Increased antihypertensive effect with other antihypertensive drugs. Monitor potassium frequently if used with potassium supplements, potassium-sparing diuretics, salt substitutes containing potassium, or drugs that may affect potassium levels (e.g. heparin, ciclosporin, drospirenone, amphotericin, corticosteroids, carbenoxolone). Reduced antihypertensive effect of valsartan and increased risk of renal impairment with NSAIDs. May reduce renal clearance of cytotoxic drugs (e.g. cyclophosphamide, methotrexate). Increased serum calcium with vitamin D or calcium salts. Potentially Fatal: Increased lithium levels with valsartan, monitor lithium levels. Increased risk of cardiac arrhythmia with dofetilide, avoid use.