Canker sores and cold sores both hurt, both show up around the mouth, and people mix them up all the time. They are different conditions, though. They have different causes, they appear in different places, and only one of them is contagious. Knowing which one you have tells you how careful to be around other people, what usually helps, and when a sore needs a professional look.
This guide compares the two using information from MedlinePlus (the U.S. National Library of Medicine) and the American Dental Association’s MouthHealthy site.
The short version
- Canker sores form inside the mouth. They are not contagious, and their exact cause is uncertain.
- Cold sores (fever blisters) usually form outside the mouth, on or around the lips. They are caused by the herpes simplex virus and are contagious.
MedlinePlus states directly that canker sores “are not the same as cold sores, which are caused by herpes simplex,” and that canker sores are not contagious.
What a canker sore is
MedlinePlus describes a canker sore (also called an aphthous ulcer) as a painful, open sore in the mouth. It is white or yellow and surrounded by a bright red area. The ADA describes canker sores as small white or gray sores with a red border that may appear alone or several at a time. They are not cancerous and do not cause cancer.
Where they appear
According to MedlinePlus, canker sores most often appear on the inner surface of the cheeks and lips, on the tongue, on the upper surface of the mouth and at the base of the gums. Most are small. MedlinePlus says they are usually under one third of an inch (1 centimeter) across, and they turn gray as healing starts.
What may trigger them
No single cause is known. MedlinePlus says canker sores may occur with viral infections, may be linked to immune system problems, and sometimes have no identifiable cause. Possible triggers it lists include:
- Mouth injury from dental work, cleaning the teeth too roughly, or biting the tongue or cheek
- Emotional stress
- Lack of certain vitamins and minerals, especially iron, folic acid or vitamin B-12
- Hormonal changes
- Food allergies
MedlinePlus also notes that women are more likely than men to get canker sores and that they may run in families.
What a cold sore is
Cold sores are caused by the herpes simplex virus (HSV). MedlinePlus explains that type 1 (HSV-1) usually causes oral herpes, and that most people in the United States are infected with this virus by age 20. After the first infection, the virus becomes dormant in nerve tissue in the face and can later reactivate, causing a new cold sore.
The ADA describes cold sores as groups of fluid-filled blisters that often erupt around the lips and sometimes under the nose or around the chin. When cold sores do occur inside the mouth, MedlinePlus says they are usually on the gums or the roof of the mouth.
How a cold sore develops
The MedlinePlus encyclopedia entry on oral herpes describes warning signs such as itching, burning or tingling near the lips before blisters appear. An outbreak may then show:
- Red blisters that break open and leak
- Small blisters filled with clear yellowish fluid
- Several small blisters that grow together into one larger blister
- A yellow, crusty blister as it heals, which eventually turns into pink skin
Triggers named by MedlinePlus include menstruation or hormone changes, being out in the sun, fever and stress.
How it spreads
Herpes viruses spread most easily from someone with an active sore. According to MedlinePlus, you can catch the virus through intimate or personal contact, or by touching an open sore or shared items that have been in contact with the virus, such as razors, towels and dishes. Parents can also pass it to children during ordinary daily activities.
Side-by-side comparison
| Canker sore | Cold sore | |
|---|---|---|
| Usual location | Inside the mouth: cheeks, inner lips, tongue, base of gums | Outside the mouth, on or around the lips; sometimes nose or chin |
| Looks like | Open ulcer with a white, yellow or gray center and a red border | Cluster of fluid-filled blisters that break, then crust |
| Cause | Uncertain; possible triggers include injury, stress, diet and hormones | Herpes simplex virus, usually type 1 |
| Contagious? | No | Yes, especially when a sore is active |
| Comes back? | Can recur | Can recur because the virus stays in the body |
What usually helps
Canker sores
Most canker sores go away on their own. MedlinePlus says the pain often eases in 7 to 10 days and that complete healing can take 1 to 3 weeks, while large ulcers can take longer. The ADA says they usually heal in one or two weeks. Home-care steps listed by MedlinePlus and the ADA include:
- Avoiding hot, spicy or acidic foods that irritate the sore
- Rinsing with salt water or a mild over-the-counter mouthwash. MedlinePlus advises against mouthwashes that contain alcohol, which can irritate the area more.
- Over-the-counter numbing products for short-term relief
- Continuing to brush twice a day and floss daily, gently
For severe cases, MedlinePlus notes that a provider may prescribe a chlorhexidine mouthwash or corticosteroid medicine. The ADA adds that a dentist may suggest treatments that reduce the chance of a secondary infection.
Cold sores
MedlinePlus says there is no cure for cold sores, but they normally go away on their own. Its oral herpes entry says symptoms usually clear in 1 to 2 weeks, and the ADA says blisters usually heal in about a week. Prescription antiviral medicines can help sores heal faster and, for people who get them often, help prevent them. MedlinePlus says these medicines work best when started at the first warning signs, before blisters develop. Comfort steps it lists include:
- Applying ice or a warm washcloth to ease pain
- Gently washing blisters with antiseptic soap and water to avoid spreading the virus to other parts of the body
- Avoiding hot drinks and spicy, salty or citrus foods
- Using sunblock lip balm, since sun exposure can be a trigger
While a cold sore is active, avoid kissing and sharing utensils, cups, towels or razors.
When to see a dentist or doctor
MedlinePlus advises contacting a provider if:
- A canker sore or mouth ulcer does not go away after 2 weeks of home care, or gets worse
- You get canker sores more than 2 or 3 times a year
- You have other symptoms with the sore, such as fever, diarrhea, headache or skin rash
For oral herpes, MedlinePlus says infection can be more serious when it occurs in or near the eye or when the immune system is weakened. Get prompt medical care in those situations.
A sore that does not heal is worth a professional look. MedlinePlus notes that some cancers can first appear as a mouth ulcer that does not heal. Our guide to oral cancer screening explains what a dentist checks for. If you are unsure whether a mouth problem is urgent, see what counts as a dental emergency.
Reducing your chances of another sore
- Brush gently. Rough brushing and mouth injuries are listed canker sore triggers. A soft brush and careful technique help.
- Notice your patterns. If sores follow stress, certain foods or your menstrual cycle, mention it to your dentist or doctor.
- Protect your lips from sun if you get cold sores.
- Ask about recurring sores at routine checkups. See how often you should see the dentist.
If you need a dentist to examine a sore that will not heal, you can search for a dentist near you.
Frequently asked questions
Are canker sores contagious?
No. MedlinePlus and the ADA both say canker sores are not contagious. Cold sores are contagious.
Can a cold sore appear inside my mouth?
Yes, though it is less common. MedlinePlus says that when cold sores appear inside the mouth, they are usually on the gums or the roof of the mouth. Canker sores typically appear on the softer, movable tissues such as the cheeks, inner lips and tongue.
Is a canker sore a sign of cancer?
MedlinePlus says canker sores are not cancer and do not cause cancer. However, a mouth ulcer that does not heal should be checked, because some cancers can first appear that way.
How long does each one last?
According to MedlinePlus, canker sore pain often goes away in 7 to 10 days, and full healing can take 1 to 3 weeks. Oral herpes symptoms usually clear in 1 to 2 weeks.
Can I stop a cold sore before it starts?
Antiviral medicines prescribed by a provider work best when taken at the first warning signs, such as tingling or itching. People who get cold sores often may be prescribed ongoing medicine.
Sources
- MedlinePlus: Canker sore (Medical Encyclopedia)
- MedlinePlus: Canker Sores (Health Topic)
- MedlinePlus: Cold Sores (Health Topic)
- MedlinePlus: Herpes – oral (Medical Encyclopedia)
- ADA MouthHealthy: Canker Sores
- ADA MouthHealthy: Cold Sores
By the Dentist Review Editorial Team.
This article is for general information only and is not medical advice. Talk to a licensed dentist about your own situation. See our disclaimer.
Our guides are written and reviewed by the Dentist Review editorial team. We research each topic using public, authoritative sources such as the CDC, the National Institutes of Health (NIH/NIDCR), the American Dental Association (ADA) and CMS, and we update guides when guidance changes. Guides are for general information only and are not dental or medical advice; please read our disclaimer and learn more about us.