Mirtazapine

What is this medication for?

• Anorexia associated with CKD/rising kidney values, CHF, stomach/intestinal disease, hepatic disease, neoplasia; nausea/vomiting incl. chemotherapy-induced; peri-operative inappetence

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Sedation/drowsiness (Main, dose-dependent)
• Vocalization, increased affection (Notable)
• Hypotension (orthostatic), unusually fast heart rate (Dose-dependent)

Worth a phone call to your veterinarian

• Blood dyscrasias (rare) (Human signal — caution in low white blood cell counts/low infection-fighting cell counts/low platelet counts)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: MAO inhibitor use within past 14 days (e.g., selegiline) — serotonin syndrome risk
• has or has had: Cardiac/cerebrovascular disease (low blood pressure); renal or hepatic impairment; diabetes (antidepressants can affect glucose); urinary retention/prostatic hypertrophy/glaucoma/stomach/intestinal obstruction (weak anticholinergic activity)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Selegiline, amitraz
  • Tramadol, linezolid
  • Fluvoxamine
  • Clonidine
  • Benzodiazepines

Storage and handling

Oral forms at room temperature, tight containers; do not freeze the suspension. Handle orally disintegrating tablets with dry hands.

Good to know

• Cats: ¼ of a 15 mg tablet every 72 hours — the every-3-day interval exists because cats glucuronidate poorly; daily dosing risks accumulation (Quimby PK data, t½ 10–15 h even in healthy cats, longer with CKD).
• Dosing conflict in the literature: the Perrin 2014 behavioral chart lists 1 mg/cat by mouth once daily — this daily scheme predates the feline PK work; prefer the Plumb every 3 days 3.75 mg regimen for cats, especially with renal disease.
• Appetite stimulation is the desired "side effect" (via H1/5-HT2 antagonism and alpha2 blockade); antiemesis comes from 5-HT3 antagonism — mirtazapine can stack with maropitant/ondansetron for refractory nausea.
• Do not exceed 30 mg/day for appetite stimulation — effect paradoxically disappears above the ceiling.