An allergic reaction Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more 
can make the lips swell. The reaction may be caused by sensitivity to certain foods or beverages, drugs, lipstick, or airborne irritants. But frequently, the cause of the swelling remains a mystery.
A condition called hereditary angioedema Hereditary and Acquired Angioedema Hereditary angioedema (a genetic disorder) and acquired angioedema (acquired C1 inhibitor deficiency) are caused by a deficiency or malfunction of C1 inhibitor, which is part of the immune system... read more 
may cause recurring bouts of swelling. Nonhereditary conditions—such as erythema multiforme Erythema Multiforme Erythema multiforme is an inflammatory skin disorder characterized by patches of red, raised skin that often look like targets and usually are distributed symmetrically over the body. (See also... read more 
, sunburn Sunburn Sunburn results from a brief (acute) overexposure to ultraviolet (UV) light. Overexposure to ultraviolet light causes sunburn. Sunburn causes painful reddened skin and sometimes blisters, fever... read more 
, cold and dry weather, or trauma—may also cause the lips to swell. Lip swelling on its own is not dangerous. However, when lip swelling is caused by angioedema, an accompanying swelling in the mouth, throat, and/or lower airways can quickly become serious or even fatal.
Treatment depends on the cause. When a cause can be identified and then eliminated, the lips usually return to normal. A corticosteroid ointment is sometimes used to reduce swelling caused by an allergic reaction. Occasionally, excess lip tissue may be removed surgically to improve appearance.
