A cold sore is the common name for a recurrent outbreak of the Herpes Simplex Virus type 1 (HSV-1). HSV-1 establishes a lifelong, latent infection in the nerve ganglia after initial exposure. While the virus is present throughout the nerves supplying the mouth and face, recurrent cold sores are highly predictable in their location. This predictability means the area under the tongue, known as the sublingual region, is an extremely rare site for a true recurrent cold sore.
Where Cold Sores Typically Appear
Recurrent cold sore outbreaks, medically termed herpes labialis, most frequently occur around the lips, specifically along the vermilion border. The virus, having lain dormant in the trigeminal ganglion, travels down the nerve axons to the surface to cause the characteristic cluster of fluid-filled blisters. Viral reactivation is most commonly triggered by factors like stress, fever, sun exposure, or hormonal changes.
Once inside the mouth, recurrent lesions show a strong preference for specific areas, namely the attached gingiva (gums) and the hard palate. The appearance of these lesions begins with a tingling or burning sensation. This quickly progresses into small, painful, grouped blisters that soon rupture to form small, clustered ulcers.
Tissue Type and Lesion Location
The predictable location of recurrent HSV-1 lesions is due to the type of tissue present in the mouth. The oral cavity is lined by two primary types of mucosa: keratinized and non-keratinized. Keratinized tissue is tough and protective, covering the lips, the hard palate, and the gingiva.
Recurrent herpetic lesions almost exclusively manifest on this keratinized tissue. The area under the tongue is covered by non-keratinized mucosa, which is softer, thinner, and more easily moved. This tissue type does not provide the necessary environment for the virus to consistently replicate and cause a recurrent lesion.
A herpes simplex lesion appears under the tongue only during a primary, or first-time, infection, often called herpetic stomatitis. This initial outbreak is widespread and severe, affecting both keratinized and non-keratinized surfaces, including the tongue, gums, and inner cheeks. Once the virus establishes latency, all subsequent, recurrent outbreaks adhere to the preference for keratinized tissues, making a recurrence under the tongue exceptionally unlikely for an immunocompetent person.
Common Non-HSV Sores Under the Tongue
If a painful sore develops under the tongue, it is likely not a cold sore but a different type of common oral ulceration. The most frequent alternative diagnosis is an aphthous ulcer, commonly known as a canker sore. Unlike cold sores, canker sores are not caused by the herpes virus and are not contagious.
Canker sores are characterized by a distinct appearance, presenting as single, round, or oval ulcers with a white or yellowish center and a bright red border. They are found exclusively on the non-keratinized, movable tissues of the mouth, such as the floor of the mouth and the sublingual area. The precise cause is unknown, but they are often linked to minor trauma, stress, dietary triggers, or underlying immune factors.
Other possibilities for a sore under the tongue include traumatic ulcers, which result from accidental biting or irritation from a dental appliance. These ulcers usually heal within a week after the source of irritation is removed. Another lesion that can appear in the sublingual area is a mucocele, a small, painless, fluid-filled sac caused by damage to a minor salivary gland duct. Because a true recurrent cold sore is rare in this location, any persistent or concerning sore should be examined by a dentist or physician for an accurate diagnosis.