Lithium is the gold standard for the treatment of bipolar affective disorder. However, its narrow therapeutic range and potential side effects limit its use. Here, we present a case of nephrogenic diabetes insipidus (NDI) developing in a patient on lithium therapy following the use of a weight-loss supplement containing borax pentahydrate.A 46-year-old woman with a 25-year history of bipolar affective disorder was admitted to our clinic due to a manic episode. She had been on regular lithium therapy for the past two years but had become irregular in the last two months while taking an online-purchased weight-loss supplement containing borax pentahydrate. The patient developed polyuria and polydipsia, and laboratory findings revealed hypernatremia, hyperchloremia, and a reduced glomerular filtration rate. Polyuria and polydipsia persisted for approximately one month.NDI can occur in patients using lithium and is generally reversible, although recovery may take several months or be incomplete in some cases. The supplement also contained Epsom salt, magnesium, zinc, mate leaf extract, and potassium sorbate; the combined renal effects of these substances are unknown. This case highlights the risks of uncontrolled supplements and underscores the importance of regular renal function monitoring in patients receiving lithium therapy.