Mirtazapine Side Effects in Dogs

Mirtazapine side effects in dogs are minimal, usually resulting in drowsiness or, less commonly, serotonin syndrome. Serotonin syndrome is when the brain levels of serotonin get too high, usually as a result of mirtazapine being taken with other serotonin increasing medications. Indicators of serotonin syndrome are dilated pupils, elevated heart rate, elevated body temperature, tremors and/or shivering, difficulty breathing, high blood pressure and general hyperactivity. Cyproheptadine can be used as an antidote in the event of serotonin syndrome. Concurrent use of mirtazapine and tramadol can also increase the risk for serotonin syndrome.

Anti-Anxiety and Anti-Nausea Properties

Originally developed as an antidepressant for moderate to severe depression for human use, mirtazapine’s side effects as an anti-nausea and appetite stimulant in dogs helps in the treatment where poor appetite and nausea are symptoms of a disease, such as:

  • liver disease
  • kidney disease
  • intestinal disease
  • stomach disease
  • chemotherapy
  • any other disease involving nausea and appetite loss

Mirtazapine also has antihistamine properties; however, since other antihistamines are more readily available, it is unlikely that this drug would be used for that purpose. Inhibitors of monoamine oxidase-A cannot be used with mirtazapine. Use of selective serotonin reuptake inhibitors (SSRIs) should not be used in conjunction with mirtazapine because serotonin levels in the brain can raise enough to create serotonin syndrome. The most commonly used SSRI would be fluoxetine or Prozac.

Mirtazapine Uses

Mirtazapine is a member of the tetracyclic class since it is composed of four chemical rings. Mirtazapine increases norepinephine and serotonin in the brain although no one is exactly sure how. Norepinephine is a stimulating neurotransmitter while serotonin is a neurotransmitter associated with comfort and relaxation. Increasing these chemicals in the brain offers treatment of depression. Although depression is sometimes treated in veterinarian medicine, it is the side effects of mirtazapine that are actually sought. Mirtazapine acts on the neuroreceptors in the stomach and intestine that communicate with the brain’s vomit center thus acting as a strong anti-nausea agent. It also acts as a strong appetite stimulant. Although mirtazapine increases central nervous system serotonin levels, it antagonizes serotonin levels in the gastro-intestinal tract.

Mirtazapine Dosage for Dogs

Mirtazapine comes in a regular formula as well as a rapid dissolving formula. Mirtazapine, also known as remeron, is given once a day. It is available in 3 sized dosages: 15 mg, 30 mg and 45 mg tablets.  In the case of kidney disease or liver disease, the clearance of the drug from the body is reduced approximately 30% so the dosage should be reduced as well.

Although mirtazapine reduces cortisol secretion by the adrenal glands, it has not been used to treat Cushing’s syndrome, in which the adrenal glands produce excess adrenal steroids.

Patients with either liver disease or kidney disease should receive a reduced dosage of mirtazapine or a reduced scheduling of receiving mirtazapine.

The safety in using mirtazapine in pregnant or lactating dogs is unknown at the present.