Dental Conditions
Mouth sores: canker sores, cold sores and ulcers
Most mouth sores are harmless and heal on their own within two weeks. The one rule that matters: any sore that lasts longer than that needs to be looked at.
Reviewed by Dr. Raj Singh, Implant Dentist, Dentistry at Vitality Health
A canker sore is a small ulcer inside the mouth, on the soft tissue of the cheeks, lips, tongue or gums. It is not contagious. A cold sore is a cluster of blisters on or around the lips, caused by the herpes simplex virus, and it is contagious. Both usually heal within seven to fourteen days. A sore of any kind that has not healed after two weeks should be examined by a dentist.
People often use the terms canker sore and cold sore interchangeably. They are different conditions with different causes, different locations and different treatments, and telling them apart is the first step in dealing with them. Mouth ulcers can also come from a sharp tooth, a rough filling, a denture edge or a burn from hot food.
This page explains how to tell the common sores apart, what speeds healing, whether to keep your dental appointment when you have a cold sore, and the warning signs that mean a sore needs a professional look.
Canker sore or cold sore: what is the difference?
| Canker sore (aphthous ulcer) | Cold sore (fever blister) | |
|---|---|---|
| Where | Inside the mouth: inner cheeks, inner lips, tongue, floor of the mouth, soft palate | Outside the mouth: on or at the edge of the lips, sometimes the nose or chin |
| Looks like | A round or oval ulcer with a white or yellow centre and a red border | A cluster of small fluid-filled blisters that break, weep and crust over |
| Cause | Not fully understood; triggers include minor injury, stress, certain foods, hormonal changes and some deficiencies | Herpes simplex virus (usually type 1), which stays in the body and reactivates |
| Contagious | No | Yes, from the first tingle until fully crusted and healed |
| Warning signs before | Usually none, or a slight tingle | Tingling, itching or burning on the lip a day or so before the blisters appear |
| Heals in | 7 to 14 days; large ones can take longer | 7 to 10 days |
What causes canker sores, and how can I speed healing?
Nobody knows exactly why some people get canker sores and others do not. Common triggers include a small injury (a bite, a sharp tooth, a toothbrush slip, braces), stress and tiredness, acidic or spicy foods, hormonal changes, and some toothpastes that contain the foaming agent sodium lauryl sulphate. Frequent or severe canker sores can occasionally point to low iron, vitamin B12 or folate, or to a digestive condition, which is worth mentioning to your physician.
To ease a canker sore and help it heal:
- Rinse with warm salt water (half a teaspoon of salt in a cup of water) two or three times a day.
- Avoid acidic, spicy, salty or rough foods until it heals.
- Use an over-the-counter protective gel or paste from the pharmacy to cover the sore and dull the sting.
- Try a toothpaste without sodium lauryl sulphate if you get sores often.
- Do not pick at it, and keep brushing gently around it.
For large, very painful or frequent canker sores, we can prescribe a steroid rinse or ointment that shortens healing time.
Cold sores: should I keep my dental appointment?
Please call us and reschedule routine treatment until the cold sore has fully crusted and healed. A cold sore is contagious; stretching the lip during treatment is painful and can spread the virus to other parts of your mouth or face, and it puts the dental team at risk. If you have urgent pain or swelling we will still see you, with extra precautions.
Cold sores heal on their own in about a week to ten days. Antiviral creams or tablets from the pharmacy or your physician work most effectively when started at the first tingle, before blisters appear. Keep the area clean, avoid touching it, wash your hands after applying anything to it, do not share cups, lip balm or towels, and avoid kissing until it has healed. If you get cold sores often, ask your physician about keeping antiviral tablets on hand.
What else causes mouth ulcers?
A sharp tooth or filling
A chipped tooth or a rough edge on a filling or crown rubs the same spot on the cheek or tongue over and over. The ulcer will keep coming back until the edge is smoothed, which takes us a few minutes.
Dentures
A denture that has loosened or has a high spot creates a sore, often along the border. Do not file it yourself. A small adjustment fixes it, and a sore under a denture that does not heal after adjustment must be examined.
Burns
Hot pizza, coffee or soup burns the roof of the mouth. It looks alarming and peels, but heals within a week or so. Cool water, soft foods and patience are the treatment.
Braces and appliances
Brackets and wires rub the inner lips and cheeks, especially in the first weeks. Orthodontic wax over the bracket protects the spot while the tissue toughens.
Bites
Biting your cheek or lip, often while eating or after dental freezing, leaves an ulcer that heals in a week or two. Repeated biting in the same place may mean a tooth is out of position.
Infections and illness
Hand, foot and mouth disease, thrush (a yeast infection), and some medical conditions and medications can cause multiple sores. Many sores at once, or sores with a fever, warrant a call to your physician.
When does a mouth sore need to be checked?
See a dentist for any sore, lump, white or red patch, or rough area in the mouth that has not healed after two weeks. Most turn out to be harmless, but a persistent sore is also the most common early sign of oral cancer, and oral cancer found early is far more treatable than oral cancer found late. Painlessness is not reassurance; early cancers usually do not hurt.
Other reasons to come in sooner: a sore that is growing, bleeds easily or feels hard at the edges; numbness or a change in sensation; difficulty swallowing or a persistent sore throat; a sore in someone who smokes, vapes, chews tobacco or drinks heavily; or a sore that keeps coming back in exactly the same place. We check the whole mouth for these changes at every routine examination. Read about our oral cancer screenings, or book a visit if something has been there too long.
Common questions
How long does a canker sore last?+
Usually seven to fourteen days. Small ones often clear within a week. A large canker sore (over a centimetre) can take several weeks and may leave a small scar. Any sore still present after two weeks should be examined.
Are canker sores contagious?+
No. You cannot pass a canker sore to anyone by kissing or sharing food. Cold sores, which appear on the lips, are contagious.
Can I go to the dentist with a cold sore?+
For routine care, please reschedule until it has fully healed. Treatment can spread the virus and is uncomfortable on a sore lip. If you have an emergency we will see you and take extra precautions.
What is the difference between a canker sore and oral cancer?+
A canker sore is painful, has a white or yellow centre with a red border, and heals within two weeks. An early cancer often does not hurt, may look like a white or red patch or a small ulcer with firm edges, and does not heal. Time is the deciding factor: two weeks without healing means an examination.
Why do I keep getting mouth ulcers?+
Frequent ulcers may come from a sharp tooth or appliance, from a toothpaste ingredient, from stress, or occasionally from a nutritional deficiency or a digestive condition. Keep a note of when they appear and where, and bring it to your visit. We will check for a local cause and may suggest blood tests through your physician.
Sources: Canadian Cancer Society: Non-cancerous conditions of the mouth · Canadian Cancer Society: Signs and symptoms of oral cancer
A sore that will not heal?
Two weeks is the limit. Book an examination and we will tell you what it is. Call (905) 479-7777.