Mirtazapine toxicity in cats: retrospective study of 84 cases (2006-2011)

Abstract

Objectives Mirtazapine is commonly used in veterinary medicine at doses of 1.88 or 3.75 mg as an appetite stimulant. The objectives of this study were to determine the most common adverse effects reported and the dose associated with these signs. Methods Records of cats with mirtazapine exposure (2006-2011) were obtained from the American Society for the Prevention of Cruelty to Animals' Animal Poison Control Center. The following parameters were recorded: signalment, weight, outcome, agent ingested, amount ingested, route of exposure, clinical signs observed, intended of use, onset time of signs and duration of signs. Results The 10 most commonly observed adverse effects reported in 84 cats exposed to mirtazapine included vocalization (56.0% of cats; mean dose 2.56 mg/kg), agitation (31.0%; 2.57 mg/kg), vomiting (26.2%; 2.92 mg/kg), abnormal gait/ataxia (16.7%; 2.87 mg/kg), restlessness (14.3%; 3.55 mg/kg), tremors/trembling (14.3%; 2.43 mg/kg), hypersalivation (13.0%; 2.89 mg/kg), tachypnea (11.9%; 3.28 mg/kg), tachycardia (10.7%; 3.04 mg/kg) and lethargy (10.7%; 2.69 mg/kg). Fifty-nine (70.2%) cases were considered accidental ingestions and 25 (29.8%) cases were given mirtazapine as prescribed. The doses associated with signs of toxicity were 15.00 mg (40 cats), 3.75 mg (25 cats), 7.50 mg (four cats), 30.00 mg (one cat), 18.75 mg (one cat), 11.25 mg (one cat), 5.80 mg (one cat) and 1.88 mg (one cat). For cats with available information, the onset of clinical signs ranged from 15 mins to 3 h, and time to resolution of clinical signs ranged from 12-48 h. Conclusions and relevance The greater number of adverse effects at 3.75 mg rather than 1.88 mg suggests that the latter may be a more appropriate starting dose for stimulating appetite while limiting toxicity. The benefit of dispensing exact doses of mirtazapine is implied given the likelihood of accidental administration of a full tablet (15 mg) and the resulting toxicity.

Figure 1

Number of cats reported to have received each intentionally or accidentally administered dose of mirtazapine (mg). The doses most commonly associated with reported adverse effects were intentionally administered 3.75 mg and accidentally administered 15.00 mg

Figure 2

Average mirtazapine dose (mg/kg) and dose range (mg/kg) associated with reported adverse effects in comparison with a 1.88 mg dose range for a cat weighing 2.5–6.0 kg. The circles represent the average mirtazapine dose associated with each clinical sign. Note that the dose range for cats weighing 2.5–6.0 kg and administered a 1.88 mg dose of mirtazapine is lower than the majority of dose ranges reported to be associated with adverse effects