My Son Has Dignosed Poycystic Kidney At The Age Of

My son has dignosed poycystic kidney at the age of 9 ( 23 kg body wt.). dr started Envas 5 mg but BP goes very low so since last couple of month he started 2.5 mg instead of 5 mg now BP is maintaining between 100 /68 .
Does 2.5 mg is enough for him or he should taken other medicines too for retarding the growth of cysts.
His BP was 115/86 when dr started envas.