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.