It is difficult to study the performance effects of morphine without considering why a person is taking morphine. Opioid-naive subjects are volunteers in a pain-free state. However, most chronic-users of morphine use it to manage pain. Pain is a stressor and so it can confound performance results, especially on tests that require a large degree of concentration. Pain is also variable, and will vary over time and from person to person. It is unclear to what extent the stress of pain may cause impairments, and it is also unclear whether morphine is potentiating or attenuating these impairments.