Reviewed by The Wellness Team. Last reviewed October 2026.
A kidney infection is a urinary tract infection that has travelled up to one or both kidneys. It responds well to antibiotics, but it is not something to sit on: treatment should start promptly, ideally the same day. Here is how to recognise it, what treatment involves and when it becomes urgent.
What is a kidney infection
Doctors call it acute pyelonephritis. Bacteria, most often E. coli from the bowel, enter the bladder first and then ascend the ureters to a kidney. It is more common in women, in pregnancy, and in anyone with a urinary catheter, kidney stones or a weakened immune system. A simple bladder infection (cystitis) stays in the bladder; a kidney infection means the infection has gone further, which is why it makes you properly unwell.
Kidney infection symptoms
Pain in your side or lower back, often on one side, just under the ribs
Fever, usually 38C or above, often with shivering and chills
Feeling sick or being sick
General exhaustion and aching, out of proportion to a cold
Urinary symptoms: needing to go often, stinging or burning, cloudy or strong smelling urine
Occasionally blood in the urine
Older adults may not get a fever at all; new confusion or a sudden dip in mobility can be the main sign. In children, look for a high fever with vomiting, poor feeding, or new bedwetting.
Kidney infection or a simple urine infection
Cystitis causes stinging, urgency and frequent urination but no fever, no back pain and no vomiting. Once a urine infection causes fever, flank pain or vomiting, treat it as a kidney infection until a clinician says otherwise. That distinction decides the treatment: kidney infections need an antibiotic that reaches the kidney tissue, which rules out some of the antibiotics used for simple cystitis.
How a kidney infection is treated
NICE guidance recommends prompt antibiotics for acute pyelonephritis, typically a 7 to 10 day course of an antibiotic that penetrates kidney tissue, chosen against local resistance patterns and your history. Paracetamol helps the fever and pain, and fluids matter. Most people start to improve within 48 hours of the right antibiotic; if you are not improving by then, you need reviewing again.
Some people need hospital assessment instead of tablets at home: anyone who cannot keep fluids down, who is pregnant, who has one kidney or a transplant, or who shows signs of sepsis. A urine sample is usually sent to the lab to confirm which antibiotic the bacteria respond to, especially if the first choice is not working.
When a kidney infection is urgent
Seek urgent medical help the same day, or emergency care, if any of the following apply:
Shivering attacks, a racing heart, confusion, or feeling faint
Vomiting that means you cannot keep fluids or tablets down
Severe pain in your side that is getting worse
You are pregnant, have one kidney, a kidney transplant, or a weakened immune system
Symptoms are improving and then suddenly worsen
Untreated, a kidney infection can lead to sepsis or permanent kidney scarring. These are the outcomes prompt treatment exists to prevent.
When to see a GP today
Any fever with back or side pain and urinary symptoms warrants a same day assessment. Waiting days to see whether it settles risks the complications above, and early treatment shortens the illness substantially. If you have had a urine infection in the last few days that has now changed character, treat that the same way.
What an online GP can do today
A GP can assess your symptoms by video or phone, confirm whether this looks like a kidney infection or simple cystitis, and prescribe the appropriate antibiotic the same day where clinically appropriate, sent electronically to your chosen pharmacy. Where a urine culture, blood tests or imaging are needed, the GP will arrange them and explain the plan, and if the picture suggests you need hospital care, you will be told directly and quickly.
Appointments cost £59 by phone, £150 by video and £220 in person at 10 Portman Square, Marylebone, with the price confirmed before you book. Same day appointments are usually available. Message us on WhatsApp: https://wa.me/447961280835
Frequently asked questions
Can a kidney infection go away on its own?
It is not safe to assume so. Unlike simple cystitis, which sometimes settles without antibiotics, a kidney infection can progress to sepsis or kidney damage. NICE guidance recommends prompt antibiotic treatment, so same day assessment is the right move.
How long does a kidney infection last?
With the right antibiotic, most people feel noticeably better within 48 hours, though the full course is 7 to 10 days and complete recovery can take up to two weeks. Not improving within 48 hours of treatment means you should be reviewed again.
Is a kidney infection contagious?
No. You cannot catch a kidney infection from someone else. The bacteria that cause it usually come from your own bowel.
Can a kidney infection come back?
It can, particularly if there is an underlying reason such as kidney stones, an incomplete course of antibiotics, or bacteria resistant to the antibiotic used. Recurrent infections are a reason for urine cultures, blood tests and sometimes a scan to look for a cause.
Sources: NICE Clinical Knowledge Summaries, Urinary tract infection (lower) in adults; NICE guideline NG111, Pyelonephritis (acute): antimicrobial prescribing.
Reviewed by The Wellness Team