Both fentanyl and the analogues have potential for use as end of life drugs. However, the narrow therapeutic index (the dosage range in which the drug is an effective analgesic) and the small quantities needed for lethal depression of respiratory function are important factors. As little as 3mg of fentanyl taken intravenously can be lethal. However, larger quantities would be needed for reliably-lethal, oral administration, unless metabolism-slowing steps (eg. taking cimetidine or the drinking of grapefruit juice) are adopted. For carfentanil, even smaller does will end life.
Interestingly, these drugs retain some of the sedative and euphoric properties of morphine. The death these drugs bring about can be painless and quick. The rapid metabolism of these drugs may also give the possibility of undetectability in terms of the cause of death.
Difficulties in using fentanyl include the usual problems of tolerance and sensitivity (common to associated all opioids). However, the fact that the drugs are available (albeit illegally) together with the fact that relatively small physical amounts are required for a lethal dose, go someway towards balancing out the pros and cons. The availability of an effective antidote (naloxone), however, means that one cannot afford to be interrupted when taking fentanyl to die.