Both fentanyl and the analogues have potential for use as end of life drugs. 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. A lethal dose of fentanyl is as little as 3mg when taken intravenously. However, larger quantities will be needed for reliable lethal oral administration unless metabolism slowing steps (eg the drinking of grapefruit juice) are adopted. For carfentanil, even smaller doses will end life.
Fig 10.5 ~3mg of Fentanyl citrate powder
These drugs retain some of the sedative properties of morphine and the death from these drugs can be painless and quick. The rapid metabolism of these drugs may also give the possibility of undetectability regarding the cause of death.