AbstractObjective The effect of premedication with butorphanol or methadone on ease of endoscopic duodenal intubation. Study Design Prospective, randomized, blinded clinical trial; Animals or Animal population Twenty client owned dogs Methods Dogs were randomly assigned to intravenous (IV) premedication with either butorphanol (0.4 mg kg-1) or methadone (0.3 mg kg-1). General anaesthesia was induced with propofol to effect, and maintained with isoflurane in 100% oxygen. Sedation score 20 minutes after premedication administration and induction dose of propofol were recorded. Heart rate (HR), mean arterial pressure (MAP), haemoglobin oxygen saturation (SaO2), respiratory rate (ƒR) and end-tidal isoflurane concentration (FE´Iso) were recorded every five minutes. Spontaneous lower oesophageal and pyloric sphincter opening, presence of gastro-oesophageal and duodeno-gastric reflux, antral peristaltic contractions and response to endoscopy were recorded as yes or no. Ease of duodenal intubation (EDI) was graded on a scale ranging from 1 (immediate entry with minimal manoeuvring required) to 4 (no entry after two minutes). Time (seconds) from the start of pyloric intubation to successfully entering the duodenum was recorded. Results Median EDI score [3 ± 1 (butorphanol), 4 ± 1 (methadone), p = 0.035], time [65 ± 36 seconds (butorphanol), 120 ± 38 seconds (methadone), p = 0.028] and number of dogs with spontaneous pyloric sphincter opening [7/10 (butorphanol), 2/10 (methadone), p = 0.035] differed between groups. No other significant differences were found. Conclusions and clinical relevance In these clinical cases duodenal intubation was performed with greater ease, shorter time and more frequent spontaneous opening of the pyloric sphincter after premedication with butorphanol in comparison to methadone. The use of butorphanol facilitated the passage of the endoscope and is therefore recommended for premedication prior to upper gastrointestinal tract endoscopy.