Nalmefene vs Naloxone for the Treatment of Recurrent Respiratory Depression After Opioid Overdose

The goal of this clinical trial is to compare naloxone to nalmefene for the treatment of opioid overdose in adults. The main questions it aims to answer are: * Does nalmefene lower the number of doses of medicine participants need to treat opioid overdose? * When participants are given nalmefene instead of naloxone, do they have fewer complications of opioid overdose such as being admitted to the hospital or having a breathing tube inserted? Researchers will compare nalmefene to naloxone for the treatment of opioid overdose. Nalmefene and naloxone are both approved medicines to treat opioid overdose. Participants who are brought to the emergency department after an opioid overdose will be given a dose of either nalmefene or naloxone if their breathing slows down again after an opioid overdose. Participants will: * Stay in the emergency department for 8 hours after receiving a dose of nalmefene or naloxone. * Receive a phone call 7 days after their emergency department to check on how they are doing. Background information: Naloxone (also known as Narcan) and nalmefene are opioid blocking medicines. When someone overdoses on an opioid, such as heroin or fentanyl, their breathing slows down or stops and they can die. By giving naloxone or nalmefene, the effect of the opioid can be blocked and the person can start breathing again. Naloxone is the most commonly used medicine to reverse an opioid overdose. The effect of naloxone lasts about an hour, and patients may need more than one dose of naloxone to keep them breathing. Sometimes patients overdose, get a dose of naloxone and wake up, and then some time later their breathing slows down again and they need another dose of naloxone. This can happen because the effect of the opioid they took lasts longer than the effect of the naloxone. The effect of nalmefene lasts longer than naloxone, about four hours. If a person gets nalmefene, their opioid may wear off before the nalmefene wears off and they might not need any more doses of a reversal medicine. Both naloxone and nalmefene are approved medicines for treating opioid overdose. Often, when a person overdoses on an opioid, someone gives them naloxone right away and then they are brought to the emergency department. In the emergency department, they are watched for several hours to make sure they don't stop breathing again when their naloxone wears off. If they do stop breathing again, they are given another dose of naloxone. In this study, participants will be given either nalmefene or naloxone if their breathing slows down while they are in the emergency department.