This study focuses on Complicated urinary tract infections, which are infections of the urinary system that are severe enough to require a stay in the hospital. The purpose of the study is to compare a new combination of medications to the standard treatments currently used in hospitals. One group will receive pivmecillinam taken by mouth and a single dose of gentamicin given through an intravenous line, which is a tube inserted into a vein to deliver medicine directly into the bloodstream. Another group will receive standard treatments consisting of either ampicillin combined with gentamicin, or a combination of piperacillin and tazobactam, both delivered through an intravenous line.

During the study, participants will be monitored to see how well the different antibiotic treatments work to clear the infection and reduce symptoms such as fever. The course of the study involves comparing the effectiveness of the oral and injection combination against the existing hospital methods to determine if the new approach is just as effective in achieving a clinical cure.