This human laboratory study will test the effects of tramadol as a step in its development as a new treatment for opioid dependence. Tramadol is a moderate mu agonist opioid that may produce low levels of opioid physical dependence. Tramadol's capacity for producing physical dependence has not been systematically studied in humans. It is important to quantify tramadol, as it provides a measure of its opioid agonist effects. This would also be informative in regards to the abuse liability of tramadol when used as an analgesic (as currently marketed), or when used in the treatment of opioid addiction (as proposed in this study). The purpose of this trial is to evaluate the level of physical dependence as well as blockade efficacy produced by chronic maintenance on oral tramadol in opioid dependent individuals.

Participants will be randomly assigned to receive different doses of tramadol or placebo for up to six weeks. Experimental sessions will take place up to three times per week during the treatment period. During challenge sessions, participants will receive an injection; four different kinds of effects may occur in a session following this injection. First, no effect may occur (a placebo). Second, an opioid agonist effect may occur (opioid agonists include heroin, morphine, hydromorphone, tramadol, and methadone), which may cause the participant to feel "high." Third, an opioid antagonist effect may occur (e.g., naloxone, naltrexone), which may cause the participant to feel a sense of opioid withdrawal.