Methylnaltrexone was discovered in 1979 by the late Dr Leon Goldberg at the University of Chicago (UChicago). To help a friend suffering from morphine-induced constipation, Goldberg had modified an established morphine (mu-opioid receptor) antagonist, naltrexone, so that it could not cross the blood–brain barrier and, consequently, did not interfere with the effect of morphine on the brain, but did block its effects on gut motility.