirtazapine blocks presynaptic alpha2-adrenergic receptors and modifies central norepinephrine and serotonin signaling. Antagonism of selected serotonin receptors, particularly 5-HT2 and 5-HT3, and H1-receptor effects contribute to increased appetite and reduced nausea. The combined receptor profile can increase food intake and weight in responsive cats, although the precise contribution of each pathway is incompletely defined.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Appetite stimulation | 0.5–1 mg/kg | PO | Every 12 h | Reported weight-based regimen. Its separate fixed daily recommendation has a different schedule and must not be treated as the same instruction. |
| Appetite stimulation | 3.75–7.5 mg/animal | PO | Every 24 h | Separate fixed daily alternative from the reported guidance; select according to patient size. |
| Appetite stimulation | 0.6 mg/kg | PO | Every 24 h | Maximum 30 mg/day; Maximum total daily amount 30 mg for this reported regimen. |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Appetite stimulation | 1.9 mg/animal | PO | Every 24 h | Healthy-cat oral regimen; use the longer interval for chronic kidney disease. |
| Appetite stimulation in chronic kidney disease | 1.9 mg/animal | PO | Every 48 h | Clinical course described for 3 weeks. |
| Historical higher oral dose | Reference only | PO | 3.75-7.5 mg/cat/day is reported, but high doses have greater adverse-effect risk. It is not merged with the lower 1.9 mg regimen. | |
| Weight-loss management | 2 mg/animal | TOPICAL | Every 24 h | Up to 14 days; 2% ointment: approximately a 1.5-inch ribbon on the inner pinna, using the actual product measuring guide. |
| Appetite stimulation | 3 mg/animal | PO | Every 72 h | Separate longer-interval oral alternative; do not combine with a daily regimen. |
| Appetite stimulation | 3.75 mg/animal | PO | Every 72 h | Separate longer-interval oral alternative; do not combine with a daily regimen. |
| Appetite stimulation | 3–4 mg/animal | PO | Every 72 h |
Indications
- Management of undesired weight loss in cats with the corresponding transdermal product.
- Appetite stimulation in selected cats with reduced intake associated with chronic kidney disease or other illness.
- Adjunctive management of nausea-associated poor appetite in selected cats.
- Extra-label appetite support in selected dogs, with a smaller clinical evidence base than in cats.
Contraindications
- Reduced renal clearance requires a longer oral interval in cats.
- Do not apply transdermal product where other animals can lick it.
- Current MAO-inhibitor treatment or administration within the preceding 14 days; use cautiously with cardiovascular disease or hypotension.
Interactions
- Avoid concurrent MAO inhibitors and SSRIs when possible.
Adverse effects
- Vocalization, restlessness, tremors and hypersalivation
- Ataxia, vomiting and cardiovascular effects at high exposure
Toxicity
Stop treatment and provide supportive care; cyproheptadine may be used for serotonergic signs. Source feline antagonist regimen is 2-4 mg/cat orally every 4-8 hours until signs resolve.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Mirtazapine 15 mg tablet | Tablet | 15 mg / tablet | PO |
| Mirtazapine 30 mg tablet | Tablet | 30 mg / tablet | PO |
| Mirtazapine 45 mg tablet | Tablet | 45 mg / tablet | PO |
| Mirataz 20 mg/g ointment 2% ointment; use product-specific ribbon measurement. | Other | 20 mg / g | TOPICAL |
| Mirtazapine 7.5 mg tablet | Tablet | 7.5 mg / tablet | PO |
Monitoring and Administration
Monitor appetite, body weight and behaviour.
Wear disposable gloves for transdermal application. Prevent people or other animals contacting the application site for 2 hours.
Apply the exact product amount to the inner pinna; do not infer equivalence of compounded transdermal vehicles.
Reported canine alternatives states 0.5-1 mg/kg every 12 hours and 3.75-7.5 mg/dog daily; they are retained as separate alternatives rather than asserted to be equivalent.
Older dog fixed-dose alternatives are under 20 lb: 3.75 mg daily; 21-50 lb: 7.5 mg daily; 50-75 lb: 15 mg daily; over 75 lb: 15 mg every 12 hours or 30 mg daily. The printed boundaries have gaps/overlap and do not constitute a verified whole-tablet weight-band product. The earlier claim that no renal adjustment is needed does not override feline renal-clearance evidence or individualized monitoring.