A four-month-old infant who keeps sucking his lower lip may have a sucking reflex, or it may be related to factors such as erupting milk teeth and stomatitis. It is recommended that parents observe the infant’s condition, and if the infant shows signs of discomfort, they should take the infant to the hospital in time to clarify the specific cause for treatment. 1. Sucking reflex: After birth, infants instinctively have the performance of sucking their fingers and biting their upper and lower lips, and through normal sucking of milk, there may be a certain period of micturition, thus there is always a sucking reflex, which easily gives the infant a sense of comfort and security and can lead to Four-month-old babies keep sucking their lower lips. It is recommended that parents can wear soothers and other items to help infants smoothly through this period; 2. Milk teeth eruption: milk teeth in the process of eruption, easy to cause itching discomfort in the gum area, four-month-old infants may have discomfort, and thus use sucking lower lip to relieve discomfort. It is recommended to use teething sticks for infants, which can effectively relieve the symptoms of local discomfort of the gums and make the teeth erupt smoothly through the gum mucosa, which can also effectively relieve the situation of infants sucking their lower lips; 3. Stomatitis: If a four-month-old baby is always sucking his lower lip, accompanied by lip ulcers, herpes, redness and swelling, it may be related to stomatitis, and can choose to use loratadine under the guidance of a doctor according to the specific condition Syrup and other drugs for treatment.