If you have sores inside your mouth, they probably aren’t cold sores. Cold sores are caused by herpes simplex virus type 1 (HSV-1) and almost always appear on or around the lips. What most people notice inside their mouth are canker sores, which look similar but have a completely different cause. That said, HSV-1 can occasionally produce sores on the hard palate or gums, especially during a first infection. Figuring out which type you’re dealing with changes how you treat it.
Cold Sores vs. Canker Sores
Cold sores and canker sores show up in different places and look different from each other. Cold sores appear outside the mouth, typically around the border of the lips, as clusters of small fluid-filled blisters. Canker sores appear inside the mouth, on the inner cheeks, lips, or tongue, and are usually single round sores that look white or yellow with a red border.
This distinction matters because cold sores are contagious and caused by a virus, while canker sores are not contagious and not caused by any infection. Canker sores can be triggered by minor injuries (biting your cheek, rough brushing), acidic foods, stress, or nutritional deficiencies. They heal on their own in one to two weeks and don’t require antiviral treatment.
There is one exception worth knowing about. During a first HSV-1 infection, the virus can cause sores on the gums, hard palate, and roof of the mouth. This is called primary herpetic gingivostomatitis and is most common in children, though adults can get it too. It often comes with fever, swollen gums, and multiple painful sores at once. Recurrent outbreaks after that first infection almost always shift to the outer lip area.
Why HSV-1 Stays in Your Body
About 3.8 billion people under age 50 worldwide carry HSV-1, roughly 64% of the global population. Most people pick it up in childhood through casual contact like a kiss from a family member. Once the virus enters through the skin or mucous membranes of the mouth, it travels along nerve fibers to a cluster of nerve cells near the base of the skull. There, it essentially goes dormant.
The virus can sit quietly in those nerve cells for months or years. When something disrupts your immune system’s ability to keep it suppressed, the virus reactivates, travels back down the nerve fiber to the skin surface, and produces a new outbreak. This is why cold sores tend to reappear in the same spot each time: the virus follows the same nerve path.
What Triggers an Outbreak
Several factors can wake the virus up. The most well-documented triggers are stress, sun exposure, fatigue, fever, and immune suppression.
Stress works through multiple pathways. When you’re under psychological pressure, your body releases stress hormones that suppress the immune cells responsible for keeping HSV-1 in check. Elevated stress hormones in the week before an outbreak can impair your body’s ability to destroy virus-infected cells, creating an opening for reactivation.
UV radiation from sunlight is another reliable trigger. Sun exposure suppresses the local immune response in your skin and activates a cellular repair process that inadvertently switches on the virus’s replication machinery. If you notice outbreaks after beach days or extended time outdoors, this is likely why. Using lip balm with SPF 30 or higher can reduce this risk.
Other common triggers include illness (anything that produces a fever), hormonal changes during menstruation, physical exhaustion, and a weakened immune system from medications or other conditions.
What a Cold Sore Outbreak Feels Like
A typical cold sore moves through five stages over one to two weeks. It starts with a tingling, itching, or burning sensation on or near the lip, before anything is visible. Within a day or two, small fluid-filled blisters form. After a few more days the blisters break open and weep, which is the most contagious and often the most painful stage. The open sore then dries out and forms a crust. Finally, the scab falls off and the skin heals underneath.
The entire process takes roughly 7 to 14 days without treatment. If a sore hasn’t cleared within two weeks, that warrants a visit to your doctor.
Treatment Options
Both over-the-counter and prescription options exist, though none eliminate the virus. They shorten outbreaks modestly.
The over-the-counter cream docosanol (sold as Abreva) can reduce healing time, but typically by less than a day. It needs to be applied multiple times daily and works best when started at the first tingling sensation. Prescription topical creams perform about the same as docosanol in clinical trials.
Prescription oral antivirals are somewhat more effective. The standard approach is a short, high-dose course started at the earliest sign of an outbreak. In clinical trials, these medications reduced healing time by roughly half a day to two days depending on the specific drug and dosing. The key is timing: starting treatment during the tingling stage, before blisters form, makes the biggest difference. If you get frequent outbreaks (six or more per year), your doctor may recommend taking a lower daily dose continuously to suppress reactivation.
Can Lysine Help Prevent Outbreaks?
Lysine is an amino acid that competes with arginine, another amino acid that the herpes virus needs to replicate. You’ll find it recommended frequently online, and there is some clinical evidence behind it, though it’s not as strong as many sources suggest.
In controlled studies, daily lysine supplementation at doses above 1 gram reduced the number of recurrences compared to placebo. One study found significantly fewer outbreaks (0.89 vs. 1.56 per period) with about 1,250 mg daily. Another found that 74% of people taking lysine reported milder symptoms compared to 28% on placebo. However, doses below 1 gram per day showed no clear benefit, and none of the studies found that lysine speeds up healing once an outbreak has started.
If you want to try it, aim for at least 1 gram daily. Some researchers suggest that keeping dietary arginine low at the same time (arginine is concentrated in nuts, chocolate, and seeds) may improve results, but this hasn’t been rigorously tested.
Transmission and Protecting Others
HSV-1 spreads through direct contact with an active sore or with saliva and skin that is shedding the virus. The tricky part is that shedding happens even when no sores are visible. Studies have found that people with HSV-1 shed the virus asymptomatically on roughly 5 to 12% of days, depending on how recently they were infected. This means transmission can occur even between outbreaks, though the risk is highest when open sores are present.
During an active outbreak, avoid kissing, sharing utensils or lip products, and oral contact with others’ skin. Be especially careful about touching your eyes after touching a cold sore. HSV-1 can infect the cornea, causing herpes keratitis, which presents as sudden eye pain, light sensitivity, blurred vision, and watery discharge in one eye. Untreated, it can lead to scarring and vision loss. Wash your hands thoroughly after touching a cold sore, and avoid rubbing your eyes during an outbreak.