Trauma: This is often the most common cause of unilateral lip swelling due to accidents, fights, or other forms of physical injury. The lip's location and prominence make it susceptible to trauma.

Infection (e.g., abscess or cellulitis): Bacterial infections can cause localized swelling, redness, and pain. The lip's rich blood supply can lead to the rapid spread of infection.
Allergic Reaction: Allergic reactions to food, insect bites, or other substances can cause swelling, which may be unilateral if the allergen was applied or came into contact with one side of the lip.
Dental Problems: Issues like a dental abscess or gum infection can cause swelling that may extend to the lip, especially if the infection is severe.

Angioedema: A potentially life-threatening condition that can cause rapid swelling of the lips, face, and airways. It often requires immediate medical attention.
Oral Cancer: Although less common, unilateral lip swelling can be a symptom of oral cancer, particularly if there's a history of tobacco use or other risk factors.
Lymphatic or Venous Obstruction: Conditions that obstruct lymphatic or venous drainage can lead to swelling. These conditions can be serious and require prompt medical evaluation.

Melkersson-Rosenthal Syndrome: A rare neurological disorder characterized by recurrent, often unilateral, facial paralysis or weakness, swelling of the face and lips, and the development of folds and furrows in the tongue.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the face, leading to swelling.
Lipoma or Other Benign Tumors: Although rare, benign tumors can grow on the lip, causing localized swelling.