Canker Sore vs Cold Sore: How to Tell Them Apart

Key takeaways

• Canker sores form inside the mouth and are not contagious.
• Cold sores usually appear on or around the lips and are caused by a virus that can spread.
• Most of both types heal within a week or two without treatment.
• A sore that lasts longer than about two weeks should be looked at by a dentist or doctor.

Canker sore vs cold sore: what are the main differences?

What is the difference between a canker sore and a cold sore?

A canker sore is a small, shallow ulcer that forms on the soft tissue inside your mouth. A cold sore is a blister caused by a virus that usually appears on or near the lips. The two get confused because both are small, sore spots around the mouth, but they are different problems with different causes.

In short:

• Canker sore: inside the mouth, not contagious, no clear single cause.
• Cold sore: outside the mouth on or around the lips, contagious, caused by the herpes simplex virus.

The table on this page puts the main differences side by side. If you are not sure which one you have, that is very common, and a dentist can take a look.

Where do canker sores and cold sores appear?

Canker sores appear inside the mouth, while cold sores appear on the outside, most often on the lip or the skin right around it. Location is usually the quickest clue.

Canker sores tend to show up on the inner cheeks, the inner lips, the tongue, the floor of the mouth, or the soft palate. They often look like a round or oval spot with a white or yellowish center and a red border. They can sting, especially when you eat or drink something salty, spicy, or acidic.

Cold sores often start as a tingling, itching, or burning feeling in one spot. Then small fluid-filled blisters form, usually along the edge of the lip. The blisters can break open, ooze, and then crust over. Sometimes a cold sore shows up near the nose or on the chin, and now and then one can appear on the gums or the roof of the mouth, which is one reason they are mistaken for canker sores.

What causes canker sores, and what causes cold sores?

Cold sores are caused by the herpes simplex virus. Canker sores do not have one confirmed cause, though several common triggers are linked to them.

Common canker sore triggers

• A small injury, such as biting your cheek, brushing too hard, or irritation from braces or a rough tooth edge
• Acidic or spicy foods
• Stress or lack of sleep
• Hormonal changes
• Low levels of certain nutrients, such as iron or B vitamins
• Some toothpastes or mouth rinses that irritate sensitive tissue

Not everyone has an obvious trigger. Some people get canker sores now and then, and others rarely do.

Common cold sore triggers

Many people carry the virus without ever knowing it. After the first infection, the virus stays in the body and can wake up later. Things that may set off an outbreak include illness or fever, stress, tiredness, strong sun exposure, hormonal changes, or a weakened immune system. A cold sore is a flare-up of a virus that is already there, not a sign of poor hygiene.

Is a cold sore contagious, and is a canker sore?

A cold sore is contagious, and a canker sore is not. This is the most practical difference between the two.

The cold sore virus spreads through close contact, such as kissing, or by sharing items like utensils, cups, lip balm, or towels. It is most contagious when blisters are present and weeping, though it can spread at other times too. While you have a cold sore, a few habits help protect the people around you:

• Avoid kissing and skin contact with the sore
• Do not share drinks, utensils, or lip products
• Wash your hands after touching the area
• Try not to pick at the blisters

Canker sores cannot be passed from one person to another. You can share a drink with someone who has one and not worry.

How long do they last, and what helps them heal?

Most canker sores and cold sores heal by themselves in about one to two weeks. Larger or more stubborn sores can take longer, and comfort care can make the wait easier.

For canker sores, these steps often help:

• Rinse gently with warm salt water
• Avoid spicy, salty, and acidic foods until the area heals
• Use a soft toothbrush and brush gently around the sore
• Ask a pharmacist about over-the-counter products made for mouth sores

For cold sores, keep the area clean and avoid touching it. Over-the-counter creams may ease discomfort, and a doctor can prescribe antiviral medicine, which often works best when started at the first tingle. If you get cold sores often, it is worth asking a doctor about a plan. Sun protection on the lips can also help if sunlight is one of your triggers.

Neither type should be popped, picked, or scraped, since that can slow healing and add irritation.

When should you see a dentist about a mouth sore?

See a dentist if a sore lasts longer than about two weeks, keeps coming back, or makes eating and drinking hard. A sore that does not heal should be checked, even if it does not hurt much.

Other signs that deserve a visit include:

• A sore that keeps growing or has a hard or raised edge
• Red or white patches in the mouth that do not go away
• Several sores at once, or new ones appearing as old ones heal
• Fever or swelling along with the sore
• A sore that appears after you started a new medicine

Most mouth sores are harmless, but a dentist can tell the difference between a common sore and something that needs more attention. Our team offers oral cancer screening as part of care, which includes a careful look at the soft tissues of the mouth. If the sore turns out to be a rough tooth edge, a broken filling, or an ill-fitting appliance, a dentist can fix that at the source. Walk-ins are welcome, and same-day emergency care is available if pain is severe.

How can you lower your chances of getting mouth sores?

You cannot prevent every sore, but a few daily habits can reduce how often they come back. The goal is to protect the soft tissue in your mouth and notice your own triggers.

• Be gentle: use a soft toothbrush and avoid brushing hard against the gums and cheeks.
• Watch your triggers: if citrus, spicy chips, or a certain toothpaste seems to bring on a sore, try cutting back to see if it helps.
• Eat a balanced diet: good nutrition supports healing in the mouth.
• Manage stress and sleep: both are common triggers for either type of sore.
• Protect your lips: lip balm with sun protection can help people who get cold sores after time outdoors.
• Keep up with checkups: a regular dental cleaning and exam lets a dentist spot rough edges, irritation, or other changes early.

Whether you have insurance or not, our team will explain your options in plain language. We accept most major dental PPO plans, and we offer CareCredit, in-house payment plans, and the Ultra Discounts plan for people without insurance. We see patients at our Peoria, North Phoenix, and Metro offices, and our Mesa office opens in December 2026.

This article is general education and not a diagnosis. A dentist can look at your mouth and advise you on your own situation.

Part of our Everyday oral health articles.

Sources

1. Mouth sores, American Dental Association (MouthHealthy)
2. Dental Health, MedlinePlus (National Library of Medicine, NIH)