In recent years Mirtazapine, a tricyclic antidepressant used in people, has been increasingly prescribed as appetite-stimulating and anti-emetic agent. Mirtazapine has many actions; however, these effects are thought to result from antagonism of 5-HT₃ receptors. Although a recent study showed mirtazapine to be effective in increasing appetite in healthy cats, compared to placebo, there is very little information available about its use in a clinical setting. This study aimed to investigate the use of mirtazapine and its effectiveness in a large referral population.

Between February 2009 and September 2011 mirtazapine was dispensed to 272 animals at our referral hospital. Case records were reviewed and data collected regarding species, breed, age, final diagnosis, reason for prescribing, number of days off food (defined as number of days reported by owner as being off food or the number of days in hospital when less than half the food offered was eaten), time until eating (defined as eating more than ¾ of the food offered on more than 2 occasions), dose and number of days dispensed and any adverse effects noted.

One hundred and sixty four dogs and sixty eight cats met the inclusion criteria of having been dispensed mirtazapine and having complete case records. The most common underlying conditions were infectious/inflammatory (84 cases) and neoplastic (58 cases) disease. Average patient age was 7 years. The starting dose used for dogs was 0.5 mg/kg every 24 hours and 3.75 mg for cats every 72 hours. The average length of course prescribed was 3 days. The average number of days that animals were off food prior to receiving mirtazapine was 6 days. Of the 232 animals, 160 were classified as having eaten and 56 were still off food, before being discharged, dying or euthanasia. Nineteen of those 56 animals died or were euthanized a few days after starting treatment. Sixteen animals were treated as outpatients and therefore no information on time to eating could be obtained. In the group of animals that ate, the average time until eating well was 24 hours. No side effects attributed to mirtazapine were documented during the course of this study.

This study appears to show that the administration of mirtazapine is associated with a temporal improvement in appetite; however, a blinded placebo controlled study is needed to further investigate this possibility.