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IMPORTANCE Environmental factors are thought to be critical in the initiation and perpetuation of multiple sclerosis disease activity. OBSERVATIONS We describe the case of a woman in her late 30s with a diagnosis of relapsing-remitting multiple sclerosis, who continued to accumulate neurological disability despite long-term natalizumab treatment. The patient continued to have visual symptoms, left leg weakness, and gait instability. In addition, she subacutely developed an encephalopathy. Our investigations revealed that the patient had a long-standing history of chewing on toilet bowl deodorizing cakes. The main ingredient in this product is 99.9% paradichlorobenzene, which is also used in mothballs. CONCLUSIONS AND RELEVANCE This case illustrates that environmental causes for neurological deterioration should be investigated in patients with multiple sclerosis who display a rapidly progressive disease course and in whom potent pharmacotherapies fail. One possible cause is the ingestion of paradichlorobenzene-containing mothballs and toilet cleaners.