Users can distinguish between some opioids, but not all. Opioids aren’t pure mu-agonists, they also have sigma agonists effects which produce non-opioid effect

Users can distinguish between some opioids, but not all. Opioids aren’t pure mu-agonists, they also have sigma agonists effects which produce non-opioid effects and also have off effects (histamine release). Combine that with the onset and duration of action, users can definitely tell if they got fentanyl versus say morphine. But not really between heroin and morphine.