Introduction 
Lithium remains the drug of choice for the treatment of bipolar disorder1 and is identified as the first-line agent for this disease in many guidelines 2. However, due to its narrow therapeutic index, small changes in the serum concentration of lithium can cause toxicity or render the treatment ineffective3. The major symptoms of lithium poisoning include tremors, hyperreflexia, gait disturbances, kidney damage, and reduced self-awareness 4; it can sometimes lead to death of the affected individual. Lithium intoxication has three different courses (acute, acute-chronic, and chronic) 5. Among these, the chronic course is the most common etiology, usually resulting from an unintentional excess of lithium intake over excretion6. Chronic lithium poisoning also presents a higher risk of serious complications than acute poisoning 5. A retrospective study on the neurotoxicity associated to lithium intake revealed that chronic poisoning mainly occurs in older patients with chronically elevated lithium levels, especially in those with acute kidney injury, and that prolonged delirium can lead to multiple physical complications 7. In addition, it was found that patients affected by chronic lithium poisoning had a median lithium concentration in serum of 2.2 mmol/L (interquartile range: 1.0–7.1 mmol/L) at presentation and received a median daily dose of 950 mg/day (interquartile range: 250–1350 mg/day). This suggests that even small doses of lithium can lead to poisoning. Here, we report a case of schizoaffective disorder in a patient with cancer at the progressive stage that was being treated with lithium at a low dose of (300 mg/day). In this particular case, the clinical symptoms were masked because of the patient’s poor physical status resulting from the progressive stage of cancer disease.