OBJECTIVES: Opioid-induced constipation persists as a challenge in the management of chronic pain treated with opioid therapy. Multiple opioid antagonists have been applied in attempt to combat the gastrointestinal side effects of opioid analgesia, however their lipid-soluble nature allows passage into the central nervous system and consequent antagonism of centrally mediated analgesia. In contrast, methylnaltrexone offers the advantage of peripheral receptor-specific opioid antagonism due to chemical alterations conferring greater polarity and less lipid solubility. We present use of enteral methylnatrexone to treat severe opioid-induced constipation in a young boy who had failed treatment with the non-specific opioid antagonist, naloxone. This case reports describes the safe transition from enteral naloxone to enteral methylnaltrexone and discusses the potential risk of relative opioid toxicity during the transition. METHODS: Though methylnaltrexone has approved for subcutaneous use, the characteristics of the patient s disease required enteral administration which had not been described in pediatric dosing. Based on conservative extrapolation of data from adult dosing, a methylnaltrexone dosing regimen was selected and the naloxone was weaned over two days in an effort to avoid a relative opioid overdose. RESULTS: The patient was successfully transitioned to methylnaltrexone from naloxone over two days. He did experience increased sedation during this time however no severe respiratory depression occurred due to the cessation of chronic central opioid antagonism causing a relative opioid toxicity. Following the institution of methylnaltrexone, his opioid requirement significantly decreased and his gastrointestinal symptoms improved. DISCUSSION: Our case report demonstrates safe transition from enteral naloxone to enteral methylnaltrexone in a pediatric patient, avoiding the serious consequences of relative opioid toxicity. This patient experienced significant improvement of opioid-induced constipation and reduction in opioid requirements and it is possible that other patients would benefit as well. The role of enteral methylnaltrexone deserves further investigation.