A reference post rather than a discussion. Corrections are the point; I would rather this be right than mine. It is about hydration and electrolytes, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
Two things drive this: people drink less because drinking makes them feel full, and they restrict salt at the same time as calories without noticing. The result reads as fatigue, headache, cramp and light-headedness on standing, all of which get blamed on the drug. Fluid plus sodium fixes most of it within a day, which is also how you can tell that is what it was.
The condition it depends on
The kidney-stone risk is the reason to take it seriously rather than just uncomfortable. Concentrated urine over months is a stone-forming condition.
The practical version
The test is quick: add fluid and salt for two days and see whether the symptom goes. If it does not, it was not hydration and it is worth a panel.
What I am not sure about
The bit I cannot resolve on my own is why hydration goes wrong on this specifically, given that nobody is telling us to drink less. If the honest answer is that nobody knows, that is a useful answer and I would rather have it.
— JennaRN · corrections welcome and will be edited into this post with credit