Lithium is the mainstay treatment for patients with bipolar disorder, but generally causes nephrogenic diabetes insipidus (NDI), a disorder in which the renal urine concentrating ability has become vasopressin-insensitive. Li-NDI is caused by lithium uptake by collecting duct principal cells and downregulation of AQP2 water channels, which are essential for water uptake from pro-urine. Recently, we found that the prophylactic administration of acetazolamide to mice effectively attenuated Li-NDI. To evaluate whether acetazolamide might benefit lithium-treated patients, we administered acetazolamide to mice with established Li-NDI and six patients with a lithium-induced urinary concentrating defect. In mice, acetazolamide partially reversed lithium-induced polyuria and increased urine osmolality, which, however, did not coincide with increased AQP2 abundances. In patients, acetazolamide led to the withdrawal of two patients from the study due to side effects. In the four remaining patients acetazolamide did not lead to clinically relevant changes in maximal urine osmolality. Urine output was also not affected, although none of these patients demonstrated overt lithium-induced polyuria. In three out of four patients, acetazolamide treatment increased serum creatinine levels, indicating a decreased GFR. Strikingly, these three patients also showed a decrease in systemic blood pressure. Altogether, our data reveal that acetazolamide does not improve the urinary concentrating defect caused by lithium, but lowers the GFR, likely explaining the reduced urine output in our mice and in a recently reported patient with lithium-induced polyuria. The reduced GFR in patients prone to CKD development, however, warrants against application of acetazolamide in Li-NDI patients without long-term (pre)clinical studies.