I didn't have the "keto flu", but from day 1 took extra salt in the form of two cups of broth that is a part of the clinic diet I used.
More on how to handle side effects: https://www.dietdoctor.com/low-carb#side-effects

I suggest you do double check that you are following all the induction rules, the biggest challenge with LC "the second time around" is remembering the latter phases and not being as strict as induction requires. Addressed by Dr. Eades in this post: https://forum.lowcarber.org/showpos...2&postcount=158