Naloxone and naltrexone are opioid antagonists because they have high affinities for all opioid receptors. They work by displacing mu and kappa agonists and by binding to but not activating the receptors. It is important to recognize clinically that they reverse the beneficial (i.e. analgesia) and undesirable effects of other opioids.
Naloxone and naltrexone are administered primarily to counter the effects of opioid overdose which can lead to profound respiratory depression or bradycardia. Naloxone has a relatively short duration of action (about 1 hour) indicating repeat doses may be required to reverse the action of an opioid agonist with a longer duration of action. Naltrexone has a longer duration of action, about 2 hours but little published evidence is available concerning its use among veterinary patients.