The common causes of low urea nitrogen include malnutrition and liver failure. First, if the low urea nitrogen is caused by malnutrition, the possible hazards include restricted growth and development or poor resistance after malnutrition, which may lead to the risk of complications such as infections. Second, if the low urea nitrogen is caused by liver failure, this can be complicated by a series of complications, such as patients may develop hepatic encephalopathy and even coma. In addition, liver failure can lead to abnormal coagulation, inducing life-threatening gastrointestinal hemorrhage and cerebral hemorrhage. Third, low urea nitrogen is also common in women during pregnancy, because women during pregnancy will lead to increased blood volume, the kidneys are in a state of high perfusion, high filtration, so the urea nitrogen will appear low, this is a normal physiological phenomenon, no special treatment is needed.