prevents the binding of opioid drugs to opioid receptors in the body. Narcannaltrexone is a morphine based opioid antagonist which can, similarly to other opioid antagonists, be used to reverse the effects, or even prepare the body for an injection of an opiate based drug. If taken prior to an opioid, the narcannaltrexone will reduce the effects of the opioid whilst is fills the opioid binding sites.