Canker Sore or Cold Sore? How to Tell and What to Do

By Lovik Mirzaeian, DDS. Updated September 2026.

A canker sore is inside the mouth, on the soft tissue of the cheek, tongue or gum, and it is not contagious. A cold sore is on the outside, usually the edge of the lip, comes as a cluster of small blisters, and is caused by a virus that spreads easily. Once you know where it is and what it looks like, telling them apart is simple.

Both are common and both usually heal on their own within a couple of weeks. What differs is what helps, what to avoid, and which one is worth having looked at.

The Quick Way to Tell

Location is the biggest clue. Inside the mouth on soft moveable tissue means canker. On the lip border, the skin around the mouth, or occasionally the hard gum right against the teeth means cold sore.

Appearance is the second. A canker sore is a single round or oval ulcer, white or yellow in the middle with a red rim, and it sits flat. A cold sore starts as tingling or itching, then a cluster of tiny fluid-filled blisters appears, which burst, weep and crust over.

Contagion is the third. Canker sores cannot be passed to anyone. Cold sores are caused by the herpes simplex virus and spread through direct contact, especially while the blisters are open.

Canker Sores

What Causes Them

Nobody knows for certain, which is honest rather than unsatisfying. Known triggers include a small injury from biting the cheek or a sharp tooth edge, stress, hormonal shifts, acidic or spicy food, and low iron, B12 or folate. Some people react to sodium lauryl sulphate, the foaming agent in most toothpastes, and switching to an SLS-free one reduces how often they get them.

What Helps

Most heal in seven to fourteen days without treatment. Warm salt water rinses, avoiding acidic and spicy food, and an over the counter numbing gel make them easier to live with. A dentist can apply a prescription gel or a chemical cautery that shortens the course noticeably for a large or stubborn one.

When to Get One Looked At

Any sore that has not healed in three weeks. Sores that are unusually large, unusually frequent, or that come with fever or swollen glands. A sore that is painless, hard at the edges, or that keeps coming back in exactly the same place. Those are the ones worth a proper look, because a small number of persistent mouth ulcers are not ulcers at all.

Cold Sores

What Causes Them

The herpes simplex virus, which most adults carry whether or not they ever get sores. It lives dormant in a nerve and reactivates with triggers: sunlight, a cold or fever, stress, fatigue, hormonal changes, and sometimes dental work that stretches the lip.

What Helps

Timing matters more than the product. Antiviral creams and tablets work best started at the tingling stage, before the blisters appear, and do little once the sore is fully out. Keep the area clean, do not pick the crust, use a lip balm with sunscreen if sun is a trigger, and wash hands after touching it.

Avoid Spreading It

No kissing, no sharing cups, cutlery, towels or razors while the sore is active. Do not touch the sore and then your eyes, because the virus can infect the cornea. Be especially careful around babies and anyone with a weakened immune system.

Why This Matters for Dental Visits

If you have an active cold sore, tell the practice before your appointment. Stretching the lip during treatment can crack the blisters and spread the virus, and it is uncomfortable for you. Rescheduling by a week is normal and nobody minds.

A canker sore is not a reason to cancel, though mention it so the area can be avoided. If a sharp tooth edge or a rough filling keeps causing sores in the same spot, smoothing it takes two minutes at a routine dental exam and stops the cycle.

Mouth Sores That Are Neither

A white patch that does not wipe off, a red patch, a lump, or an ulcer that is painless and not healing needs to be seen rather than watched. Oral cancer screening is part of every exam here, and the American Dental Association gives three weeks as the point at which any sore should be checked. Most turn out to be nothing. The ones that are something are far easier to treat when found early.

Frequently Asked Questions

How can I tell if it is a canker sore or a cold sore?

Location first. Inside the mouth on soft tissue is a canker sore, on the lip or the skin around it is a cold sore. Canker sores are a single flat ulcer with a white centre and red rim. Cold sores start as tingling, then a cluster of small blisters that burst and crust. Only cold sores are contagious.

Are canker sores contagious?

No. Canker sores are not caused by a virus or bacteria and cannot be passed to anyone by kissing, sharing cups or any other contact. Cold sores, which are caused by the herpes simplex virus, are contagious, particularly while the blisters are open.

How long does a canker sore last?

Most heal within seven to fourteen days on their own. Large ones can take three to four weeks and may leave a small scar. Any sore still present after three weeks should be looked at by a dentist to rule out other causes.

Should I cancel my dental appointment if I have a cold sore?

Usually yes, and just call to reschedule. Treatment stretches the lip, which can crack the blisters, spread the virus and be painful. A canker sore is different and is not a reason to cancel, though it is worth mentioning so the area can be avoided.

Got a Sore That Will Not Heal? Let Us Take a Look

A quick look tells us whether it is a canker sore, a cold sore, an irritation from a sharp edge, or something that needs more attention. It takes a few minutes and it is a common reason for a visit. Contact us to set up a consultation.

Lovik Mirzaeian, DDS has achieved extensive training in multidisciplinary dental treatments. His credentials include Dental Implants, Cosmetic Dentistry, Full Mouth Reconstruction, Endodontics, Oral Surgery, Orthodontics, and Sleep Sedation. Our office puts your comfort and smile first, and we are dedicated to giving you top-notch dental care. Get ready to show off your perfect smile!

Looking for a trusted family dentist in Clovis? Dr. Mirzaeian welcomes new patients of all ages.