Buprenorphine, a treatment for opioid dependence, can be mixed with another drug, naloxone, to limit abuse potential. Parenteral administration (intravenous or intramuscular injection) of buprenorphine/naloxone causes withdrawal symptoms in opioid dependent individuals. However, naloxone does not cause withdrawal symptoms in non-dependent opioid abusers. This study will investigate whether naloxone decreases the opioid agonist effect from injected buprenorphine, hence decreasing the abuse potential of buprenorphine/naloxone, in non-dependent opioid abusers.