Fentanyl and its analogues are highly potent and clinically widely used as narcotic analgesics and
represent a particular class of ì agonist. These compounds are known as non-lethal chemical
warfare agents. A very large number of fentanyl analogues (FA) have been synthesized, with the
aim of establishing structure-activity relationship (SAR) and finding clinically more useful drugs
with better pharmaceutical profile such as high potency and less side effects (e. g. respiratory
depression and lower addiction potential). The most well-known FA are 3-methyl fentanyl,
ohmefentanyl, lofentanil, carfentanil, alfentanil and sufentanil. ED50 of different FA are reported but
these data are not considered all together to deduce structure-activity relationship. In this
contribution, the effect of 5 groups contributing to the analgesic activity of FA (70 compounds) are
reinvestigated. The best groups are determined and more effective fentanyl analogue is suggested.
We have tried to determine the relative weight of effective groups on ED50 of FA, but comparison
of different data indicates that actions of different groups on ED50 are not independent. In another
part, atomic electron density and conformational studies of FA by abinitio methods are correlated
with their respective ED50.