An injectable slow-release methadone, partial opioid agonist or opioid antagonist formulation is provided comprising methadone, a partial opioid agonist or opioid antagonist in a poly(D,L-lactide) excipient with a small amount of residual ethyl acetate. Upon intramuscular or subcutaneous injection of the composition, methadone, a partial opioid agonist or opioid antagonist is released in a controlled manner over an extended period of time. The composition finds use in the treatment of heroin addicts and alcoholics to reduce consumption of the abused substances. Of particular interest are the drugs buprenorphine, methadone and naltrexone.