A sore inside the mouth may be from various causes. The most common type of mouth sore is a “canker sore,” which may be stress-related, or the product of a dietary deficiency (usually vitamin B12, iron, zinc, or folic acid), or from an injury to the mouth, or from salty, hot, acid, or irritating foods.

A canker sore is not a cold sore, and the two are often confused. A cold sore or fever blister properly so called is a viral infection from the herpes simplex virus. Cold sores usually occur outside the mouth, on or near the lips. In a minority of cases, however, cold sores can break out inside the mouth, erupting from the mucus membranes, especially the hard palette (roof of the mouth), or the gums.

All cold sores are highly infectious, unlike normal canker sores. A cold sore inside the mouth behaves exactly like one outside the mouth, but leads to some problems not associated with external cold sores due to the position.

In all cases, including when the cold sore occurs inside the mouth, the disease goes into remission but does not disappear.

Rather, the virus retreats into a dormant stage on the ganglia of the sensory nerves near the point of outbreak, and further outbreaks may recur periodically.

As far as medical science knows, there is no “cure” for herpes simplex infections. Rates of recurrence vary from person to person, but once infected, recurrence is a possibility for anyone.

Special Problems With Cold Sores Inside Mouth

A cold sore inside the mouth will not heal more slowly than one on the exterior. However, the lesion may be irritated by eating or drinking, especially of salty, acidic, hot, or spicy food, or by toothpaste or other substances put into the mouth.

This does not change the cycle of recovery and return to remission, but it can produce pain and sometimes difficulty eating or drinking. The discharge from a cold sore inside the mouth may produce a bad taste, and the crust is more easily disturbed due to the action of saliva and tongue motion.

It is also more difficult to treat a cold sore inside the mouth, as topical antiviral drugs such as aciclovir (Zovirax) and docosanol (Abreva) are not intended for ingestion and may be contraindicated.

A cold sore inside the mouth is marginally less contagious than an external cold sore, due to the lowered likelihood of the viral discharges being transmitted to the hands and hence to others.

An inside-the-mouth cold sore can still result in contagion, however, particular if the sufferer kisses someone else or otherwise makes mouth contact.

There may be some alternative cold sores treatment, such as lysine (an amino acid), various herbal treatments (extracts of echinacea and of black currants), and other substances that can might be used safely to treat cold sores inside the mouth.

Most such treatments have not been approved by the FDA, nor their effectiveness confirmed by clinical studies. However, that by no means makes it certain that they aren’t efficacious. It merely makes it less certain that they are.

Consult a physician about possible side effects of such treatments, and whether they are safe to use inside the mouth; if they are, it is safe to use them and possible that they may shorten the duration of an outbreak.

In any case, a cold sore, regardless of where the lesion occurs, will disappear after approximately two weeks from the first onset of symptoms, or one week from the eruption of the open lesion phase of the disease.

This happens without treatment, which acts to shorten the duration of an outbreak, not to cure the disease.