Background: Lithium is the primary mood stabilizer used in bipolar disorder. Its renal, thyroid and neurological toxicities are well documented, but optic nerve involvement is rare and its mechanism uncertain. Case summary: A 37-year-old woman on long-term lithium developed progressive bilateral visual loss and headache after her dose was increased from 400 to 600 mg/day. A 12-hour trough serum lithium of 1.5 mmol/L was recorded at a general hospital, without systemic signs of toxicity. After dose reduction the level fell to 0.5 mmol/L, but three days after discharge her vision deteriorated to hand movements in both eyes, with bilateral Frisén grade 2 optic disc swelling and grossly delayed pattern-reversal visual evoked potentials (VEP). Neuroimaging including magnetic resonance venography was normal, and the infectious and autoimmune screen was negative apart from a weakly positive myelin oligodendrocyte glycoprotein antibody (MOG-IgG). Lithium was stopped and no corticosteroids were given. Tomography improved within three days and reached 6/6 and 6/7.5 by day 22, with resolution of disc swelling on optical coherence tomography (OCT). At approximately 4 months acuity was 6/6 in both eyes, with mild temporal disc pallor and reduced macular thickness. The Naranjo score was 6 (probable adverse drug reaction). Conclusion: Optic neuropathy followed a period of elevated lithium exposure and continued to progress after serum lithium had returned to the therapeutic range, improving only after lithium was withdrawn. Clinicians should evaluate visual symptoms promptly in patients taking lithium, including those whose serum levels are within the target range.