Pain in the side, also known as flank pain, has many causes, including injury to the muscles in the side and back, or dysfunctions of nearby organs.
Some causes of flank pain, such as a kidney infection or pancreatitis, are very serious. Others, including muscle tension or stiffness, can be painful but are mostly harmless.
In this article, learn about the causes of flank pain and the treatment options.
Six possible causes of flank pain include:
1. Muscle problems
The muscles of the stomach, the back, and even the chest may cause flank pain. Some common causes of muscle pain include:
- muscle injuries, such as strains or sprains
- a sedentary lifestyle, meaning that a person does not move enough
- remaining in an unusual or physically stressful position for a long time
The flank pain is usually on the side of the injured muscle. However, people who have muscle pain due to a sedentary lifestyle may experience pain on both sides or pain that moves back and forth between sides.
Muscle injuries are among the most common causes of flank pain. Muscle pain can be very intense, but the intensity of the pain is not necessarily a measure of the severity of the injury.
2. Urinary tract infection
Urinary tract infections (UTIs) develop when harmful bacteria enter the urinary tract, infecting the bladder, urethra, or kidneys. Most UTIs stay in the lower urinary tract. These infections are more common in women than in men.
Without treatment, a UTI can spread to the kidneys and cause a more severe kidney infection.
In addition to flank pain, symptoms of a UTI include:
- painful urination
- frequent or constant need to urinate
- blood in the urine
Pain from a UTI can affect both flanks or just one.
3. Kidney problems
The kidneys act as the body’s filter. They sit in the mid-back, just under the ribs. People may feel pain from the kidneys in their back or flanks.
Some kidney problems develop when an infection spreads from the bladder. Kidney disease can also be genetic or occur as a result of a problem with another organ.
Kidney problems that may cause flank pain include:
- kidney stones
- kidney diseases, such as polycystic kidney disease
- a blood clot or bleeding in the kidneys
Anyone who has flank pain and other symptoms of kidney problems, such as urinary symptoms, should see a doctor.
Kidney pain usually only appears on the same side of the body as the affected kidney. When an infection or disease affects both kidneys, a person may experience pain on both sides.
Shingles is an infection that causes a painful, blistering rash. Anyone who has had chickenpox can develop shingles, though this infection is more common in older people and people with a weakened immune system.
Shingles usually begins as a deep burning feeling or prickly pain from the nervous system. Over several days, a rash of fluid-filled blisters appears.
In some people, shingles can be very severe and cause life-threatening infections. People living with HIV, those taking immunosuppressants, and older people should see a doctor immediately if they think that they have shingles.
As shingles usually affects just one side of the body, people will notice symptoms in either the left or right flank but not both.
5. Pancreas or liver problems
The pancreas and liver sit deep within the abdomen, just under the rib cage. Sometimes, problems with these organs cause flank pain. The pain can also radiate to the back.
As the liver and pancreas work together to help the body digest food, a problem with one organ may eventually affect the other.
Many different health issues can affect the pancreas and liver. Some of these, such as blocked bile ducts due to gallstones, are highly treatable. Others, including autoimmune hepatitis, are chronic diseases.
Some liver and pancreas conditions become life-threatening if a person does not get treatment, but they always present with other symptoms in addition to flank pain.
Pain from problems with the pancreas or liver occurs on the right side of the body. Other symptoms include:
- dark urine
- pale bowel movements
- nausea or vomiting
- sudden attacks of pain
- yellow eyes or skin
6. Spinal health issues
Spinal health issues, including spinal arthritis or herniated discs, may cause pain that radiates to the flank. Spinal arthritis is a type of chronic inflammation of the vertebrae.
Disc diseases cause swelling and pain in the discs that cushion the bones of the spine. Some other conditions, such as a fracture in the spine, may also cause flank pain.
The pain from spinal problems typically occurs on either the right or left flank, but it is possible for both sides to be painful.
People with spinal health issues may also have:
- sharp, shooting pain that extends down one side and into the leg
- unexplained numbness or tingling, especially in the leg or foot
- chronic back pain
- difficulty moving
A doctor can usually narrow down potential causes of flank pain by asking about a person’s other symptoms and taking a complete medical history. They may also order tests to help make a diagnosis.
Some common tests include:
- imaging scans to look at the kidneys, liver, pancreas, and spine
- urine analysis to check for signs of a UTI
- a physical examination to identify muscle issues or rashes
- blood tests
Treatment for flank pain depends on the cause. Treatment options may include:
- antibiotics for infections, including kidney infections, UTIs, or pancreatitis from an infection
- treatment to dissolve kidney stones
- removal of the gallbladder or medication to dissolve gallstones
- antiviral drugs for shingles
- liver disease medications
- liver transplant for severe liver disease
- kidney transplant for some forms of kidney disease
- dialysis for kidney disease, especially while awaiting a kidney transplant
- stretching, physical therapy, ice, compression, and rest for minor muscle injuries
- hospitalization for monitoring and IV fluids, especially for pancreatitis, gallstones, kidney disease, and liver disease
- medication for arthritis
- surgery or physical therapy for disc diseases
In some cases, a person might not need treatment. Small kidney stones often pass without treatment, and muscle pain from cramps and sitting too long may disappear after a short time.
A doctor may also recommend lifestyle changes, especially for liver disease that is due to an unhealthful diet or for muscle pain resulting from sedentary habits.
People should go to the emergency room or seek urgent care for flank pain that occurs with:
- fever, pale stool, or yellow eyes and skin
- intense vomiting
- blood or urine
- flank pain following an injury, especially a blow to the side or back
People may wish to see a doctor within a day for:
- pain urinating
- intense pain that is getting worse
- a painful rash
- numbness or tingling in the legs
If the pain is mild or moderate and there are no other symptoms, flank pain is most likely to be due to a muscle strain, which means that it should resolve with rest.
Flank pain is a common symptom, and the presence of pain alone does little to indicate the underlying cause. Most causes of flank pain are treatable, but proper treatment requires an accurate medical diagnosis.
People should see a doctor for flank pain on the right or left side, especially if the pain is severe, gets steadily worse, or returns after disappearing.