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Teriflunomide and its prodrug, leflunomide, are disease-modifying medications used to treat relapsing-remitting multiple sclerosis (RRMS) and rheumatoid arthritis (RA), respectively. Peripheral neuropathy is a rare side effect associated with both medications, although the incidence rate and exact pathological mechanism remain unknown. We present a retrospective case series of three patients with RRMS, who developed painful small fiber neuropathy at various timeframes (<6 >months, one year, and four years, respectively) while on teriflunomide treatment (14 mg/day); we also engage in a literature review of small and large fiber neuropathy associated with teriflunomide and leflunomide use. All three patients developed small fiber neuropathy following teriflunomide exposure. Laboratory workup was negative for metabolic, infectious, vitamin deficiency-related, and autoimmune etiologies, except for one patient who had chronic metabolic syndromes (impaired glucose, hyperlipidemia) before medication intake. However, the patient developed neuropathy following teriflunomide treatment. Electrophysiological findings were negative for large fiber neuropathy in all three patients with positive skin biopsy, with reduced epidermal nerve fiber density (ENFD) in two of the three patients. Teriflunomide was discontinued in all cases, after which symptoms stabilized. Current literature on leflunomide supports a direct neurotoxic effect or buildup of toxic intermediates from uridine synthesis inhibition. Cessation of teriflunomide use in the described cases resulted in symptom stabilization. Early recognition and treatment may lead to good clinical outcomes in these patients.

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