Physiologic and morphologic changes and incidence of neoplasms in mice and rats fed naltrexone HCl for 24 months

Short-term (acute oral LD50 and 90-day oral subchronic) studies in mice and long-term (24 months) carcinogenesis bioassays were performed in B6C3F1 mice and Fischer 344 rats given naltrexone. The oral LD50 was approximately 1500 mg/kg; convulsions, hypopnea, and cardiac failure were dose-related. Naltrexone mixed with feed over 90 days did not evoke definitive signs of gross toxicity, and histopathology was unrelated to drug treatment. Similar drug/feed admixtures given for 24 months to mice or rats did not disturb behavior. In mice, naltrexone reduced growth rates 5-10% and food intake 9-19%, but survival rates were 70-82% for treated mice and controls. The frequency and location of predominant tumors were similar in treated and untreated mice. In the rat, the same dosages had little effect on growth or food intake. The majority of all sacrificed rats had neoplasms. Neither neoplasms nor nonneoplastic lesions in mice or rats were associated with drug treatment. It is concluded that naltrexone is not a carcinogen.