Mirtazapine
6-azamianserin, Org-3770, mepirzapine
Also known as: Remeron SolTab · Zispin · Mirataz · 6-azamianserin · Org-3770 · mepirzapine · Mirtazapinum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceChemotherapy-induced anorexia
- q24-72h
NA
Governing clinical context (2)
- Management of weight loss (label dosage; FDA-approved): Using the transdermal formulation, apply 1.5-inch ribbon (≈2 mg/cat) to the inner pinna of the cat’s ear every 24 hours for 14 days. 1 The person applying medication should wear gloves. Alternate application of ointment between the left and right inner pinnae of the ears.
- ■ This medicine may be given with or without food. If your animal vomits after receiving the medication on an empty stomach, give it with food or a small treat. If vomiting continues, contact your veterinarian.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Mirtazapine is an atypical tetracyclic antidepressant widely utilized in veterinary medicine primarily as a potent appetite stimulant and antiemetic for dogs and cats.
- Clinical Pearl: It is particularly beneficial for patients with chronic kidney disease (CKD), congestive heart failure, GI disorders, liver disease, or neoplasia, where it addresses both nausea and inappetence.
- It can be safely combined with other antiemetics for refractory vomiting or chemotherapy-induced nausea and vomiting (CINV).
- The primary dose-limiting side effect is sedation, though paradoxical excitation (vocalization, increased affection) is frequently observed in cats.
Important: Always use the lowest effective dose to minimize sedative properties. Do not exceed 30 mg/day in dogs, as higher doses paradoxically lose appetite-stimulating effects.
Mechanism of action
Mirtazapine has a complex, multi-receptor pharmacological profile (often classified as a NaSSA - Noradrenergic and Specific Serotonergic Antidepressant):
- Central pre-synaptic α2-adrenergic receptors: Antagonism blocks the negative feedback loop → increases Norepinephrine (NE) release → stimulates appetite via other α-receptors.
- 5-HT3 receptors: Potent antagonism in the Chemoreceptor Trigger Zone (CRTZ) and GI tract → provides robust anti-nausea and antiemetic effects.
- 5-HT2 receptors: Antagonism contributes to its anxiolytic profile.
- H1 (Histamine) receptors: Potent antagonism → produces prominent sedative effects.
- Peripheral α1-adrenergic & Muscarinic receptors: Moderate antagonism → may cause occasional orthostatic hypotension and mild anticholinergic effects.
Safety & warnings
Contraindications
- Hypersensitivity to mirtazapine
- Use of monoamine oxidase inhibitors (MAOIs, e.g., selegiline) within the past 14 days
- Pre-existing haematological disease
Adverse effects
- Drowsiness/sedation
- Increased affection (cats)
- Hypotension
- Tachycardia
- Sedation (common and can be profound)
- Increased vocalization (especially in cats)
- Altered behavior and increased interaction with others
- Blood dyscrasias (reported in humans)
Precautions
Use with caution in patients with known cardiac disease or cerebrovascular disease that could be exacerbated by hypotension.
- Monitor patients with renal impairment, renal failure, or hepatic disease closely, as clearance may be reduced.
- Use with caution in patients with diabetes mellitus, as antidepressants may indirectly affect blood glucose concentrations.
- Due to weak anticholinergic activity, use vigilance in patients susceptible to these effects (e.g., urinary retention, prostatic hypertrophy, acute closed-angle glaucoma, increased intraocular pressure, GI obstruction, or ileus).
- May impair concentration and alertness in active animals.
- Use cautiously in patients with pre-existing hematological disease (leukopenia, neutropenia, thrombocytopenia), as it has been rarely associated with blood dyscrasias in humans.
Drug interactions
Mirtazapine may cause increases in blood pressure
Minimal effects on mirtazapine blood levels, but may cause additive impairment of motor skills
May cause increased serum concentrations of mirtazapine
Increased risk for serotonin syndrome
Increased risk for serotonin syndrome; MAO inhibitors considered contraindicated with mirtazapine
Increased risk for serotonin syndrome
Increased risk of serotonin syndrome
Monitoring
- Increased appetite
- Decreased episodes of vomiting
- Weight gain
- Adverse effects (sedation, vocalization, tachycardia)
- Appetite and body weight
- Behavioral changes (sedation, vocalization)
- Complete blood count (due to human risk of blood dyscrasias)
- Hepatic and renal function parameters
Pharmacokinetics
Half-life
Absorption
Rapidly and completely absorbed following oral administration. Food has minimal effects on rate and extent of absorption. Oral bioavailability is about 20% for dogs and 50% for humans.
Distribution
Plasma protein binding is approximately 70-72% for dogs, mice, and rats.
Metabolism
Metabolized via multiple pathways including 8-hydroxylation, N-oxidation, and demethylation followed by conjugation. Cats have a limited capacity for glucuronidation, leading to less rapid clearance and requiring extended dosing intervals.
Elimination
Elimination occurs primarily via urine (75%) and feces (15%). Renal or hepatic impairment may reduce clearance.
Overdose
Ingestion of upwards of 10-fold to 30-fold the therapeutic dose in humans exhibits minimal toxicity requiring no acute intervention other than observation.
Treatment: Despite the wide safety margin, standard overdose protocols (including the administration of activated charcoal) and monitoring are recommended in acute overdose situations.
Available products
Formulations
- Oral Tablets
- Orally Disintegrating Tablets (ODT)
- 15 mg tablets
- 15 mg/ml oral solution
- Transdermal formulation (named patient basis / authorized veterinary preparations)
Veterinary
- None
- Mirataz 20 mg/g transdermal ointment
Human-labeled
- Mirtazapine Oral Tablets: 7.5 mg, 15 mg, 30 mg & 45 mg (Remeron, generic)
- Mirtazapine Orally Disintegrating Tablets: 15 mg, 30 mg & 45 mg (Remeron SolTab, generic)
- Zispin 15 mg tablets
- Zispin 15 mg/ml oral solution
Regulatory status
Mirataz transdermal ointment is authorized for cats.
Mirataz transdermal ointment is authorized for cats.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store coated and orally disintegrating tablets at 25°C (77°F) with excursions permitted to 15-30°C (59-86°F). Protect from light and moisture. Orally disintegrating tablets should be used immediately once removed from the blister pack.
Client information
- Administration: Give exactly as prescribed. May be given with or without food.
- Orally Disintegrating Tablets (ODT): If using the melting tablets, ensure your hands are completely dry before handling. Place the tablet under your pet's tongue and hold their mouth closed for a few seconds until it dissolves. Offer water immediately after.
- What to Expect: You should see an improvement in appetite and decreased vomiting.
- Side Effects to Report: Contact your veterinarian if you notice excessive drowsiness, severe weakness, rapid heart rate, or unusual behavior.
Note for Cat Owners: It is very common for cats to become unusually vocal (meowing loudly) or overly affectionate after taking this medication. This is generally harmless but let your vet know if it becomes disruptive.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.