Fingolimod has a pharmacologic half-life of 6–9 days7 and lymphocytes would be expected to normalize 2–4 weeks after discontinuation.3 However, there are case reports of prolonged lymphopenia following prolonged drug exposure, up to 37 months after discontinuation.8 Following discontinuation, there is also a risk of rebound disease activity 2–4 months from the time of withdrawal.3 In animal models, rebound is preceded by a burst of S1P1 overexpression in trapped lymphocytes, which also correlates with subsequent massive lymphocyte egress and CNS infiltration. Here, S1PR1 is linked to lymphopenia.