Cod liver oil contains mainly vitamin A and vitamin D. However, cod liver oil is not the only source of vitamin A and D for children. Infants and children need 1000 international units (IU) of vitamin A and 400 international units (IU) of vitamin D daily. Vitamin A is primarily responsible for maintaining normal growth, reproduction, vision, epithelial tissue integrity, and anti-infective immune function of the body. Vitamin A deficiency can cause damage to the epithelial tissue structure of the body, delayed development of enamel and bones in children, and also cause low immune function, presenting a susceptible state. The main function of vitamin D is to promote the absorption of calcium and phosphorus by the mucosa of the small intestine and to promote the reabsorption of calcium and phosphorus by the renal tubules. Vitamin D deficiency can cause a decrease in calcium and phosphorus absorption through the intestinal tract and impairment of calcification of bone-like tissue, leading to rickets. The content of vitamin A in human milk is higher than that in cow’s milk. If the mother has adequate nutrition and a balanced diet, most of the vitamin A in milk can meet the needs of the infant, and vitamin A can be stored in the body and does not need to be supplemented daily. Vitamin D is less in breast milk and cow’s milk, so newborns should start sun exposure as early as possible, especially premature babies, twins and artificially fed babies or winter-born babies. Vitamin D can also be given orally at 400-800 IU continuously starting 1-2 weeks after birth. Those who cannot adhere to the oral dose can inject 100,000-200,000 IU of vitamin D once (can be maintained for 1-2 months). After the age of 2-3 years, the growth rate of children slows down, the variety of diet and outdoor activities increases, and additional cod liver oil supplementation is generally not necessary. A single dose of vitamin A in excess of 300,000 IU can cause acute toxicity in children. Foods containing high concentrations of vitamin A, such as large doses of concentrated cod liver oil and white bear liver, can also cause acute poisoning. Chronic poisoning can be caused if 100,000 IU is taken daily for more than 6 consecutive months. After poisoning, nausea, vomiting, poor appetite, weakness, thirst, abdominal pain, diarrhea, hepatomegaly, headache, dizziness, drowsiness and sluggishness, etc. Infants with active ventricular membrane secretion and increased cerebrospinal fluid leading to increased intracranial pressure may have bulging fontanelle, irritability and mild meningeal irritation symptoms. In addition, conjunctival congestion, subconjunctival hemorrhage, blurred vision, diplopia, and optic papilloedema may occur. Skin flushing, peeling, hyperpigmentation, hair loss, hair loss, and itchy skin may also be observed. It should be treated at a regular hospital. Generally speaking, if children take vitamin D supplementation of more than 4000 IU per day for several months, or repeatedly and repeatedly injected in large doses intramuscularly, it can also trigger poisoning. It causes excessive absorption of calcium by the body, resulting in hypercalcemia and calcium deposition in bones, blood vessels, and organs, leading to functional abnormalities in the corresponding tissues or organs. Early manifestations of vitamin D toxicity include anorexia, nausea, fatigue, irritability, low fever, pallor, no or little sweating, followed by gastrointestinal disorders, weight loss, high neuroexcitability, even convulsions, elevated blood pressure, cardiac arrhythmia, and a tendency to drink water and urinate more often, and some children may develop dehydration and acidosis. If the diagnosis of vitamin D overdose is confirmed by examination, the child will soon return to normal after immediately stopping vitamin D and calcium supplements, and the appetite will improve significantly. There are many concentrated cod liver oil preparations, parents and doctors should read the detailed instructions, according to the ratio of vitamin A and vitamin D can generally be divided into 2:1 type, 3:1 type and 10:1 type. 2:1 type of concentrated cod liver oil contains 10,000 IU of vitamin A and 5,000 IU of vitamin D per ml, so parents can choose this type of cod liver oil as long as they feed their children 3 drops daily. 3:1 type of concentrated cod liver oil capsules contain 1,800 IU of vitamin A and 1,800 IU of vitamin D per capsule. The 10:1 cod liver oil capsules contain 10,000 IU of vitamin A and 1,000 IU of vitamin D. These capsules contain too much vitamin A and vitamin D for infants and are not suitable for infants. Parents should note that when feeding various infant formulas and fortified foods, it is important to read the vitamin A and D content of the formula to avoid overdosing by mistake.