Aphthous Ulcer: This is the most common cause of painful oral ulcers, characterized by recurrent, self-limiting ulcers that can appear on the tongue, gums, or inside the cheeks. The exact cause is unknown, but stress, trauma, and certain foods can trigger them.

Herpes Simplex Virus (HSV): HSV can cause painful oral ulcers, especially during the initial outbreak. These ulcers are typically vesicular and can appear on the lips (cold sores) or inside the mouth.
Traumatic Ulcer: Accidental bites, dental work, or sharp edges on teeth or dental appliances can cause traumatic ulcers, which are painful and can appear anywhere in the oral cavity.
Oral Lichen Planus: This chronic inflammatory condition can cause painful ulcers, especially the erosive form, which presents with ulcers and areas of mucosal loss.

Squamous Cell Carcinoma: Although less common, oral cancer can present as a painful ulcer that does not heal. It's crucial to consider this diagnosis, especially in patients with risk factors like smoking or tobacco use.
Tuberculosis: Oral tuberculosis can cause painful ulcers, especially in immunocompromised patients. It's a critical diagnosis to consider due to its implications for public health and the need for specific treatment.
Syphilis: The secondary stage of syphilis can cause oral ulcers, which are typically painless but can be painful in some cases. Missing this diagnosis can lead to severe complications if left untreated.

Behçet's Disease: A rare autoimmune disorder that can cause recurrent oral ulcers, among other symptoms like genital ulcers and eye inflammation.
Pemphigus Vulgaris: An autoimmune disease that causes blistering of the skin and mucous membranes, leading to painful oral ulcers.
Erythema Multiforme: A skin condition that can also affect the mucous membranes, causing painful ulcers in the mouth, among other symptoms.