What do antidiuretic hormones do?

2 things: Hi. ADH (vasopressin) prevents free water loss in the urine (i.e., concentrates the urine) and increases vasoconstriction and availability of Von Willebrand Factor, which augments blood clotting. In my experience (and I'm not a hematologist), the big therapeutic use of the ADH (vasopressin) analog, DDAVP, is in diabetes insipidus. It makes their lives comfortable and livable.