People who have had a kidney stone should drink enough fluids to produce two liters of urine per day in order to prevent more kidney stones from forming, according to new guidelines from the American College of Physicians.

If drinking more fluids doesn’t work, patients can try “water pills” (known as diuretics) or other drugs to reduce stone formation. But both of these recommendations are weak, since the evidence for them is moderate to low quality, the authors write.

“On average, drinking enough fluid so that you have two liters of urine per day significantly decreases your risk of having a kidney stone,” said Dr. Robert Centor, a practicing internist and Chair of the American College of Physician’s Board of Regents, which approved the new recommendation.

Kidney stones form from solid crystals, usually including calcium oxalate, and adding more fluids decreases the concentration of solids in the urine, making it harder for stones to form, he said.

“I want patients’ urine to be clear, not yellow,” Centor told Reuters Health by phone.

More than 10 percent of men and seven percent of women in the U.S. will develop a kidney stone in their lifetime, according to a 2012 study, and many who have one will have more in the future, Centor said. They can be incredibly painful and may require surgery, he said.

Researchers reviewed papers published on recurrent kidney stones between 1948 and 2014 to form the new guidelines.

There is some evidence that drinking more fluids may prevent subsequent stones without causing side effects. If that doesn’t work, taking a thiazide diuretic, which increases the amount of urine, may help. Other options include citrate, which attaches to calcium in the urine and keeps crystals from forming, or a drug called allopurinol, the authors write in the Annals of Internal Medicine.

Patients should try the first two options before allopurinol, which is also used to treat gout and can cause drowsiness, diarrhea and stomach pain, Centor said.

People may need to drink more than two liters of fluids to produce two liters of urine, he said, depending on how hot the day is and how active the person is.

“You might need to drink three liters on a hot day,” Centor said.

There is also some evidence that soda intake may be related to recurrent kidney stones, the authors write.

Cutting down on dark sodas, particularly sweetened ones, may help, Centor said. Dark sodas are made with phosphoric acid, which makes urine more acidic.

Clear or yellow citrus sodas, made with citrate, may actually help prevent stones, he noted.

There are plenty of reasons to avoid sweetened soft drinks, but as far as kidney stones are concerned, people who’ve never had one don’t need to worry about cutting out soda, he said.

There are different types of kidney stones, but in general people at high risk can reduce their sodium intake, reduce animal protein, and try to get five fresh fruits and vegetables per day, said Dr. John A. Sayer, Senior Clinical Lecturer in Nephrology at the Institute of Genetic Medicine at Newcastle University in the U.K.

Anyone can get a kidney stone, but they are more common in hot climates or working environments, Sayer told Reuters Health by email.

Family history makes stones more likely, Centor said. A patient with a kidney stone will be in so much pain they will go to the emergency room, he said. At a follow-up appointment after the stone has passed, doctors can suggest strategies for preventing more.

“When a patient comes in with a kidney stone, they can’t hear me because they’re in so much pain,” Centor said.

“We don’t have a perfect treatment for preventing kidney stones, every patient is different and responds differently,” he said.