Mirtazapine As An Appetite Stimulant
... increasing the animal's appetite without the unwanted side effects of prednisone or megesterol.
Mirtazapine (Remeron®) is a tetracyclic antidepressant. It is used in people to treat moderate-to-severe depression. It works by blocking presynaptic alpha-2 adrenergic receptors that normally inhibit the release of the neurotransmitters norepinephrine and serotonin, thereby increasing active levels in the synapse. Mirtazapine is a potent antagonist of 5-HT2 and 5-HT3 receptors. Mirtazapine has no significant affinity for the 5-HT1A and 5-HT1B receptors. Since it has activity against the 5-HT3 receptors, it can also act as an antiemetic.
Its use in dogs and cats is off-label. It appears to increase appetite through its ability to increase serotonin activity. Additionally, the anti-emetic and anti-nausea effect that can be seen is the result 5-HT3 receptor blockade similar to ondansetron (Zofran®) and dolasetron (Anzemet®).
The drug should not be used in combination with monoamine (MAO) inhibitors. It also should not be used in conjunction with selective serotonin reuptake inhibitors (SSRIs), such as Prozac, since it could lead to serotonin syndrome. Serotonin syndrome is a potential side effect should brain levels of serotonin get too high, but this syndrome generally requires a combination of serotonin-increasing medications. Elevated heart rate, tremors/shivering, dilated pupils, difficulty breathing, elevated body temperature, or high blood pressure can all be signs of serotonin syndrome. Animals with serotonin syndrome sometimes demonstrate general hyperactivity.
The dose is empirical at this point, extrapolated from the human dose. Cats are usually given 1/4 15mg tablet Q72 hrs. The dose in dogs is 3.75mg SID less than 15 lbs.; 7.5mg SID 15-30 lbs.; 15mg SID 30-60 lbs.; 30mg SID greater than 60 lbs. The drug is available in 15mg and 30mg tablets. There is currently a study underway in cats with chronic renal disease to establish the optimum dose and to evaluate the drug's effectiveness as an appetite stimulant and anti-emetic agent. We will pass on those findings to you once the study is completed.