A multimodal approach is recommended to optimize perioperative pain control in animals, although opioid use in horses remains limited due to the risks of central nervous system (CNS) stimulation and reduced intestinal motility. A group of 19 healthy, male, mixed-breed horses were divided into two groups and medicated with acepromazine (0.05 mg kg⁻¹) and detomidine (10 µg kg⁻¹), with methadone (0.05 mg kg⁻¹) (ADM) or saline (ADS) administered intravenously (IV). Physiological variables, intestinal motility, gastric distention, and facial pain (EQUUS-FAP) were evaluated one day before (DB), before the surgical procedure (BS), and at 1, 2, 4, 6, and 8 h (T1h–T8h) after administration (ADM-ADS). Results are presented as means with standard deviation or medians with an interquartile range. Analysis of variance, the Mann–Whitney, and Durbin tests were applied (p < 0.05). Intestinal motility was reduced at T1h and T2h, returning to baseline by T6h and T8h in both groups. Ultrasonographic examination revealed reduced motility, with less significant changes in the left ventral colon (LVC), right ventral colon (RVC), and cecum. Gastric dilatation was more pronounced in the ADM group at T1, 4, 6, and 8h. EQUUS-FAP scores were significantly lower in ADM at T2, 4, and 6h. ADM protocol may aid chemical restraint and analgesia without increasing hypomotility.

Boa constrictor snakes represent a suitable model for studying the absorption, metabolism, and elimination of tramadol due to their distinct physiological characteristics. The objective of this work was to provide preliminary data on the pharmacokinetics of tramadol and its active metabolite, O-desmethyltramadol (M1), in the plasma of Boa constrictor using liquid chromatography with fluorescence detection. Ten snakes received tramadol (5 mg kg⁻¹) both into the epaxial musculature (TRIM) and into the paravertebral vein (TRIV) with a 45-day interval between the two administration methods. Blood samples were taken at specified time points to analyze the pharmacokinetics. Data were evaluated with an independent pharmacokinetic model (R software version 4.3.0). A paired Student’s t-test was used for all parametric variables, except clearance, which was analyzed with the Wilcoxon test. A significance level of 5% was applied. The mean (range) maximum concentration of tramadol, volume of distribution, clearance, and elimination half-life for the TRIM group were 2.58 µg mL⁻¹, 10.58 ± 2.91 L kg⁻¹, 0.36 L kg⁻¹ h⁻¹, and 19.96 ± 8.34 h, respectively. For the TRIV group, these values were 3.39 µg mL⁻¹, 5.60 ± 1.69 L kg⁻¹, 0.22 L kg h⁻¹, and 17.32 ± 7.55 h⁻¹, respectively. M1 achieved maximum concentration and elimination half-lives of 0.58 µg mL⁻¹ and 49.89 ± 10.8 h, respectively, for TRIM and 0.59 µg mL⁻¹ and 35.66 ± 10.85 h for TRIV. The bioavailability of intramuscular tramadol was 61%, and M1 remained at similar concentrations for 20 min after tramadol administration in both treatments. Tramadol is rapidly biotransformed into M1 in Boa constrictors, maintaining high concentrations over an extended period. The pharmacokinetic characteristics, particularly the sustained plasma concentrations of M1, suggest potential for effective analgesia in the Boa constrictor. Furthermore, the intramuscular route provides the additional advantage of ease and practicality of administration.