nalmexone nalmexone nalmexone (inn) (code names en-1620a, um-592), or nalmexone hydrochloride (usan), is a semisynthetic, opioid partial agonist or mixed agonist-antagonist with both analgesic and narcotic antagonist properties that was never marketed. in clinical studies it was found to have comparable analgesic efficacy to morphine, though with several-fold reduced potency. in addition, nalmexone's side effects, the most common of which were sleepiness and sweating, were reported to be similar to those of morphine, albeit with a noticeably higher degree of incidence. not to be confused with naltrexone. nalmexoneclinical dataatc codenoneidentifiers iupac name (5α)-3,14-dihydroxy-17-(3-methyl-2-buten-1-yl)-4,5-epoxymorphinan-6-one cas number16676-26-9 16291-05-7 (hcl)pubchem cid5360516chemspider4514525uniiaib278ukigcomptox dashboard (epa)dtxsid80168172 chemical and physical dataformulac21h25no4molar mass355.434 g·mol−13d model (jsmol)interactive image smiles o=c45oc1c2c(ccc1o)c3n(cc253(o)cc4)c\c=c(/c)c inchi inchi=1s/c21h25no4/c1-12(2)6-9-22-10-8-20-17-13-3-4-14(23)18(17)26-19(20)15(24)5-7-21(20,25)16(22)11-13/h3-4,6,16,19,23,25h,5,7-11h2,1-2h3/t16-,19+,20+,21-/m1/s1key:ohkcloqpslqcqr-mbpvovbzsa-n synthesis nalmexone synthesis: lowenstein, m. j.; fishman, j.; 1967, u.s. patent 3,320,262