The best antibiotic depends on urine culture, local resistance rates, allergy, pregnancy, kidney function, and whether illness is severe or complicated. A urine culture should be taken before treatment when possible.

For a nonpregnant adult with uncomplicated pyelonephritis who can take oral medicines:

For hospitalized, septic, vomiting, obstructed-stone, immunocompromised, or otherwise complicated cases, initial IV therapy may include ceftriaxone, cefepime, piperacillin-tazobactam, or a carbapenem if resistant organisms such as ESBL-producing bacteria are a concern. Treatment should then be narrowed to culture results.

Do not use nitrofurantoin or oral fosfomycin for pyelonephritis. They do not achieve dependable kidney-tissue concentrations. Medical textbooks similarly identify fluoroquinolones and TMP-SMX as useful oral choices and caution against nitrofurantoin for kidney infection.

Pregnancy: suspected pyelonephritis generally requires hospital assessment and IV antibiotics, often a second- or third-generation cephalosporin, then culture-directed therapy. Do not self-treat in pregnancy.

Seek urgent care today or emergency care if there is fever with flank/back pain plus persistent vomiting, confusion, faintness, low urine output, pregnancy, diabetes/immunosuppression, known kidney stones, or no improvement within 48 hours. An infected obstructed kidney is an emergency.

A practical review from the American Academy of Family Physicians supports fluoroquinolone-based outpatient therapy in selected uncomplicated cases and IV treatment for patients needing hospitalization. Recent systematic reviews also compare oral therapy with an initial IV dose followed by oral therapy, including PMID 41222820 and PMID 41443283.