bodies do not always metabolize many kinds of drugs at the same rate: individuals differ, and their own metabolism rates for drugs, alcohol, etc can change during times of illness, and probably other changes of life.
When my Dad was dying, his prescription for pain suppressed breathing at the end, because even though the dose was on time and as prescribed, there was too much "on board", still in his system.
This is a very serious concern, thanks for bringing it up.