Selective serotonin reuptake inhibitor (SSRI) antidepressants are gaining popularity for treating depression. Increasing reports of hyponatremia led New Zealand researchers to investigate the incidence, time course and risk factors complicating treatment with fluoxetine (Prozac) or paroxetine (Paxil). Wilkinson and colleagues found that the incidence of hyponatremia was 4.7 per 1000 people treated per year for fluoxetine and 6.3/1000 people treated for paroxetine. Both older age (70 and over) and low body weight were identified as risk factors. Given the above incidence rates, researchers concluded that routine monitoring for hyponatremia was unjustifiable. If monitoring were deemed necessary, they recommend it be done 3 to 4 weeks after initiation of treatment. They also recommend keeping a closer eye on older people with a low body weight (e.g. body mass index (BMI) < 20) who are taking SSRIs.
Source: Wilkinson TJ, Begg EJ, Winter AC, Sainsbury R. Incidence and risk factors for hyponatraemia following treatment with fluoxetine or paroxetine in elderly people. British Journal of Pharmacology 47(2):211-217.