I have been on IVIG infusions every 3 weeks for just over a year. I haven't heard anything about reduced effect over time. Mine seemed to be more impactful after about 10 months. I have heard that sometimes Medicare will want to try to wean you off of these after you've been on for a couple of years to see if they are still needed. Also, my neurologist said the reason for 3 weeks between them is that it has a half life of 3 weeks (I haven't looked anywhere to confirm this).