This is a single-center, randomized, 7-way crossover, double-blind, active and placebo-controlled study to evaluate the subjective effects of oxycodone combined with ultra-low dose naltrexone in comparison to oxycodone alone in non-physically dependent subjects with a history of opioid abuse. Approximately 14 subjects will be randomized to one of fourteen sequences selected from a balanced 7x7 Latin square design and its mirror image.