Compositions containing opioid antagonists, particularly alvimopan and its active metabolite, with improved solubility and bioavailability for oral or parenteral administration, injectable dosage formulations, kits, and methods of making and using same are disclosed. In preferred embodiments, invention provides injectable formulations containing opioid antagonists, particularly alvimopan and its active metabolite, having low solubility that may be readily prepared, are stable during storage, and provide maximum levels of opioid antagonists when administered parenterally, particularly via injection. The results are achieved by a combination of processing techniques and component selection.