Naltrexone is an opioid antagonist. It works by blocking the euphoric feelings (highs) that you may experience after taking opiates and alcohol. This reduces craving and helps you remain off of them.Naltrexone belongs to a class of drugs known as opiate antagonists. It works in the brain to prevent opiate effects (such as feelings of well-being, pain relief). It also decreases the desire to take opiates. This medication is also used to treat alcohol abuse.