Single most likely diagnosis
- Cold sore (Herpes Simplex Virus): This is the most common cause of blisters on the lips, characterized by recurrent episodes of painful vesicles that crust over and heal within a few days.
Other Likely diagnoses
- Angular cheilitis: Inflammation of the corners of the mouth, which can cause blisters or cracks, often due to fungal or bacterial infections.
- Impetigo: A highly contagious bacterial skin infection that can cause blisters or sores on the lips.
- Contact dermatitis: An allergic reaction or irritation from substances like cosmetics, food, or metals, leading to blisters on the lips.
Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Primary syphilis: The initial stage of syphilis, which can manifest as a single, painless blister or sore (chancre) on the lips.
- Oral herpes zoster (shingles): Reactivation of the varicella-zoster virus in the trigeminal nerve, causing a painful rash or blisters on one side of the face, including the lips.
Rare diagnoses
- Pemphigus: A group of rare autoimmune diseases that cause blistering of the skin and mucous membranes, including the lips.
- Stevens-Johnson syndrome: A severe skin and mucous membrane disorder, usually a reaction to medication or infection, which can cause blisters on the lips and other areas.
- Erythema multiforme: An immune-mediated disorder that can cause target-shaped lesions or blisters on the lips and other mucous membranes.