A 29-year-old man presents two days after running out of prescribed oxycodone, which he had been taking in increasing amounts for a year. He is yawning, restless and complaining of abdominal cramps; examination shows watering eyes, gooseflesh, loose stools and enlarged pupils, but he is afebrile and fully orientated. There is no muscle rigidity, inducible clonus or reduced respiratory rate. Which one of the following is the most likely diagnosis?