Medically reviewed guide · Last updated: 23 June 2026 · Reading time: approx. 10 min

Flank pain: causes, how to tell them apart and when to see a doctor

A pulling, stabbing or cramping in the side can be harmless or an important warning sign. Many people are unsure at first, because flank pain is easily confused with back pain and it is not immediately clear whether an organ or the muscles are affected. Flank pain is not a disease of its own but a symptom, behind which very different causes can lie, from the kidney through the urinary tract to the muscles. That is exactly what makes the classification tricky. This guide helps you tell the three most common directions apart, namely renal colic, kidney infection and muscular complaints, shows the most important warning signs and explains when you should not wait but seek medical advice. This way you can judge your risk better and go into a conversation at an appointment more purposefully.

Where is the flank, and does the pain come from the kidney?

The flanks lie on both sides of the trunk, between the last rib and the hip, roughly where the hands rest in the side when propping up. The kidneys lie in this area, but many muscles, nerves and vessels also run here, and neighbouring organs can radiate to this spot. This very variety explains why flank pain can feel so different, from dull and dragging to sharp and cramping. A helpful first distinction is the question of whether the pain depends on movement. If it changes clearly when you turn, bend, cough or press on the spot, that points more to a muscular cause. If, on the other hand, it persists independently of movement and is joined by fever, nausea or blood in the urine, the kidney comes to the fore. A reliable distinction is not possible for laypeople, however, which is why unclear or strong symptoms should be medically assessed. The rule of thumb is a first clue, not a diagnosis, because a kidney condition can sometimes feel different from the textbook too.

Colic, infection or muscular?

The overview below compares the three most common directions. It does not replace a diagnosis but helps you classify your symptoms better and prepare the conversation at the practice. Pay attention above all to the pain character, the accompanying signs and the question of whether the pain changes with movement. In reality the pictures sometimes mix, and not every case fits neatly into one column, which is why the medical judgement always stands at the end.

The three most common directions in detail

Behind flank pain most often lie the kidney, an infection or the muscles. The following sections go into more detail on how you recognise each direction and what it means for treatment.

Renal colic from stones

When a stone comes loose from the kidney and moves into the ureter, it can wedge and block it. The body tries to push the stone along with strong muscle movements, which triggers the typical, very strong renal colic. Such a pain surge often lasts twenty to sixty minutes and comes in waves, frequently with nausea, vomiting and blood in the urine. Many of those affected describe renal colic as one of the strongest pains they have ever experienced. It is characteristic that those affected are restless and find no position that brings relief. As soon as the stone reaches the bladder, the pain often eases abruptly. Small stones often pass on their own, larger ones sometimes have to be broken up with shock waves or removed. Read more in our article on kidney stones.

Kidney pelvis infection

When a bladder infection ascends through the ureters to the kidney, a kidney pelvis infection can arise. It causes a rather dull, constant flank pain, usually one-sided, together with fever, chills and a marked feeling of illness. Often painful urination and a frequent urge come first, because the infection started in the bladder. A clear tapping pain is also typical when the practice taps on the kidney area from the side. This form is urgent, because untreated it risks blood poisoning. It is usually treated quickly with an antibiotic, accompanied by physical rest and plenty of fluid. Pregnant women, older people and those with a weakened immune system in particular should not hesitate here. How a bladder infection arises and can be treated is described in our article on the urinary tract infection.

Muscular and orthopaedic causes

Very often flank pain is simply muscular, for example from tension in the lower back muscles, one-sided strain, prolonged sitting or poor posture. The spine, too, for example a slipped disc or degenerative changes, can radiate into the flank. An irritated nerve root then sends the pain along its course into the side, which makes the assignment to the kidney harder. It is typical that this pain changes with movement, pressure or certain postures and that no fever or urinary symptoms occur. Often a trigger can also be named, such as heavy lifting, an unaccustomed movement or prolonged sitting at a desk. This is the most common harmless cause, but with persistent symptoms it should still be assessed.

Acute or longer-standing? What the course reveals

The timing also helps with classification. Sudden, very strong pain that comes in waves points to colic. Acute, persistent pain with fever points more to an infection. Symptoms that come and go over weeks and change with posture and strain, on the other hand, are often muscular or from the spine. A harmless stitch in the side after exercise usually disappears on its own after a short rest and is no cause for concern. Persistent or recurring flank pain should be medically assessed, even if it is not very strong, because it can be a sign of a treatable underlying condition. It is best to note when the pain occurs, how long it lasts and what improves or worsens it, as this helps with classification.

Other causes and the question of left or right

It is not always the kidney or the muscle behind it. The bowel, for example with bloating or diverticulitis, the gallbladder with gallstones, the pancreas, the spleen or shingles can also cause flank pain. With gallstones, the colicky pain typically occurs on the right, often after a fatty meal, while pancreas complaints tend to radiate band-like into the back. The side gives first clues here: on the left lie, among others, the left kidney, the spleen and parts of the bowel, on the right the right kidney, the gallbladder and the appendix. That is why right-sided flank pain can also come from the gallbladder, for example, and left-sided pain from the bowel. Pain on both sides can point to an inflammation of both kidneys. This assignment is not conclusive, however, and does not replace an examination. More rarely, shingles, which often shows with a burning, band-shaped pain and later a skin rash, or nerve irritation from the spine are behind it.

Warning signs: see a doctor now

If the flank pain occurs together with blood in the urine, you should always have this clarified by a doctor. What can be behind it and why painless blood in particular should be taken seriously is covered in our article on blood in the urine. Even if the bleeding occurs only once and the pain is mild, the combination of flank pain and blood in the urine is a clear signal to make an appointment.

What you can do with muscular complaints

If serious causes have been ruled out and the complaints appear muscular, you can do quite a bit yourself. Warmth in the form of a hot water bottle, a cherry stone cushion or a warm bath relaxes the muscles and can also release cramping tension. Take it easy without slipping fully into a protective posture, because gentle movement is good for the muscles and prevents new tension. If needed, an anti-inflammatory painkiller helps. In the long term, strong core muscles, ergonomic sitting and avoiding one-sided strain prevent the symptoms from returning. If the pain does not improve or warning signs appear, this belongs in medical hands. It is also important not to slip into a permanent protective posture, because this can tense the muscles further.

What the doctor does

The assessment begins with a conversation about the medical history and a physical examination in which, among other things, the kidney beds are tapped and the abdomen and back are palpated. A urine test shows whether blood or signs of an infection are present. The ultrasound is usually the first imaging tool and makes stones, urine congestion or signs of a kidney pelvis infection visible. If that is not enough, for example with very small ureteral stones, a computed tomography often follows. The treatment then depends on the cause. With colic, antispasmodic and pain-relieving remedies are often in the foreground, with an infection the antibiotic, and with muscular complaints warmth, movement and physiotherapy.

Prevention: getting ahead of stones and infections

Depending on the cause, flank pain can be prevented in different ways. Anyone prone to kidney stones can counteract them with plenty of fluid, as long as no heart or kidney failure speaks against it, because a good drinking amount helps to flush out small crystals before they become stones. In some cases the medical practice also recommends an adjustment of the diet, such as less salt or less animal protein. A bladder infection should be treated and cured consistently so that it does not ascend into a kidney pelvis infection. Against muscular complaints, regular movement, strong core muscles, ergonomic sitting and avoiding one-sided strain help in the long term. Warm clothing in the cold season can also prevent muscular tension in the flank area. Anyone who has already had renal colic or a kidney pelvis infection should discuss the respective preventive measures with their medical practice, as the risk of recurrence can be increased.

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The bottom line: flank pain ranges from a harmless muscle twinge to an urgent emergency. The rule of thumb about movement-dependence and a look at accompanying signs such as fever or blood in the urine help with the first classification. Anyone who observes and describes their symptoms well makes the diagnosis easier for the practice and the path to the right treatment easier for themselves. Certainty, however, only comes from a medical assessment, and with the warning signs mentioned you should not wait. When in doubt, it is always better to have a harmless cause confirmed than to recognise a serious one too late.