Kidney stones form when certain minerals in your urine become so concentrated that they crystallize and stick together into solid masses. This usually happens over several months, though people prone to stones can develop them in just a few weeks. The process involves a combination of not drinking enough fluid, eating certain foods, having specific medical conditions, or simply being genetically predisposed to producing urine that’s heavy in stone-forming minerals.
What Happens Inside Your Kidneys
Your kidneys filter waste products from your blood and flush them out in urine. Normally, your urine contains enough water to keep minerals like calcium, oxalate, and uric acid dissolved. When the balance tips, either because there’s too much of these substances or too little water to dilute them, crystals begin to form. Those crystals can cling to the inner lining of the kidney and gradually accumulate into a stone.
The American Urological Association recommends producing at least 2.5 liters of urine per day to keep minerals diluted enough to prevent crystallization. For most people, that means drinking roughly 3 liters of fluid daily, since you lose some water through sweat and breathing. Falling consistently below that threshold is one of the most common and controllable reasons stones develop.
The Four Main Types of Stones
Not all kidney stones are made of the same material, and what causes one type differs from what causes another.
- Calcium oxalate stones are by far the most common. Your liver produces oxalate naturally, and you also absorb it from foods like spinach, nuts, chocolate, and certain fruits. When oxalate levels in your urine climb too high, it binds with calcium and forms crystals. High doses of vitamin D, intestinal bypass surgery, and metabolic disorders can all push calcium or oxalate concentrations higher.
- Uric acid stones form when urine becomes too acidic. They’re more common in people who eat a lot of animal protein (especially organ meats and shellfish), those with diabetes or metabolic syndrome, and people who lose a lot of fluid through chronic diarrhea or malabsorption conditions. Genetic factors also play a role.
- Struvite stones are triggered by urinary tract infections. Certain bacteria produce an enzyme that breaks down urea in the urine, generating ammonia and raising the pH. That alkaline environment lets magnesium, ammonium, and phosphate crystallize together rapidly. These stones can grow large with little warning.
- Calcium phosphate stones are less common on their own but sometimes form alongside calcium oxalate stones. They tend to appear in people with conditions that make their urine more alkaline.
Dehydration and Diet
Chronic low fluid intake is the single biggest modifiable risk factor. When you don’t drink enough, your urine becomes a concentrated solution where minerals are more likely to crystallize. This is why kidney stones are more common in hot climates and during summer months, when people sweat more without compensating with extra water.
Diet plays a direct role, but not always in the ways people expect. Eating too much sodium forces your kidneys to excrete more calcium, raising stone risk. High animal protein intake increases uric acid production and also makes urine more acidic. Oxalate-rich foods like spinach, beets, rhubarb, and almonds can contribute to calcium oxalate stones, though the effect is usually modest unless you’re eating large quantities. Interestingly, getting enough dietary calcium actually helps prevent stones, because calcium binds oxalate in the gut before it ever reaches the kidneys. Cutting calcium from your diet can backfire by allowing more oxalate to be absorbed.
Your Gut Bacteria Play a Role
One of the less obvious factors in stone formation involves your gut microbiome. Certain intestinal bacteria break down oxalate before your body absorbs it. When those bacterial populations are depleted, more oxalate ends up in your bloodstream and eventually in your urine. One well-studied species associated with lower urinary oxalate levels is commonly lost after antibiotic use or dietary changes that alter gut flora.
Research published in Kidney International found that stone formers had significantly fewer oxalate-degrading bacteria compared to healthy controls. The imbalance wasn’t limited to a single bacterial species. Multiple groups of gut bacteria appear to work together to manage oxalate, which means broad disruptions to gut health, from repeated antibiotics to poor diet diversity, could increase your stone risk in ways that aren’t immediately obvious.
Medical Conditions That Raise Risk
Several health conditions make kidney stones more likely by changing the chemical makeup of your urine.
Hyperparathyroidism is a classic example. The parathyroid glands regulate calcium in your blood, and when one or more of them becomes overactive, calcium levels rise. That excess calcium gets filtered through the kidneys, creating an environment ripe for calcium stone formation. People who form recurrent calcium stones are sometimes screened for this condition because treating the overactive gland can stop stones from coming back.
Obesity, type 2 diabetes, and metabolic syndrome all shift urine chemistry toward lower pH (more acidic), which promotes uric acid stones. Chronic digestive conditions like Crohn’s disease or a history of bariatric surgery can increase oxalate absorption in the gut, sometimes dramatically. Recurrent urinary tract infections set the stage for struvite stones, which is why these stones are more common in people with structural urinary tract abnormalities or those who use catheters.
Medications That Can Trigger Stones
Certain medications increase stone risk as a side effect. Topiramate, commonly prescribed for migraines and seizures, lowers citrate levels in the urine. Citrate normally acts as a natural inhibitor of stone formation, so when it drops, calcium phosphate stones become more likely. The risk rises with higher doses and is especially pronounced in younger adults. Some seizure medications in the same drug class carry the same risk.
High-dose vitamin C supplements (typically above 1,000 mg per day) can increase oxalate production, since the body converts excess vitamin C into oxalate. Calcium-based antacids, certain diuretics, and some HIV medications have also been linked to stone formation.
Who Gets Kidney Stones Most Often
Men develop kidney stones roughly twice as often as women, though the gap has been narrowing in recent decades as obesity rates have risen among women. Your risk increases significantly if you’ve had a stone before: about half of first-time stone formers will develop another stone within five to ten years without preventive changes.
Family history matters. If a parent or sibling has had kidney stones, your risk is higher, partly because of shared genetics that influence how your kidneys handle calcium, oxalate, and uric acid. Being overweight independently raises risk, both through metabolic changes and through the higher sodium intake that often accompanies a larger body size.
How Quickly Stones Develop
Most stones take several months to grow large enough to cause symptoms. For people with strong risk factors, though, a stone can reach a noticeable size in just a few weeks. Small stones under about 4 millimeters often pass on their own without you ever knowing they were there. It’s the ones that grow to 5 millimeters or larger that tend to get stuck in the ureter (the tube connecting the kidney to the bladder), causing the intense flank pain that sends people to the emergency room.
Stones don’t always cause problems right away. Some sit in the kidney for months or years before shifting into a position where they block urine flow. That’s why a stone might seem to appear suddenly even though it’s been forming slowly in the background.