Drinking a popular summertime beverage may cause a painful problem for some—kidney stones.

According to urologist John Milner, M.D., iced tea contains high concentrations of oxalate, one of the key chemicals that leads to the formation of kidney stones, a common disorder of the urinary tract that affects about 10 percent of the population in the United States. Though hot tea also contains oxalate, it isn’t as easy to consume a quantity large enough to encourage the formation of stones.

Kidney stones are small crystals that form from the minerals and salt normally found in the urine in the kidneys or ureters, which are the small tubes that drain urine from the kidney to the bladder. Most of the time kidney stones are so small that they are harmlessly expelled from the body. But on some occasions, the stones grow to the point that they can become lodged in the ureters.

The best way to avoid kidney stones is to stay hydrated, especially by drinking water or lemonade. Avoiding foods high in oxalates such as spinach, chocolate, rhubarb and nuts is also recommended.

In addition, eating foods high in calcium can help reduce the amount of oxalate the body absorbs.