Hyponatraemia is the most commonly encountered electrolyte abnormality in clinical practice. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) accounts for nearly 60% of all hyponatraemias. Selective serotonin reuptake inhibitors (SSRIs) are well known to have side effects of SIADH. There have been few reported cases of serotonin norepinephrine reuptake inhibitors (SNRIs) causing SIADH-induced hyponatraemia. Duloxetine is one type of SNRI used to treat several conditions, including depression and diabetic neuropathy. We present a case of a 76-year-old woman with a history of fibromyalgia who had recently been prescribed duloxetine for her condition. On admission to the hospital, her sodium decreased to a low of 118 mmol/L. Evaluation for other causes of hyponatraemia yielded negative results. Duloxetine was discontinued and after 3 days the patient's sodium increased to 130 mmol/L. The purpose of this case report is to highlight the importance of having suspicion for rare but real side effects of medications such as duloxetine.
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