I have suffered from leg cramps while on IVIG. One episode was so bad my wife took me to the ER. The cramps were causing my leg to contract as if in a seizure. The Physician treated me with Valium (muscle relaxer) and hydrocodone (pain). The relief was almost immediate.
Since I have learned of an "over the counter" remedy made by Hylands called Leg Cramps PM. My Pharmacist recommends taking these tablets before going to bed.
I hope this helps.