It is unexpected that patients with stable lymphocytes on one fumarate would experience significant changes when switching to another fumarate; however, this was being observed in clinical practice. We wanted to identify the frequency and severity of lymphopenia after switching from dimethyl fumarate to either diroximel fumarate (DRF) or monomethyl fumarate (MMF). Our study determined that patients switching to DRF were more likely to have decreased lymphocyte counts post switch compared to MMF.