phenobarbital boosts toxicity of ifosfamide by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Use Caution/Observe. CYP3A4 inducers may improve the metabolism of ifosfamide to its Energetic alkylating metabolites.If indications of withdrawal occur on the first working day of treatment, a loading dose of 100 to two hundred mg of phenobarbital