Science: Desmopressin is a synthetic form of antidiuretic hormone (ADH or vasopressin), which helps retain water in the body. Damage to the pituitary may cause a deficiency of this hormone, a condition called diabetes insipidus (not related to diabetes mellitus though both cause excessive urination and usually thirst). There is often a deficiency of ADH in CFS/ME.
In one study desmopressin improved HPA axis function in CFS/ME. Because it can increase blood volume it is sometimes used to treat low blood pressure. Anecdotally some people have reported improved digestion.
Results: I get strange episodes when I just pee out everything and have to go to the bathroom often 5-6 times an hour, peeing just clear liquid as is typical of diabetes insipidus, quickly leading to dehydration which because of my adrenal insufficiency is dangerous. At their worst an episode has lasted for 3-4 days. Sometimes during such an episode when I try to drink water, my body actually reacts to it (even in small quantities) as if it was poison - stomach cramps, nausea and feverish chills. These episodes used to be rare (a few times a year) and appear mostly at random or triggered by flying or excessive drinking of liquid at once, but these days they can appear several times a week and are usually triggered by acute stress.
I also very easily get dehydrated if I don't drink any water even for just 1-2 hours (even if I try to drink more later in compensation). Antidiuretic hormone helps abort an episode, though takes a while to work (I should probably have a nose spray instead of the pills). However, it does not help my adrenal insufficiency, and it does not really help my nocturia - it greatly reduces urine output at night, but even if it is like 1/3 of normal, I still wake up at night to go to the toilet (probably out of habit), but it's a good thing as I need to take hydrocortisone at night, anyway. In 2012 the diabetes insipidus worsened and now I have to take desmopressin all the time, usually 1/2 tablet twice a day, but sometimes once a day is enough and sometimes I need a full tablet.
Watch out for: Can cause headaches, edema and stomach upset. One is supposed to limit intake of fluids while on desmopressin, as otherwise there may even be brain edema. (However, I've noticed that if I take it when I'm not dehydrated, water actually feels "yucky" and I can't imagine drinking it, something I never experience otherwise, but if I get dehydrated before it starts acting, I still feel very thirsty and can safely drink quite a lot as long as I feel thirsty.)
Source:
Anecdotal evidence of different CFS/ME treatments