Trauma or Minor Injury: This is the most likely cause of an outer lip ulcer, as the lip is a common site for minor injuries, such as cuts, bites, or burns, which can lead to ulcer formation.

Herpes Simplex Virus (HSV): HSV infection can cause lip ulcers, particularly on the outer lip, and is a common condition that can be triggered by stress, sun exposure, or other factors.
Impetigo: A bacterial infection that can cause ulcers on the lip, often accompanied by crusting and redness.
Allergic Contact Dermatitis: An allergic reaction to substances like lip balm, cosmetics, or food can cause lip ulcers and inflammation.

Squamous Cell Carcinoma: Although rare, lip ulcers can be a sign of squamous cell carcinoma, a type of skin cancer that requires prompt diagnosis and treatment.
Lupus: Systemic lupus erythematosus (SLE) can cause lip ulcers, among other symptoms, and is an important diagnosis to consider due to its potential for serious complications.
Syphilis: Secondary syphilis can cause lip ulcers, and it is essential to consider this diagnosis due to the potential for serious health consequences if left untreated.

Erythema Multiforme: A rare skin condition that can cause lip ulcers, often accompanied by other symptoms like fever and target-shaped lesions.
Mycobacterial Infection: Rare infections like tuberculosis or leprosy can cause lip ulcers, although these are relatively uncommon in most populations.
Autoimmune Disorders: Conditions like pemphigus or pemphigoid can cause lip ulcers, although these are relatively rare and often require specialized diagnosis and treatment.