BackgroundDue to the recurrence of disease activity in multiple sclerosis (MS) patients, a washout period of <3 months has been suggested for the transition from natalizumab (NTZ) to fingolimod (FTY). However, very short transition periods of <1 month may be more beneficial. MethodsRetrospective analysis of patients from the Rocky Mountain MS Center at the University of Colorado who were: a) on NTZ for ≥6 months prior to switching to FTY; b) had a transition period ≤ 6 months; and c) initiated FTY treatment prior to November 2013. Transition periods were grouped as follows: <1 month, 1–2 months, and 3–6 months. Outcomes assessed include clinical and MRI measures within one year of FTY initiation. ResultsThirty-seven, 56 and 24 patients had a transition period < 1 month, 1–2 months and 3–6 months, respectively. Baseline characteristics were well matched: mean age 45–49 years (p = 0.17), disease duration 11–13 years (p = 0.42), and ~70% women (p = 1.00). Following the switch (including transition period), clinical relapses were observed in 0% (<1 month), 12.5% (1–2 months), 37.5% (3–6 month) (p < 0.001) of patients. New gadolinium enhancing lesions occurred in 3.3% (<1 month), 13% (1–2 months), 21.4% (3–6 months) (p = 0.13) patients. New T2 lesions were observed in 11.1% (<1 month), 16.3% (1–2 months), 33.3% (3–6 months) (p = 0.28) of patients. There were no unexpected adverse events or PML observed. ConclusionsMinimizing transition times from NTZ to FTY was beneficial and safe.