Tehran University of Medical Sciences Iranian Journal of Psychiatry 1735-4587 5 3 2010 09 15 113 116 Fereshteh Shakibaei Behavioral Sciences Research Center, Isfahan University of Medical Sciences, Isfahan, Iran Ali Gholamrezaei Poursina Hakim Research Institute, Isfahan, Iran Mahmood Alikhani Behavioral Sciences Research Center, Isfahan University of Medical Sciences, Isfahan, Iran Khalil Talaeizadeh Behavioral Sciences Research Center, Isfahan University of Medical Sciences, Isfahan, Iran 2015 10 14 Objective: Fluoxetine, a widely used antidepressant, can affect the serum sodium level. The aim of this study was to evaluate and compare changes in the serum sodium level of depressive patients taking fluoxetine according to age.    Methods: This quasi-experimental study was conducted on 126 patients with depression, dividing the patients into two age groups of 15 to 35 years, and above 55 years, who referred to psychiatric clinics of the Isfahan University of Medical Sciences. Serum sodium level was measured prior to fluoxetine therapy and at the first and third week after; adverse symptoms were recorded. Serum sodium level, hyponatremia, and other adverse effects were compared between the two age groups.  Results: There was a significant decrease in serum sodium levels in the older patients at the first and third week after the therapy; but the serum sodium levels decreased only after the third week of therapy in the younger patients (p < 0.05). Serum sodium level was significantly lower in older than in younger patients at the third week of the therapy (140.8±2.26 vs. 135.2±2.06; p < 0.05), and hyponatremia was detected only in the older patients after the first and third week of therapy, 4.7% and 15.8%, respectively. Conclusions: Using fluoxetine can decrease serum sodium level, which is more frequent, more severe, and more clinically manifested in older than younger patients. Monitoring serum sodium level is recommended in early weeks of fluoxetine therapy especially for patients older than 55 years of age.  https://ijps.tums.ac.ir/index.php/ijps/article/view/360 https://ijps.tums.ac.ir/index.php/ijps/article/download/360/356