In those days when I had IVIG 2 days per month I sometimes got the headaches up to two weeks later. During the 5, or so, months I had IVIG 3 days every 3 weeks I sometimes got the headache up to 2.5 weeks later. Sometimes the headache, while low level, lasts for a week.
Any proof it’s the IVIG? Nope, how could I prove that?
In the years when I didn’t get IVIG I usually didn’t get headaches.
When I drink enough water during and between IVIG treatment events and, I mean every day, day after day, I don’t get headaches.