Hello, 640mg is very high dosage. The only reason to increase the dosage so much (that I can think of) is trying to reduce albuminuria in a patient with type 2 diabetes. There is a study suggesting that 640mg of valsartan may normalize protein excretion in more patients than 320mg of valsartan. I suppose this was the reason that your doctor changed the dosage. I can t say whether this is appropriate for you or not because I don t have that much data. In most patients 640mg are well tolerated. Some patients experience headache and/or dizziness and other side effects though. So in conclusion, this is something to discuss with your doctor. If you didn t have any side effects then there are no immediate concerns. You d better talk with your doctor first before changing your regimen. I hope it helps! Kind Regards!