In the second part the three formulations will be selected from those meeting the criteria for stability, solubility, particle size, color and concentration of 2-butanol in the first part of the study. Each of these formulations will be prepared by a certified pharmacy. In addition to the three selected naltrexone formulations, a fourth dosage form, a capsule-in-capsule (CIC) design, will be prepared in the pharmacy by incorporating 40 or 80 mg (determined from the first part of the study) of 2-butanol which is encapsulated (size 3 capsule) plus 50 mg of naltrexone (HCI form) in a size 0 double blind hard gel capsule. This capsule-in-capsule design will serve as a formulation-free positive control.