Renal colic is a sudden, severe flank pain caused by a blockage in the urinary tract, most commonly a kidney stone. It’s one of the most intense types of pain people experience, often compared to childbirth, and it sends millions of people to emergency departments every year. About 13% of people will develop a kidney stone at some point in their lives, making this a remarkably common condition.
Why It Hurts So Much
The pain of renal colic isn’t caused by the stone itself scraping the walls of the ureter (the narrow tube connecting each kidney to the bladder), though that’s a common misconception. The real driver is pressure. When a stone blocks urine flow, fluid backs up into the kidney and the pressure inside the renal pelvis rises sharply. That rising pressure triggers the local release of prostaglandins, inflammatory compounds that do two things at once: they dilate blood vessels in the kidney, which increases urine production and pushes the pressure even higher, and they cause the smooth muscle of the ureter to spasm around the stone.
This is why the pain comes in intense waves. The ureter contracts rhythmically, trying to push the stone downward, and each contraction amplifies the pressure against an already stretched system. Because the kidney and intestines share nerve pathways, the swelling of the kidney also commonly triggers nausea and vomiting, which can make the experience feel even more overwhelming.
What Renal Colic Feels Like
The classic presentation starts as a sharp pain in the flank, the area between your lower ribs and hip on one side. From there it typically radiates forward and downward toward the lower abdomen, groin, or testicle. The pain tends to arrive abruptly, often waking people from sleep or striking without any warning during normal activity. Unlike most abdominal pain where lying still helps, people with renal colic are often restless, unable to find a comfortable position.
More than half of people experience nausea or vomiting alongside the pain. Blood in the urine, either visible or detectable under a microscope, appears in about 85% of cases. You might notice your urine looks pink, red, or tea-colored. Some people also feel an urgent need to urinate frequently or experience a burning sensation, especially as the stone moves closer to the bladder.
Pain location can shift as the stone travels. A stone high in the ureter usually causes flank and back pain. As it moves into the middle section, pain often wraps around to the front of the abdomen. Once it reaches the lower ureter near the bladder, pain may settle in the groin or inner thigh.
How It’s Diagnosed
A non-contrast CT scan is the gold standard for diagnosing renal colic. It picks up stones with a sensitivity and specificity approaching 95 to 100%, and it can identify the stone’s exact size and location, both of which matter for deciding on treatment. The scan also reveals whether the kidney is swollen from backed-up urine and can rule out other causes of sudden flank pain, such as an aortic aneurysm or ovarian torsion. A urine sample is typically checked for blood, signs of infection, and crystals that hint at the stone’s composition.
Will the Stone Pass on Its Own?
The single most important factor in whether a stone will pass without intervention is its size. Stones under 3 mm pass spontaneously about 98% of the time. At 4 mm, the rate drops to roughly 81%. At 5 mm, it falls to about 65%. Once a stone reaches 6 mm, only about a third will pass on their own, and stones 6.5 mm or larger have a spontaneous passage rate of just 9%.
Location matters too. Stones already in the lower third of the ureter, closer to the bladder, have a better chance of passing than stones still lodged near the kidney. Most stones that are going to pass do so within a few weeks, though the process can occasionally stretch longer.
Pain Relief: What Works Best
Anti-inflammatory pain relievers (NSAIDs) are more effective than opioids for renal colic, and this isn’t true for most other types of severe pain. A pooled analysis of six clinical trials found that patients treated with NSAIDs had greater reductions in pain scores, were less likely to need additional pain medication in the short term, and experienced significantly fewer side effects. The advantage makes biological sense: since prostaglandins are a primary driver of the pain, drugs that block prostaglandin production attack the root cause rather than just masking the sensation.
Opioids, by contrast, were associated with notably higher rates of vomiting, which is particularly unhelpful given that nausea is already a major symptom. They remain an option for people who can’t tolerate anti-inflammatories, but they’re no longer the first choice.
Medications That Help Stones Pass
For stones that are small enough to pass on their own but large enough to cause trouble, an alpha-blocker medication called tamsulosin is commonly prescribed. It relaxes the smooth muscle in the ureter wall, widening the channel and reducing spasms. A large meta-analysis found that tamsulosin increased stone clearance rates from about 70.5% to 80.5% and shortened the time to passage by several days on average. The benefit was most pronounced for stones larger than 6 mm. Both the American Urological Association and the European Association of Urology recommend it for patients managing a stone conservatively.
When a Stone Won’t Pass
Larger stones, or stones that fail to move after a reasonable period, require a procedure. The two most common options are shock wave lithotripsy, which uses focused sound waves from outside the body to break the stone into smaller fragments, and ureteroscopy, where a thin scope is passed through the urethra and bladder to reach the stone directly and either extract it or laser it into pieces. The choice depends on the stone’s size, location, and composition.
Certain situations call for urgent intervention regardless of stone size. A kidney stone combined with a urinary tract infection is a true emergency, because infected urine trapped behind a blockage can lead to sepsis rapidly. A stone blocking the only functioning kidney, uncontrollable pain despite medication, and persistent vomiting that prevents keeping fluids down are also reasons for prompt surgical management.
Preventing Another Episode
If you’ve had one episode of renal colic, your risk of a recurrence is substantial. The single most effective preventive measure is drinking enough fluid. Major urology guidelines consistently recommend a daily fluid intake of at least 2.5 liters, with the goal of producing more than 2 liters of urine per day. Water is the preferred choice. Sodas, especially colas that contain phosphoric acid, and sugar-sweetened beverages that are high in fructose can actually increase stone risk.
Dietary changes also make a meaningful difference. A diet rich in vegetables and fiber, with normal calcium intake (not reduced, which is a common misconception), limited salt, and moderate animal protein lowers the likelihood of forming new stones. Cutting calcium from your diet can paradoxically increase stone risk by allowing more oxalate to be absorbed in the gut. For people with specific stone types like cystine stones or those caused by a genetic condition, fluid targets are even higher, often 3.5 to 4 liters per day.