This is the important bit:
The problem is that a lot of people are in pain and opiates can help to a massive degree but you do have the cognitive side effects and to measure them can be difficult. They key bit is the limit - I have patients who have side effects on anything as little as 8/500 cocodamol and yet others are fine with oxycodone.
It's not quite right but I sometimes advise patients to take their car down a quiet road (with someone) and do an emergency stop (a bit like in the driving test). If they cannot stop adequately/in time, they shouldn't be driving.