Desmopressin is synthetic vasopressin (AKA anti diuretic hormone). Your body is supposed to produce increased vasopressin at night, but some people don't. Desmopressin is essential taken as a supplement, to bring the ADH levels up to normal. The catch is, if you have to much to drink, your body normal cuts off vasopressin production, and makes more urine, to balance your blood chemistry. If you are taking desmopressin, you have to refrain from drinking excessive quantities of liquid, because your body won't have the option to eliminate it, and too much water in your blood can cause serious damage, or even death.
Basically, any night you've had too much to drink before bed, is also the night you probably shouldn't be taking desmopressin either.