Hyponatremia is a common electrolyte disturbance associated with the use of medication and can have serious consequences. The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the cause in the majority of cases. This article describes a 53-year-old patient with the following complaints: restlessness, urinary complaints, increased urgency, thirst and confusion. Hyponatremia was diagnosed. A restrictive fluid policy in combination with reduction of the causative medication was initiated, as a result of which the patient made a full recovery. This case illustrates the potential of SIADH on quetiapine. Early recognition of this syndrome is vital for timely initiation of correct treatment, which can prevent further and more permanent consequences.