Hyponatremia, defined as a serum sodium <135 mmol/l, is the most common clinical disorder of water and electrolyte balance. Hyponatremia occurs in approximately 15-20% of hospitalized patients and up to 20% of patients in critical condition. It can lead to a wide range of clinical symptoms, from mild to even life-threatening, and is associated with increased mortality and longer duration of hospitalization in patients affected by many different syndromes. The diagnosis and treatment of hyponatremia in patients’ is still a major problem. Hyponatremia is a disorder observed commonly in toxicological practice. It most often occurs in people who abuse alcohol, narcotics - mainly ecstasy, drugs, as well as the so-called water intoxication in athletes and the people who are mentally ill. In view of the complex pathomechanisms and a variety of symptoms observed in poisoned patients, hyponatremia should be considered as one of the reasons and the exponent of the general condition of the patient.