Naloxone has been used for many years as an IV or IM injection for the reversal of opioid effects (following opioid overdose) and has been evaluated as an oral formulation to manage opioid-induced constipation. Immediate release oral naloxone preparations have however led to reversal of opioid effects and withdrawal. This has initiated the development of prolonged (slow release) naloxone preparations which prevent the systemic levels of naloxone reaching levels where the central opioid effects may be reversed. Naloxone has a high first pass metabolism (98%) and short half life (~1hr).