Recurrent Aphthous Stomatitis (Canker Sores): This condition is the most common cause of ulcers around the mouth, characterized by recurrent, painful, round or oval ulcers that can appear on the lips, tongue, cheeks, and gums. The exact cause is unknown, but stress, minor mouth injury, and certain foods can trigger them.

Herpes Simplex Virus (HSV) Infection: HSV can cause painful ulcers around the mouth, known as cold sores or fever blisters. These are highly contagious and can be triggered by stress, sunlight, or illness.
Impetigo: A bacterial infection that can cause ulcers or sores around the mouth, often accompanied by redness, swelling, and pus. It's highly contagious and common in children.
Angular Cheilitis: A condition that causes red, swollen patches in the corners of the mouth, which can sometimes develop into ulcers. It's often associated with vitamin deficiencies, ill-fitting dentures, or excessive saliva production.

Squamous Cell Carcinoma: Although rare, ulcers around the mouth can be a sign of oral cancer, especially if they are persistent, grow in size, or are accompanied by other symptoms like difficulty swallowing or a lump in the neck. Early detection is crucial for effective treatment.
Lupus or Other Autoimmune Disorders: Systemic lupus erythematosus (SLE) and other autoimmune diseases can cause oral ulcers as part of their symptomatology. These conditions require prompt diagnosis and treatment to manage symptoms and prevent complications.
Syphilis: The secondary stage of syphilis can cause mucous patches or ulcers in the mouth, which are highly infectious. Missing this diagnosis can lead to serious health consequences if left untreated.

Behçet's Disease: A rare disorder that causes inflammation throughout the body, including oral ulcers, genital ulcers, and inflammation in parts of the eye. It's more common in certain parts of the world and can be challenging to diagnose.
Erythema Multiforme: A skin condition characterized by target-shaped lesions and ulcers, which can appear around the mouth. It's often triggered by infections or medications.
Pemphigus Vulgaris: A rare autoimmune disease that causes severe blistering of the skin and mucous membranes, including the mouth. It requires immediate medical attention due to the risk of infection and fluid loss.