Severe hyponatremia associated with sildenafil (Viagra)

Dear Editor,

Until now, hyponatremia was an unknown endocrinological side‐effect of sildenafil (Viagra; Pfizer, New York, New York, United States). We report the case of a 69‐year‐old man who took one sildenafil tablet (50 mg) every day over a period of several months. He had not taken any other drug, and had no other history of disease. One day, after taking his medication, he suddenly developed cramps and lost consciousness at his home. Although he was hemodynamically stable, he drifted in and out of consciousness (Glasgow Coma Scale [GCS] score of 9 (E2V2M5)) and was confused; he did not experience cramps again. His blood glucose, serum triglyceride, and total protein levels were normal. Neurological examination revealed that both pupils were normal. Chest radiography and electrocardiography showed no abnormalities. Brain computed tomography and magnetic resonance imaging showed mild atrophy only. The typical spike‐and‐wave pattern was not observed on electroencephalography. Renal and thyroid functions were normal. Whole‐body computed tomography scanning did not indicate the presence of tumors or infections. Serum and urinary sodium levels were 113 mEq/L (135–150 mEq/L) and 176 mEq/L, and serum and urinary osmolalities were 237 mOsm/kg (260–290 mOsm/kg) and 490 mOsm/kg (40–1,500 mOsm/kg), respectively. Cortisol and antidiuretic hormone levels were 18.4 μg/dL (4.0–19.3 μg/dL) and 2.2 pg/mL (0.3–4.2 pg/mL), respectively. With these data, we speculated that the mechanism of hyponatremia would be caused by the syndrome of inappropriate secretion of antidiuretic hormone, according to the criteria of Bartter and Schwartz.1, 2 Saline was administered from the time of admission, and the patient's serum sodium level was 139 mEq/L after 4 days of hospitalization. He was responsive with a Glasgow Coma Scale score of 15 (E4V5M6), and was discharged after 8 admission days. Sildenafil is suspected to have caused the severe hyponatremia in this case.

This case raises two important points. First, we must consider the possibility of severe hyponatremia occurring in patients taking sildenafil. Convulsions and confusion are important symptoms of hyponatremia. Four cases of epileptic seizures and one case of hyponatremia that were encountered during the clinical trials of sildenafil were reported to Pfizer, the manufacturer of the drug. Gilad et al. reported two cases of epileptic seizures in patients taking sildenafil.3 However, hyponatremia as a side‐effect of sildenafil has not been reported. Moreover, a good correlation between hyponatremia and the effects of the drug has not been found.

Conflict of Interest

None.