I have been getting IVIG for a year now, every 8 weeks and felt pretty unprepared - this site has been immensely helpful. I seem to react differently every time, the first was my worst because I did not know how to prepare, and when I asked at the infusion center and my Dr, they said people don't usually have a reaction. I usually have a heavy, dull headache the afternoons of 2 day treatment, diarrhea starting during the 1st day; the 2nd day may turn into migraine, vomiting, and have had times where I felt hungover for several days. It can also be fine one day and fall back to the hangover (did I drop off my fluid intake. . . ?)
From this site, I have learned to greatly increase fluid intake 3-4 days before, during and after treatment. I pre-medicate with benadryl and tylenol 1/2 hour before treatment starts, and then at bed again, just in case. I sometimes will continue taking the tylenol for a few days to attempt to fend off headaches (I tend to wake up with them). I try to eat as simply and cleanly as possible, and get extra sleep. I see an Integrative Health Specialist and she has recommended a magnesium (chelate) supplement a week before and during, which does seem to help. I also have nausea bracelets that I put on before treatment begins. (trying to cover all my bases :-)
With all of this being said, it is worth EVERYTHING - I had a chronic leak for 10 years, ER visit every 12-18 months for dehydration, then a very different episode in February 2019 (just updated in my profile) which put me in ICU for almost a week. Since I started IVIG, no episodes and no need for a diuretic at all - I feel so much better, not constantly puffed out and bloated. My feeling is that learning to prep for/cope with/manage these side effects is a good way to take care of me and (maybe) feel a little more in control of my health.
Very best of luck to you -