Hyponatraemia is a well-known side effect of desmopressin therapy. Nocturia in elderly patients can be treated with desmopressin, which usually induces mild and reversible hyponatraemia. This case report is about two patients, in which severe hyponatriaemia, at least partly induced by desmopressin, was observed. The patients were affected by nausea, vomiting and confusion. These symptoms are caused by brain swelling, which can result in coma and death. The case reports illustrate the importance in measuring baseline P-sodium concentration during therapy and examining current medication before prescribing desmopressin.