Naltrexone, is an opioid antagonist approved by the FDA in 1984 in a 50mg dose for the treatment of opioid and alcohol abuse. By blocking opioid receptors, naltrexone also blocks reception of the endogenous opioid peptides: beta-endorphin, metenkephalin and dynorphin. Virtually every cell of the immune system has receptor for these mediators. These peptides regulate a wide range of biological activities, including immune responses through interaction with G-protein coupled transmembrane opioid receptors.