Recurrent Aphthous Stomatitis: This condition is the most common cause of frequent oral ulcers, characterized by recurring, benign, and non-contagious ulcers in the mouth. The exact cause is unknown, but it is thought to be related to immune system dysfunction, stress, and genetic predisposition.

Herpes Simplex Virus (HSV) Infection: HSV can cause oral ulcers, especially during primary infection. These ulcers are typically painful and can be accompanied by other symptoms like fever and swollen lymph nodes.
Hand, Foot, and Mouth Disease: This viral illness, common in children, can cause oral ulcers along with a rash on the hands and feet.
Oral Lichen Planus: A chronic inflammatory condition that can cause oral ulcers, along with white patches and redness in the mouth.
Nutritional Deficiencies: Deficiencies in vitamins like B12, iron, or folate can lead to oral ulcers among other symptoms.

Cancer (Oral Squamous Cell Carcinoma): Although rare, oral ulcers can be a sign of oral cancer. It's crucial to rule out cancer, especially in patients with risk factors like smoking or tobacco use.
Autoimmune Disorders (e.g., Systemic Lupus Erythematosus, Behçet's Disease): These conditions can cause oral ulcers as part of their symptomatology and require prompt diagnosis and treatment.
HIV/AIDS: Oral ulcers can be an early sign of HIV infection, making it a critical diagnosis not to miss.

Erythema Multiforme: A rare skin condition that can cause oral ulcers, along with skin lesions.
Pemphigus Vulgaris: A rare autoimmune disease that causes blistering of the skin and mucous membranes, including the mouth.
Graft-Versus-Host Disease (GVHD): In patients who have undergone bone marrow or stem cell transplants, GVHD can cause oral ulcers among other symptoms.
Crohn's Disease and Ulcerative Colitis: Although primarily affecting the gastrointestinal tract, these inflammatory bowel diseases can occasionally cause oral ulcers.