A pustule on the side of the mouth may be caused by sebaceous cysts and boils. 1. Sebaceous cysts: Due to the blockage of sebaceous gland excretory ducts, sebum secreted by sebaceous glands cannot be discharged and gathers in the glands. The main manifestation is skin elevation and pain. If accompanied by infection, the area around the cyst may be visibly red and swollen, with elevated skin and pain, and containing pus. If it occurs in the area around the mouth, it can cause a pustule around the mouth. 2. Boil: it is a suppurative inflammation of the hair follicle in the deep part and the surrounding tissues, and its main pathogen is Staphylococcus aureus, which is mainly manifested as a localized hard lump, accompanied by redness, swelling and pain, and if it occurs in the area around the mouth, it can also cause a pustule on the side of the mouth. There are many other causes of pus on the side of the mouth, you should consult a doctor in time to identify the cause of the disease, and follow the doctor’s advice for further treatment or therapy.