Lithium salts are a commonly used mood stabiliser with a narrow therapeutic index. Changes in dosage and long-term treatment bring the risk of acute and chronic complications from the cardiovascular, gastrointestinal, urinary and endocrine systems. One of the most frequently occurring complication is hypercalcaemia, which can be an independent cause of systemic signs. The article analyses the case of a 66-year-old long-term lithium user with bipolar affective disorder, who presented with neuropsychiatric symptoms and accompanying markers of acute kidney injury. In the course of diagnosis, hypercalcaemia and lithium overdose were identified. In addition, the patient presented with multiple disorders that might have resulted from acute lithium intoxication as well as chronic toxicity, including: hypothyroidism, bradycardia, and parkinsonian syndrome. After lithium withdrawal and conservative treatment of hypercalcaemia and acute kidney injury, significant clinical improvement was achieved, while renal parameters and serum calcium levels did not return to normal until six months after hospital discharge. The paper aims at outlining the profile of possible adverse effects of lithium treatment, which create a complex clinical picture. It presents diagnostic and therapeutic management in hypercalcaemia and lithium intoxication, as well as methods of preventing their occurrence.