A small erythema on the lips may be caused by allergic lipitis, as follows. 1. It occurs mostly in allergic individuals, and erythema is formed after the lips come into contact with allergens. The allergens may be methyl methacrylate, denture base made of nickel-chromium alloy or cobalt-chromium alloy, silver amalgam or resin-based filling materials, metal inlays, toothpaste, lipstick, chewing gum and so on. 2. In addition, topical antibiotic ointments, such as erythromycin ointment, gargles, such as chlorhexidine gargle, and analgesics, such as lidocaine gel, may also cause allergic lipitis. Medications should be used under medical supervision. There may be other reasons for a small red spot on the lips, it is recommended to go to the hospital in a timely manner, to clarify the cause of the disease and early targeted treatment.