Unfortunately, I have the same problem. I had a pituitary apoplexy in 2009 with secondary panhypopituitarism & diabetes Insipidus. Long story short...I was started on Desmopressin nasal spra, one spray each nostril before bed. This worked for a while and my body adapted and required more and more. Now, I am taking both the nasal spray ( 2 sprays each nostril in the morning and before bed) and oral Desmopressin morning and night. My Endocrinologist reports I have doubled the recommended dose and that for some individuals the body just metabolizes this medication more rapidly than others making it less effective. Unfortunately, since this is a vasoconstrictor...the medication is also dangerous...too much can constrict the blood vessels in the the major organs such as the heart and kidneys resulting in a greater of two evils: heart or kidney failure versus dehydration from the diabetes Insipidus.