3. Active medical illness that might make participation hazardous (e.g., untreated hypertension, hepatitis with AST or ALT >3 times upper limit of normal, unstable diabetes or heart disease). Adequately treated medical conditions are acceptable; 4. female subjects who are pregnant or lactating, or female subjects of childbearing potential who are not using birth control (oral contraceptives, barrier (diaphragm or condom) plus spermicide, or levonorgestriel implant); 5. Liver failure or liver function test levels greater than three times normal; 6. History of allergic reaction to naltrexone; 7. History of a drug overdose in the past 3 years; and 8. Current diagnosis of chronic pain disorder for which opioids are prescribed for pain relief.