- Previous therapy with agents with significant systemic myelosuppressive, neurotoxic, pancreatoxic, hepatoxic or cytotoxic potential within 3 months of study start or the expected need for such therapy at the time of enrollment of therapy with methadone or ribavirin/interferons or treatment with neurotoxic drugs or drugs that affect Cytochrome P450 3A4 (CYP3A4).