E) Certain drugs will be used with caution or avoided. Barbiturates and phenytoin induce the hepatic enzymes that metabolize cyclophosphamide and therefore if these medications are required, cyclophosphamide doses may need to be increased to achieve a comparable immunosuppressive effect. Drugs that inhibit cyclophosphamide metabolism include allopurinol, imipramine, and phenothiazines, chloramphenicol and chlorpromazine; these drugs will be avoided. NSAID increase the risk of hyponatremia; these drugs will be avoided.