A kidney stone, also known as a renal calculus or nephrolith, is a calculus formed in the kidneys from minerals in the urine. Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size (usually at least 3 millimeters (0.12 in)) they can cause blockage of the ureter. This leads to pain, most commonly beginning in the flank or lower back and often radiating to the groin or genitals. This pain is often known as renal colic and typically comes in waves lasting 20 to 60 minutes. Other associated symptoms include: nausea, vomiting, fever, blood in the urine, pus in the urine, and painful urination.In those who have previously had stones, prevention is recommended by drinking fluids such that more than two liters of urine is produced per day. If this is not effective enough, thiazide diuretic, citrate or allopurinal may be taken. It is recommended that soft drinks containing phosphoric acid (typically colas) be avoided.