The margin of safety of a single application of transdermal fentanyl solution when administered at multiples of the therapeutic dose to laboratory dogs.

Previous studies have demonstrated that a single, topical application of a novel, long-acting transdermal fentanyl solution provides analgesic fentanyl concentrations for at least 4 days. The objective of this study was to describe the margin of safety following application at multiples of the therapeutic dose. Twenty-four laboratory dogs were administered a single placebo or 1×, 3×, or 5× multiple of the dose of 2.6 mg/kg (50 μL/kg) to the ventral abdominal skin and observed for 14 days. Plasma fentanyl concentrations increased in proportion to dose. Adverse reactions in the 1× group were transient and included a low prevalence (≤ 33%) of mild sedation, reduced food intake, modest weight loss, and minimal reductions in heart rate and rectal temperature. Moderate to severe sedation emerged in the 3× and 5× groups, which was associated with a dose-limiting reduction in food and water intake, necessitating maintenance fluid replacement for the first 2 days following application. Also observed in the higher-dose groups were an increased prevalence of abnormal stools and transient lens opacities. All abnormal health observations were completely resolved prior to necropsy on day 14, and there were no histological abnormalities identified. These data support the safe use of the 1× dose and describe the outcome of an overdose of up to 5× dose in the absence of opioid reversal.