Pain is a subjective state – that’s why practitioners typically ask us to ‘scale’ our experience of it, from a ‘1’ (slight) to a ’10’ (unbearable). And while it’s easy to see why the experience of pain is useful in an evolutionary sense (‘keep your hand out of the fire!’) it’s difficult to account for what mechanism is responsible for this experience. Different analgesics may have different effects (and affects) for different people, and some don’t seem to contraindicate – it seems you can ingest an opioid at the same time as paracetemol.
Enter the placebo effect! Here’s a recent research paper on placebos and pain. And also a fascinating article on ‘leveraging the Placebo Effect to Reduce Opioid Requirements’.