Mirtazapine is an antidepressant medication commonly used in both dogs and cats to stimulate appetite, particularly in animals who are not eating due to illness or medication side effects. It works by increasing the levels of serotonin and norepinephrine in the brain, which can help stimulate appetite. Mirtazapine can be administered orally or transdermally, and the dosage may need to be adjusted based on the animal’s response and any potential side effects such as sedation or restlessness.