Why are kidney stones so painful?

A kidney stone forms if chemicals concentrated in your urine form a crystal in your kidney, and the crystal grows over time. It's not always clear why this happens. A stone isn't usually painful while it's still in your kidney.

When the stone leaves your kidney, it travels down your ureter so it can leave your body. Some stones are tiny and pass through without causing symptoms. But because the ureter is such a small tube (about 1/8 of an inch wide), a kidney stone can get stuck and block the ureter. If this happens, urine backs up and flows back to the kidney.

The pressure on the kidney caused by the backflow of urine -- combined with spasms in the ureter as your body tries to clear the stone -- can cause intense pressure and pain in the lower back, side, groin or all three. In fact, kidney stone pain is one of the most severe types of pain physicians treat. Kidney stones can also cause sweating, nausea, and vomiting. You might see blood in your urine. And if the stone causes an infection, you can experience fever and chills.

Once the stone passes out of your ureter and into your bladder, you usually don't have any problem passing it the rest of the way out of your body. The urethra is at least twice as wide as the normal ureter, so the stone doesn't usually block it.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.