All of the chronic diseases of advancing age in cats--hyperthyroidism, diabetes, heart disease, chronic renal failure and cancer--can cause nausea and loss of appetite. But cats who do not eat for a couple of days are at risk of hepatic lipidosis (fatty liver disease), which is even more serious and potentially fatal. Pancreatitis, chronic feline lower urinary tract disease and chronic upper respiratory disease, too, can cause loss of appetite. An appetite stimulant, such as mirtazapine, can encourage a cat to start eating again.
Mirtazapine (brand name, Remeron) is a human drug prescribed "extra label" by veterinarians. In people, it's used as an antidepressant that is especially helpful for patients with anxiety and sleep disturbances, and as a sleep aid and appetite stimulant in patients who may have trouble with both due to an illness such as cancer or due to a treatment with strong side effects, such as chemotherapy. While other medications are more effective for anxiety in cats, veterinarian and VeterinaryPartner.com's educational director Wendy C. Brooks says that mirtazapine's side effects can relieve nausea and act as a strong appetite stimulant in cats.
Veterinarians choose mirtazapine as an appetite stimulant for cats with chronic disease because of its antinausea properties.
One of the advantages of mirtazapine over other appetite stimulants for cats is that it is given just twice a week, Dr. Brooks says. The drug can be compounded into a flavoured liquid or gel to rub inside the cat's ear tip, if the cat is difficult to pill.
Although side effects are rare, mirtazapine may cause an increase in blood pressure, heart rate and cholesterol levels and can make some cats restless and agitated. A more serious but even less common side effect in cats is serotonin syndrome, which occurs when brain levels of serotonin are too high. It is usually caused by a combination of drugs that increase serotonin, Dr. Brooks says. Symptoms include elevated heart rate, shivering, dilated pupils, difficulty breathing, elevated body temperature and high blood pressure.
In cats with liver or kidney disease, the clearance of mirtazapine from the body is reduced by about 30 per cent, Dr. Brooks says. Those cats should receive a very small dose "or the dosing schedule can be stretched out." Mirtazapine should not be used with fluoxetine (brand name, Prozac) or other serotonin reuptake inhibitors.