This is a randomized, placebo-controlled, double blinded phase 2 exploratory clinical trial of intravenously administered pooled human immunoglobulin (IVIG) in anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) immune mediated necrotizing myopathy (IMNM). Planned enrollment is 12 individuals with active anti-HMGCR IMNM meeting inclusion and exclusion criteria. Assuming 20% drop-out, the investigators anticipate 10 participants will complete all study assessments. Enrolled participants will be randomized 1:1 to either IVIG 2g/kg or placebo (0.9% sodium chloride at equivalent volume) at weeks 0, 4, and 8. The primary efficacy and co-primary safety and tolerability endpoints will be assessed at week 12. After the randomized phase of the trial, all participants will be offered to continue on to an open-label extension phase in which participants will receive IVIG at weeks 12, 16, and 20. Participants will then return at week 24 for a final non-infusion visit to reassess safety, tolerability, and efficacy outcome.