Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. 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
So the question, as narrowly as I can put it: why hydration goes wrong on this specifically, given that nobody is telling us to drink less. Practical detail welcome, however dull — the duller the better.
— PharmD_Rodriguez · corrections welcome and will be edited into this post with credit