Cold sores in your mouth come from herpes simplex virus type 1 (HSV-1), spread through direct contact with an infected person’s saliva, skin, or sores. Most people pick up the virus during childhood from a kiss, a shared cup, or close contact with a family member. Once you’re infected, the virus stays in your body permanently and can reactivate to cause sores on or inside your mouth throughout your life.
How HSV-1 Spreads
HSV-1 passes from person to person through contact with the virus in saliva, sores, or skin surfaces in and around the mouth. Kissing is the most common route, but you can also pick it up by sharing utensils, drinks, lip balm, or towels with someone who carries the virus. The Mayo Clinic specifically warns against sharing lip products because people can carry HSV-1 in their mouth without any visible sores and still pass it along.
The greatest risk of transmission is when someone has active blisters, but the virus can spread even when the skin looks completely normal. This is called asymptomatic shedding: the virus quietly reaches the skin or saliva without causing symptoms. Research from the University of Washington found that people with HSV-1 shed the virus on about 12% of days in the first few months after infection, dropping to around 7% of days by 11 months. In most of those instances, the person had no symptoms at all.
What Happens During Your First Infection
The first time HSV-1 infects your mouth, it can cause a condition called gingivostomatitis, which is especially common in young children but can happen at any age. This initial outbreak is usually the worst one. Symptoms include painful sores on the lips, gums, tongue, or inner cheeks, along with red and swollen gums, fever, headaches, swollen lymph nodes in the neck, and loss of appetite from the mouth pain. Some people also develop bad breath and dry mouth.
Not everyone gets noticeable symptoms during their first infection, though. Many people contract HSV-1 in childhood and never realize it. The virus quietly travels to a cluster of nerve cells near the base of the skull, where it goes dormant and waits.
Why Cold Sores Come Back
Once the virus settles into your nerve cells, certain triggers can wake it up and send it back to the surface. Known reactivation triggers include:
- Sun exposure: Ultraviolet light is one of the most well-documented triggers. In controlled studies, UV exposure reactivated herpes infections within about four to five days.
- Stress: Physical or emotional stress suppresses immune function enough to let the virus reactivate.
- Illness or fever: This is why cold sores are sometimes called “fever blisters.” A weakened immune system during a cold or flu gives the virus an opening.
- Fatigue and lack of sleep
- Hormonal changes, such as during menstruation
- Injury to the mouth area, including dental work or windburn
How a Cold Sore Develops
An outbreak follows a predictable pattern over roughly 7 to 10 days.
It starts with a tingling, burning, or itching sensation around or inside the mouth. This prodromal stage is your earliest warning sign. Within a day or two, small fluid-filled blisters appear on the skin’s surface. These blisters can show up on your lips, around your mouth, or inside it, including the throat. The skin underneath and around them turns red.
After a few days, the blisters break open into shallow, red sores. This weeping stage is when the sore is most contagious. The open sore then dries out and forms a yellowish or brown crust. Over the following days, the scab gradually flakes away as the skin heals underneath. Scarring is rare.
Cold Sores vs. Canker Sores
If you notice a sore inside your mouth, it may not actually be a cold sore. Canker sores (aphthous ulcers) are far more common inside the mouth and are often confused with herpes-related sores. The differences are straightforward:
- Location: Cold sores typically appear on or around the lips. They can occur inside the mouth, but this is more common during a first infection. Canker sores appear only inside the mouth, usually on the inner cheeks, lips, or tongue.
- Appearance: Cold sores look like clusters of small, fluid-filled blisters. Canker sores are single, round sores with a white or yellow center and a red border.
- Contagiousness: Cold sores are contagious. Canker sores are not caused by a virus and cannot be passed to another person.
- Cause: Cold sores are caused by HSV-1. The exact cause of canker sores is unknown, though stress, minor mouth injuries, and certain foods seem to trigger them.
If you’re seeing clusters of tiny blisters inside your mouth along with swollen gums and fever, that points more toward a herpes infection, particularly a first outbreak. A single, round, non-blistering sore on the inside of your cheek is almost certainly a canker sore.
Reducing Outbreaks and Spread
You can lower your chances of triggering an outbreak by wearing lip balm with SPF in the sun, managing stress, and getting enough sleep. These won’t eliminate outbreaks entirely, but they reduce their frequency for many people.
To avoid spreading the virus, don’t kiss or share drinks, utensils, or lip products when you have an active sore or feel the tingling that signals one is coming. Keep in mind that transmission can happen even without symptoms, so sharing lip balm is always a gamble. Washing your hands after touching a sore helps prevent spreading the virus to your eyes or other parts of your body.
Antiviral medications can shorten an outbreak by a day or two if taken early, ideally during that initial tingling stage. For people who get frequent outbreaks (six or more per year), daily antiviral therapy can significantly reduce how often sores appear.