Flank pain — pain on one side of your body, at waist or abdomen height — can be acutely debilitating. Are you dealing with a medical emergency that requires immediate treatment, however, or can you just ignore the pain? While it is, as always, impossible to diagnose you over the internet, reading about the possible causes of pain in the side should at least help you realize that flank pain can be caused by serious, and sometimes even potentially fatal, health conditions.
Pain In The Side At A Glance: A Quick Overview Of Flank Pain Causes
The most common and likely causes of flank pain are :
- Urinary tract problems. These can include ureteral obstruction caused by a blood clot or kidney stone, a history of surgery in the relevant area, and kidney inflammation (pyelonephritis). A kidney tumor, vesicoureteral reflux, or kidney infarction can also be to blame.
- Traumatic injury — sustained during an accident or fight, for instance — is another possible cause of flank pain.
- Other causes include ectopic pregnancy, appendicitis, gallbladder disorders, and gastrointestinal disorders.
The nature of the pain you are experiencing is among the most important clues that can reveal what's wrong with you. Your pain may be acute or chronic, sharp or dull, and it may stick to one location or radiate outward. Those people who are trying to rule causes of their flank pain in or out should pay special attention to the nature of pain associated with particular conditions, which we'll get to below.
Flank pain is very frequently caused by urinary tract disorders, and a ureteral obstruction is one of the main culprits. The ureters are the ducts or tubes that transport urine from the kidneys to the bladder . Though you won't consciously realize you even have ureters under normal circumstances, their walls are rather rich in nerve endings. That means that, if something obstructs the ureters that causes them to contract violently in an effort to move things along, you will experience a very sharp, intense pain on the affected side, which may be left or right.
Kidney stones are the most common cause of a ureteral obstruction. They typically get "stuck" at one of three points — the renal pelvis, while crossing over the iliac artery, and at the entrance to the bladder wall.  A blood clot can produce similar symptoms. These can form as the result of a kidney biopsy or in people with rare blood disorders such as Glanzmann thrombasthenia .
People with a ureteral obstruction may feel:
- A very intense, overwhelming, pain.
- While its source is unilateral, coming from one side (which you may or may not be able to identify), this pain can radiate to the whole abdomen. The pain may radiate downward towards your scrotum or labia, because it crosses three nerves — the iliohypogastric, ilioinguinal, and genitofemoral nerves.
Your pain will usually be so bad that you're really not in any doubt as to whether you need medical attention. Analgesics and spasmolytic medications will often be enough to help patients manage their pain, though additional measures will be required if you have bigger kidney stones.
Other Causes Of Flank Pain Originating In The Urinary Tract
A kidney inflammation, medically called pyelonephritis, is usually caused by viral or bacterial infections that have made their way up from the lower parts of the urinary tract. It can also be caused by systemic infections. In this case, you will feel pain in your upper flank that is likely to be dull and continuous. Appropriate antibiotics are required to clear up bacterial infections. 
Kidney tumors, too, can result in flank pain. In this case, the pain will appear so gradually that you'll hardly notice, and you are unlikely to have any other obvious symptoms. The possibility of a kidney tumor makes it quite clear why even people with barely-there pain present over a longer period of time should seek medical attention. 
A kidney infarction is another cause of flank pain. This condition involves the obstruction of the renal artery, which supplies the kidneys with blood. You may notice severe pain that radiates to your back, as well as diarrhea and vomiting. Immediate medical attention is required. 
In some cases, rare congenital conditions like vesicoureteral reflux, in which urine flows backward and puts pressure on the kidneys, or even simply a history of surgery in the area, can also be responsible for urinary-tract related flank pain. These conditions usually affect only one kidney, so the pain is unilateral and can occur on either side of the body.
Extra-Urinary Causes Of Flank Pain
The gallbladder, a small organ where bile is stored, can be affected by gallstones, tumors, or inflammatory processes. As a result, you can experience pain in the upper flank, typically on the right side. Indigestion, nausea, and bloating are also possible. 
Appendicitis, an inflammation of the appendix, is really quite tricky to diagnose as the length and location of the appendix vary greatly. This makes symptoms variable from person to person. In most cases, the pain initially appears around the navel region, and then moves to the lower right portion of the abdomen, but it can really be spread across the entire front abdominal wall. While flank pain is not indicative of appendicitis in itself, this disorder still needs to be ruled out if you suffer from pain in your side. 
Ectopic pregnancy, in which an embryo implants itself outside the uterus and typically in one of the fallopian tubes, can likewise lead to a very sharp pain in one side. The pain may be accompanied by vaginal bleeding, nausea, dizziness, and shoulder pain. Women experiencing an ectopic pregnancy will have missed a period, but they may have interpreted vaginal bleeding as being their menstrual period. This is a life-threatening condition if the fallopian tube in question ruptures, and needs immediate treatment. 
In addition, any gastrointestinal tract disease can lead to flank pain, unsurprisingly along with any injury. In the latter case, you'll already be aware of the cause, but you should see your doctor to have the extent of the damage assessed and to get the treatment you need.