Effective analgesia opioid use in cats

WE were interested to read the recent letter about presumed opioid toxicity in an eight-year-old cat (VR, 4/11 January 2025, vol 196, pp 39–40). However, we were also concerned that this letter might be interpreted as a recommendation to avoid the use of full μ-agonist opioids in cats, particularly older cats, thereby potentially denying many animals adequate analgesia. This case sounds challenging with high doses of non-veterinary licensed medications – trazodone (50 mg) and gabapentin (100 mg) – required to facilitate treatment. This was followed by premedication with medetomidine (5 μg/kg) and methadone (0.3 mg/kg). The anaesthetic protocol is similar to many used clinically in the UK; however, in this case it may have been advisable to further reduce premedication doses as it is known that trazodone can potentiate the effect of other anaesthetic medications.1 Although unlikely in this case, there is a small risk of serotonin syndrome when trazodone and methadone are combined as they are both serotonin reuptake inhibitors.2 The description of respiratory depression and forelimb rigidity is more likely attributable to propofol, as mentioned, rather than to opioid toxicity.3 We believe there may also be some confusion regarding the cat's pupils. The case described showed ‘extreme miosis’ (ie, pupil constriction). While most species show miosis4 following opioid administration, cats are unique in that opioids cause mydriasis (ie, pupil dilatation).5 In this case, miosis would be unlikely to be attributable to administration of an opioid. In people, methadone has a long duration and a more variable analgesic effect so tends to be used as a treatment for drug dependence rather than a clinical analgesic,6 making comparison to human medicine inappropriate. Opioids remain the cornerstone of human perioperative analgesia although, as in veterinary medicine, there is a move towards provision of multimodal analgesia rather than over-reliance on high doses of opioids.7 The British Geriatric Society suggests opioids should be considered for older people with moderate/severe pain. Before prescription, it suggests considering age-related physiological changes and polypharmacy.8 Anaesthetic protocols should be tailored to the American Society of Anesthesiologists status and temperament of the patient, taking the expected pain level and duration of the intended procedure into account.9 There may be benefits to performing preanaesthetic blood testing in older patients to help guide medication choice and dose.10 Any medications administered before premedication should be considered and reducing the dose of individual agents used is vital.11 “We believe that not including effective μ-agonist opioids as part of anaesthetic protocols for routine procedures in cats, particularly older cats, presents significant welfare implications As with any adverse reaction coming to our attention involving one of Dechra's veterinary medicinal products, and regardless of any possible causality link, this case has been declared to the competent authorities. Hence it will enter the pharmacovigilance signal detection process ensuring that if any new safety information is confirmed, it is included in the product literature.