One known adverse event associated with dimethyl fumarate (DMF) is grade III lymphopenia which usually resolves within 2–3 months upon DMF discontinuation. Here, we report a case of a 50-year-old woman with MS who developed grade III lymphopenia within 6 months of DMF initiation, and despite treatment cessation within the next 6 months, she has continued to have severe persistent lymphopenia for over 5 years. Our observation suggests prolonged and possibly irreversible lymphopenia as a possible adverse event of DMF, and it emphasizes the need for monitoring lymphocyte numbers, and to cease dosing promptly after onset of grade III lymphopenia.
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