Opioid antagonists are used to treat opioid overdose, opioid use disorder, and opioid-induced constipation. These medications work by blocking the effects of opioids on your brain and body, which can help reverse an overdose, reduce cravings in opioid use disorder, or relieve constipation. Opioid antagonists come in different forms, including oral pills and injections. Examples include naltrexone (Vivitrol), naloxone (Narcan, Kloxxado, Zimhi), and methylnaltrexone (Relistor).