The U.S. Food and Drug Administration has approved a new use for leucovorin calcium tablets, allowing the drug to treat cerebral folate deficiency (CFD), an extremely rare neurological condition. The medication is a synthetic version of vitamin B9 and has long been used to lessen the toxic side effects of chemotherapy or boost its effectiveness in cancer care. The drug—sometimes sold under the name Wellcovorin—was originally marketed by GlaxoSmithKline beginning in 1983, though the company later stopped selling it when generic versions became available. According to the FDA, the decision was based on “a systematic review of the published literature...