Benzodiazepine Abuse

In this study, the investigators will measure affective, neurocognitive and behavioral outcomes related to chronic use of opioids and benzodiazepines (screening phase), and in response to the administration of the opioid morphine, the benzodiazepine alprazolam, morphine then alprazolam, alprazolam then morphine, morphine+alprazolam simultaneously, and placebo (laboratory pharmacology experiment). The latter will enable the investigators to assess the effects of an opioid alone, benzodiazepine alone, concurrent and simultaneous administration of opioid+benzodiazepine, relative to a placebo control.