sentences:  Nalmefene is an opiate derivative similar in both structure and activity to the opioid antagonist naltrexone.Advantages of nalmefene relative to naltrexone include longer half-life, greater oral bioavailability, and no observed dose-dependent liver toxicity.Like other drugs of this type, nalmefene may precipitate acute withdrawal symptoms in patients who are dependent on opioid drugs, or more rarely when used post-operatively, to counteract the effects of strong opioids used in surgery.