Canker Sore vs. Cold Sore: How to Tell Them Apart, Which Is Contagious, and What Heals Each Faster

Sore on or near your lip and not sure what it is? Dr. Liza Wakim explains how to tell a canker sore from a cold sore, which one is contagious, and what helps each heal faster.

People mix these two up all the time, and I understand why. Both hurt, both can show up around the mouth, and both seem to appear at the worst possible moment. But they are completely different problems, and the difference matters: one is contagious and the other is not.

The simplest clue, and the one I use chairside, is where the sore is. In this post I will walk you through that rule, plus how each one looks, what causes it and what actually helps it heal.

The quick answer: canker sore vs. cold sore at a glance

A canker sore is a small, painful ulcer inside the mouth that is not contagious. A cold sore is a cluster of fluid-filled blisters, usually on or around the lips, caused by the herpes simplex virus, and it is contagious. Canker sores usually heal in one to two weeks; cold sores usually crust over and heal in about one to two weeks too.

What a canker sore is (and common triggers)

A canker sore, also called an aphthous ulcer, is a shallow, round or oval sore with a white, gray or yellow center and a red border. It forms on the soft, movable tissue inside the mouth and can make eating and talking surprisingly painful for its size.

The exact cause is not fully understood, but canker sores are not contagious, and the ADA notes they usually heal on their own in one to two weeks. StatPearls lists common triggers including:

• Minor injury, like biting your cheek, a sharp tooth edge or brushing too hard
• Stress and lack of sleep
• Certain foods, such as acidic or spicy foods
• Hormonal changes
• Some nutritional deficiencies, such as iron, folate or vitamin B12
• Some ingredients in toothpaste, such as sodium lauryl sulfate (SLS)

Frequent or very large canker sores can sometimes be linked to other health conditions, which is one reason recurring sores are worth mentioning to your dentist or doctor.

What a cold sore is (HSV-1) and how it spreads

A cold sore, sometimes called a fever blister, is caused by the herpes simplex virus, usually type 1 (HSV-1). According to MedlinePlus, cold sores are small blisters that typically appear on or around the lips, and they spread through close contact such as kissing or sharing cups, utensils or lip balm.

After the first infection, the virus stays dormant in the nerves and can reactivate. Common triggers for a new outbreak include:

• A cold, fever or other illness
• Sun exposure on the lips
• Stress or fatigue
• Hormonal changes, such as menstruation
• Injury to the lip area, including some dental procedures

Many people feel a tingling, itching or burning spot a day or so before the blisters appear. That early warning matters, because it is the best time to start treatment.

Where it is tells you a lot: inside vs. outside the mouth

This is the rule of thumb I use chairside:

• Inside the mouth, on soft, movable tissue (inside of the lips, inner cheeks, under the tongue, floor of the mouth): most likely a canker sore
• On the lip, the border of the lip or the skin around the mouth: most likely a cold sore

There is one important exception. Herpes can occasionally cause small clusters of sores inside the mouth on the firmer tissue that sits tight against the bone, like the gums or the roof of the mouth. MedlinePlus notes that oral herpes sores can appear on the gums, the inside of the mouth and the roof of the mouth, especially with a first infection. If you are unsure, a dentist can usually tell by looking.

If what you have is a raised bump rather than an open sore, it may be something else entirely, like a gum boil or a mucocele.

How to treat each

Canker sore relief

Most canker sores heal on their own. The goal is to stay comfortable and avoid irritating the sore while it heals:

• Rinse gently with warm salt water or an alcohol-free mouthwash
• Avoid acidic, spicy, salty or crunchy foods for a few days
• Use an over-the-counter numbing gel or protective paste, following the label
• Brush gently with a soft toothbrush, and avoid catching the sore
• For frequent or severe sores, a dentist or doctor may prescribe a medicated rinse or gel, according to StatPearls

Cold sore antivirals and timing

Cold sores also heal on their own, but treatment can shorten an outbreak and ease discomfort. Timing is everything: MedlinePlus explains that antiviral medicines work best when started early, ideally at the first tingle before blisters appear. The American Academy of Dermatology notes that both prescription antiviral pills and creams are options, and that people with frequent outbreaks may benefit from a plan to start treatment at the first sign.

While a cold sore is active:

• Avoid kissing and sharing cups, utensils, towels or lip balm
• Do not pick, squeeze or pop the blisters
• Wash your hands after touching the area
• Use lip balm with sunscreen to help prevent sun-triggered outbreaks

Can mouthwash or toothpaste make them worse?

For canker sores, possibly. Many toothpastes contain sodium lauryl sulfate (SLS), a foaming agent. A 2019 systematic review summarized by the ADA found that switching to an SLS-free toothpaste may reduce canker sore outbreaks and pain in people who get them often. If you get canker sores frequently, an SLS-free toothpaste is an easy change to try.

Strong, alcohol-based mouthwashes can sting an open sore. My post on whether mouthwash is bad for mouth ulcers explains what to look for in a rinse.

For cold sores, toothpaste and mouthwash are not the trigger, but stretching the lips, for example during long dental visits, can sometimes irritate the area.

When to see a dentist

Any sore that does not heal is also worth checking as part of an oral cancer screening.

Frequently asked questions

Are canker sores contagious?

No. Canker sores are not caused by a virus that spreads between people, so you cannot catch one from kissing or sharing a drink. Cold sores are different: they are caused by the herpes simplex virus and are contagious, especially while blisters are present.

Can a cold sore be inside your mouth?

Usually cold sores appear on or around the lips. Herpes can occasionally cause small clusters of sores inside the mouth on firmer tissue, like the gums or the roof of the mouth, especially with a first infection. A single round sore on the inside of the lip or cheek is much more likely to be a canker sore.

How long does each last?

Canker sores usually heal on their own in one to two weeks. Cold sores usually move from tingling to blisters to crusting and heal in about one to two weeks as well. Antiviral treatment started at the first tingle can help shorten a cold sore outbreak. Any sore lasting longer than two weeks should be checked.

Should I pop a cold sore?

No. Popping or picking a cold sore can spread the virus to other areas or other people, slow healing and increase the risk of infection or scarring. Keep it clean, avoid touching it, wash your hands often and let it crust over and heal on its own, or ask about antiviral treatment.

Can I get dental work with a cold sore?

It is best to call the office first. Many dental offices prefer to reschedule routine or cosmetic treatment until an active cold sore has healed, to avoid spreading the virus and because stretching the lips can make it worse. Urgent dental problems can still be treated.

What toothpaste is best if I get canker sores?

If you get canker sores often, try a toothpaste without sodium lauryl sulfate (SLS), a common foaming agent. Research suggests SLS-free toothpaste may reduce canker sore outbreaks and pain for some people. Check the ingredient list, and choose a fluoride toothpaste that is SLS-free.

Still not sure which one you have?

A quick look usually settles it, and we can help you treat it and prevent the next one. If you are in or around Washington, PA, you can request an appointment at Enhanced Wellness or call us at (724) 225-5070.

Sources

• American Dental Association, MouthHealthy. Canker Sores.
• StatPearls. Recurrent Aphthous Stomatitis. NCBI Bookshelf.
• MedlinePlus. Cold Sores.
• MedlinePlus. Herpes, oral.
• American Academy of Dermatology. Cold sores: diagnosis and treatment.
• ADA, JADA Specialty Scan. Oral pathology: SLS-free toothpaste systematic review summary. 2020.

Elizabeth Wakim

Dr. Elizabeth Wakim, DDS, is the founder of Enhanced Wellness. She’s a compassionate and highly-regarded dentist with her own practice in Washington, Pennsylvania, known for providing modern, comprehensive dental care, botox and facial aesthetics with a focus on patient comfort and anxiety reduction, serving general, cosmetic, and pediatric dentistry needs.