Used primarily to treat urinary tract infections caused by susceptible Gram-negative bacteria, especially Escherichia coli. It is particularly effective in uncomplicated UTIs in women and children. Mecillinam works by interfering with bacterial cell wall synthesis at the penicillin-binding protein 2 site, leading to rapid bactericidal activity. Due to its unique mechanism and narrow spectrum, it helps preserve normal flora and reduces the risk of broad-spectrum antibiotic resistance. Often prescribed in oral form, it achieves high concentrations in urine, making it ideal for lower urinary tract infections. In some regions, it is used in combination with other antibiotics for enhanced coverage in complicated cases.