Canker Sores vs Cold Sores: How to Tell Them Apart

You have a sore spot in or around your mouth, it stings when you eat, and you want to know two things: what it is, and whether you can pass it on. The two most common culprits, canker sores and cold sores, are often confused, but they are very different problems with different causes and different care.

The fastest way to tell them apart is location. Canker sores form inside the mouth, on soft, moving tissue. Cold sores usually form on or around the outside of the lips. That one clue gets most people to the right answer.

One thing matters more than which kind you have: any sore in the mouth that has not healed within two weeks should be looked at by a dentist or doctor. More on that below.

What a Canker Sore Is

A canker sore, known clinically as an aphthous ulcer, meaning a small, shallow, non-infectious mouth ulcer, is a break in the lining of the mouth. Canker sores are not caused by a virus and are not contagious. You cannot catch one from someone, and you cannot give one to anyone by kissing or sharing a cup.

They are typically:

Round or oval, with a white, grey or yellowish centre and a red border
Found on the inside of the lips or cheeks, under or on the sides of the tongue, on the soft palate, or at the base of the gums
Painful out of proportion to their size, especially with acidic, salty or spicy food

Most canker sores are small and heal on their own in about one to two weeks without scarring. Less commonly, people get larger, deeper sores that take longer and may leave a scar, or clusters of many tiny sores at once. Both of those are worth showing to a dentist.

What triggers canker sores

The exact cause is not fully understood, and it seems to involve the immune system reacting to the lining of the mouth. Known triggers include:

Minor injury, such as biting your cheek, a sharp tooth edge, braces, or brushing too hard
Stress and lack of sleep
Hormonal changes, for some people around their period
Certain foods, often acidic fruit, tomatoes, chocolate, nuts or very spicy dishes
Toothpastes containing sodium lauryl sulfate (SLS), a foaming agent that some people find irritating
Low iron, vitamin B12 or folate
Some medical conditions, including coeliac disease and inflammatory bowel disease

Frequent, severe canker sores can occasionally be a sign of an underlying condition, which is another reason to mention them to your dentist or doctor rather than just putting up with them.

What a Cold Sore Is

A cold sore, sometimes called a fever blister, is caused by the herpes simplex virus, usually the type known as HSV-1. Many people carry this virus, often picked up in childhood, and it stays dormant in the nerves for life. From time to time it reactivates and causes a sore, usually in the same place.

Cold sores are typically:

On the outer edge of the lip or the skin just around it, sometimes under the nose or on the chin
Preceded by tingling, itching or burning for a day or so before anything appears
A small cluster of fluid-filled blisters that break, weep, then crust over into a scab

Most cold sores heal in about one to two weeks. Less often, the virus can cause sores inside the mouth, typically on the firm gums or the hard palate, and a first infection in a young child can cause a much more widespread, feverish illness in the mouth that needs a doctor.

Cold sores are contagious

Cold sores spread through direct contact, such as kissing, sharing cups, utensils, lip balm or razors, and contact with the fluid from the blisters. They are most contagious from the first tingle until the sore has fully crusted and healed, though the virus can occasionally spread without a visible sore.

While you have one, avoid kissing, oral contact, and sharing anything that touches your mouth. Be especially careful around newborns, anyone with a weakened immune system, and people with eczema, as the virus can be more serious for them. Wash your hands after touching the sore, and avoid touching your eyes.

What triggers cold sores

Common triggers include:

Colds, flu and fevers, hence the name
Strong sunlight or wind on the lips
Stress and tiredness
Hormonal changes
Injury to the lips, including some dental treatment

If you get cold sores and have a dental appointment coming up during a flare, call the practice. Many will reschedule non-urgent work, both for your comfort and to avoid spreading the virus.

Canker Sore vs Cold Sore: Side by Side

Feature	Canker sore	Cold sore
Where it appears	Inside the mouth, on soft moving tissue: cheeks, inner lips, tongue, soft palate	Outside, on or around the lips; occasionally on gums or hard palate
What it looks like	A single shallow ulcer, or a few, with a pale centre and red rim	A cluster of small blisters that break and then scab
Warning before it appears	Sometimes a sore or burning spot	Often tingling, itching or burning for a day or so
Cause	Not fully understood; immune reaction, injury and other triggers	Herpes simplex virus
Contagious	No	Yes, especially until fully healed
Typical healing time	About one to two weeks	About one to two weeks
What helps most	Protecting it, numbing gels, avoiding trigger foods, gentle rinses	Early antiviral treatment, lip protection, avoiding contact

If you are still unsure, the location and whether it began as blisters are usually the deciding clues.

What Helps a Canker Sore Heal

There is no instant cure, but you can make a canker sore much less painful while it heals:

Numbing gels from the pharmacy, often containing benzocaine, applied to the sore before meals.
Protective pastes or patches that cover the sore so food and your tongue cannot rub it.
Salt water rinses, about half a teaspoon of salt in a glass of warm water, swished gently a few times a day.
Antiseptic or anti-inflammatory mouthwashes, used as directed. Our mouthwash guide explains the different types.
Avoiding trigger foods for a few days: citrus, tomatoes, crisps, spicy and very salty food.
A soft toothbrush and an SLS-free toothpaste, which some people find reduces how often sores appear.

If you get canker sores often, or they are large and slow to heal, a dentist or doctor can prescribe stronger treatments, such as a steroid mouth rinse or paste, and may suggest blood tests for iron, B12 and folate.

What Helps a Cold Sore Heal

Cold sores respond best to treatment started early, ideally at the first tingle:

Antiviral creams from the pharmacy can shorten an outbreak a little if applied as soon as you feel it coming.
Prescription antiviral tablets can work better, and people with frequent or severe outbreaks sometimes take them at the first sign, or even preventively. Ask your doctor or pharmacist.
Cold compresses and over-the-counter pain relief can ease discomfort.
Lip balm with sunscreen helps prevent sun-triggered outbreaks.
Hands off. Picking the scab slows healing and risks spreading the virus.

See a doctor if a cold sore has not healed in about two weeks, if sores keep coming back, if they spread toward your eyes, or if you have a weakened immune system.

Other Mouth Sores That Are Easy to Confuse

Not every sore is one of these two. Others include:

Trauma ulcers, from a sharp tooth, a broken filling, a denture rubbing or a hot food burn. These usually heal within a week or two once the cause is removed.
Oral thrush, a yeast infection that leaves creamy white patches that may wipe off and leave a red area underneath.
Hand, foot and mouth disease, common in young children, with mouth sores plus a rash on hands and feet.
Lichen planus, an inflammatory condition that can cause lacy white lines or sore red patches.

If a sore does not fit either description above, keep an eye on it, and if in doubt ask your dentist.

When a Mouth Sore Needs Checking

Most mouth sores are harmless and heal on their own. But a sore that does not heal can occasionally be an early sign of something more serious, including oral cancer, which is often painless at first.

Make an appointment with your dentist or doctor if you notice:

Any sore, ulcer, lump or patch in the mouth that has not healed within two weeks
A red or white patch that does not rub off
A sore with raised, hard edges
Numbness, or a sore that bleeds easily
Difficulty swallowing or a persistent sore throat on one side
A lump in the neck

This does not mean you should panic about every ulcer. It means two weeks is the line. Our guide to oral cancer screening explains what that check involves and who is at higher risk.

Seek care more urgently if a sore comes with a high fever, you cannot eat or drink, or a cold sore spreads near your eye.

Frequently Asked Questions

Can I spread a canker sore by kissing? No. Canker sores are not caused by a virus or bacteria you can pass on, so they are not contagious. Cold sores are a different matter and do spread through kissing and shared items.

Why do I keep getting canker sores in the same place? Repeated sores in one spot often point to a local cause, such as a sharp tooth edge, a rough filling, braces or a habit of biting that area. A dentist can often smooth the edge or fix the restoration. Frequent sores all over the mouth are worth raising with a doctor.

Can a cold sore appear inside my mouth? It can, though it is less common. When the herpes virus causes sores inside the mouth in adults, they usually appear on the firm tissue of the gums or roof of the mouth, while canker sores tend to appear on soft, moving tissue.

Does stress really cause mouth sores? Stress is a well-known trigger for both. It does not create the herpes virus, but it can reactivate it, and it is one of the most commonly reported triggers for canker sores.

How long is too long for a mouth sore? Two weeks. Most canker sores and cold sores heal within that time. Anything still there after two weeks should be examined by a dentist or doctor, even if it does not hurt.

What to Do Next

Look at where the sore is and how it began. Inside the mouth, a single shallow ulcer, no blisters: very likely a canker sore. Protect it, avoid trigger foods, and use a numbing gel or salt water rinse while it heals. On the lip, beginning as tingling and then blisters: very likely a cold sore. Begin an antiviral early, avoid contact, and wash your hands.

Make a note of the date you first noticed it. If it is still there in two weeks, or it keeps coming back, book an appointment with your dentist and ask them to take a look.