The purpose of this study is to determine if the opioid agonist effects of doses of 4/1 mg, 8/2 mg, and 16/4 mg of buprenorphine/naloxone can be completely blocked by 50 mg of oral naltrexone. This study will provide data to support the design of a similar study of the depot formulation of naltrexone and ultimately to study this combination for the treatment of cocaine dependence.

Inclusion Criteria:

• Provide written informed consent
• Be male/female between the ages of 18-55
• Be in good physical and mental health as determined by interview and physical exam
• Have a body mass index between 18 and 30, inclusive
• Have no oral cavity pathology that would interfere with sublingual absorption of buprenorphine/naloxone or placebo
• Have negative qualitative urine toxicology screen for opioids, cocaine, benzodiazepines, and barbiturates prior to administration of the first dose of naltrexone
• Be able to comply with protocol requirements
• If female, have a negative pregnancy test and agree to use an acceptable method of birth control from date of consent and until two weeks after the last administration of study drug or placebo

Exclusion Criteria:

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