I had POIS for at least 15 years.
My main symptoms were:
- brain fog
- extreme fatigue
- flu
- concentration issues
Generally, it last for 3-4 days.
I tried several treatments:
- paracetamol, antihistaminics
- nyacin
- vitamins
- ...
But no one helped me improve my symptoms.
Then, I decided to try meditation. After lot of different meditations, I can say today, almost 80% of symptoms disapeared (mostly brain fog that was so annoying at work...).
Even if it's Placebo effect, I can only recommend you to try this unorthodox method, especially, if you have tried lot of things and no ones worked for you.
I will advise to begin with basic meditations (concentrating on your breath and focusing to empty your mind), you can find lot of such meditations on internet.
Then, when I was more confident with basic meditation, I continued trying to cure myself with more sex-oriented meditations (improving sexual energy, removing shame around sex/masturbation, releasing chastity vows from past lives, ...).
Even, if this can sound totally weird, I think people here will mostly want to cure themselves than to focus on the method used.
Give it a try, it can't harm you