May precipitate symptoms of acute withdrawal in opioid-dependent patients, including pain, hypertension, sweating, agitation, and irritability. Animal studies indicate nalmefene may not completely reverse buprenorphine-induced respiratory depression. Nalmefene structurally similar to both naltrexone and naloxone (patients with hypersensitivity to these agents may also react to nalmefene). Use with caution in cardiovascular disease or in patients receiving medications with potential adverse cardiovascular effects (e.g. hypotension, pulmonary edema or arrhythmias). Pulmonary edema and cardiovascular instability, including ventricular fibrillation, reported in association with abrupt reversal when using narcotic antagonists. Use with caution in renal impairment. Concurrent use of flumazenil and nalmefene may increase risk of seizures. Patients are at greater risk of overdose when nalmefene wears off if megadosing of opiates occurred. Risk of recurrent respiratory depression if opioid involved is long-acting. Excessive dosages should be avoided after use of opiates in surgery. Abrupt postoperative reversal may result in nausea, vomiting, sweating, tachycardia, hypertension, seizures, and other cardiovascular events (including pulmonary edema and arrhythmias).