Background: Hyponatremia is one of the most common electrolyte abnormalities seen in hospitalized patients. In cancer patients, it's generally related to syndrome of inappropriate ADH secretion (SIADH).Case Report: Here, we report a breast cancer patient with hyponatremia related to high dose trimethoprim-sulfamethoxazole use for pneumonitis cariini pneumonia.Conclusion: It is important to be aware that all hyponatremias are not related to SIADH in cancer patients. For proper treatment strategies, a proper differential diagnosis is needed.

Keywords:

Cancer, Hyponatremia, Trimethoprim-sulfamethoxazole