Used primarily to treat urinary tract infections caused by susceptible strains of Gram-negative bacteria such as Escherichia coli, Klebsiella, and Proteus species. It is effective in treating both acute and chronic urinary tract infections due to its ability to concentrate in urine. Nalidixic acid was one of the first quinolone antibiotics and is often used in research settings to study antibiotic resistance mechanisms. It also serves as a selective agent in microbiological media to isolate and identify pathogenic bacteria by inhibiting the growth of non-resistant organisms. Though largely replaced by newer fluoroquinolones in clinical use, it remains valuable in laboratory applications and in regions where cost-effective treatments are needed.