A paradigmatic case of acute combined spinal cord degeneration and delirium due to inappropriate administration of folic acid in the context of chronic cobalamin deficiency is described. Rapid improvement was obtained with immediate cessation of folate administration and parenteral cobalamin supplementation. Folic acid and cobalamin prescription rules are recalled. Pathophysiological hypotheses tentatively explaining the neurotoxicity of folic acid in case of vitamin B12 deficiency are summarized.