200mg of morphine, plus 50mg of of any benzodiazepine (like valium, ativan, xanax) will kill most any adult human.
They will have an amazing high, followed by unconsciousness, followed rapidly by respiratory arrest.
Add say, 20mg of potassium, 5 minutes after the first cocktail and you’ll get a nearly instant cardiac arrest that the prisoner will never feel.
Use the same method the Euthanasia advocated recommend. Plastic bag and helium.