A 29-year-old man is brought in shortly after his partner administered naloxone for suspected heroin overdose. He is yawning repeatedly, has abdominal cramps, diarrhoea, gooseflesh and enlarged pupils, and is shouting that he feels unbearably unwell. His temperature is 37.1°C, muscle tone is normal and ankle reflexes are not brisk. Which one of the following is the most likely diagnosis?