Canker Sores vs. Cold Sores: What’s in Your Mouth and When to See a Dentist

You notice a painful spot inside your mouth. Maybe it showed up after a stressful week, or perhaps it appeared out of nowhere while you were eating lunch. Your first thought is probably the same one that brings thousands of Canadians to Google every month: is this a canker sore or a cold sore, and should I be worried?

The two conditions look similar at first glance, but they are fundamentally different in their causes, how they spread, where they appear, and how they should be treated. Confusing one for the other can lead to unnecessary anxiety (canker sores are not contagious) or, worse, to ignoring warning signs that do require professional attention.

At Markham Smile Centre, oral sores are among the most common concerns patients bring up during routine checkups and emergency visits alike. This guide will walk you through everything you need to know about both types of mouth sores, so you can identify what you are dealing with, manage the discomfort at home, and recognize when it is time to see your dentist.

Key Takeaways

• Canker sores appear inside the mouth (gums, tongue, inner cheeks) and are not contagious. Cold sores appear outside the mouth, usually on or around the lips, and are contagious.
• Canker sores have no single known cause, whereas cold sores are caused by the herpes simplex virus (HSV), which affects an estimated 60% or more of the global population.
• Most minor mouth sores heal on their own within one to two weeks.
• You should see a dentist if a mouth sore lasts longer than two weeks, is unusually large, keeps coming back, or is accompanied by fever, difficulty swallowing, or spreading swelling.
• Your dentist can prescribe medicated rinses, topical treatments, or antiviral medications that over-the-counter products cannot match.

What Are Canker Sores?

Canker sores, known medically as aphthous ulcers, are small, shallow ulcers that develop on the soft tissues inside the mouth. They can appear on the gums, the inner lining of the cheeks and lips, the tongue, or the soft palate. Unlike cold sores, canker sores never appear on the outer surface of the lips or on the skin around the mouth.

According to the American Dental Association (ADA) , canker sores are among the most common oral lesions and are neither contagious nor precancerous.

What Do Canker Sores Look Like?

A canker sore typically appears as a small, round or oval white or yellowish spot with a clearly defined red border. The surrounding tissue may be slightly swollen. Even a small canker sore can cause significant pain, especially when it is irritated by food, drinks, or the movement of your tongue and cheeks during talking and eating.

Canker sores usually measure less than one centimetre across and tend to appear in groups of two to four at a time.

Common Causes and Triggers

The exact cause of canker sores remains uncertain. Research has identified several factors that may contribute to or trigger outbreaks, including:

• Minor oral trauma. Accidentally biting the inside of your cheek, irritation from braces or a rough tooth edge, or overly aggressive brushing can all trigger a canker sore.
• Stress and fatigue. Emotional or physical stress is one of the most frequently reported triggers.
• Certain foods. Acidic or spicy foods, such as citrus fruits, tomatoes, and hot peppers, can irritate the mouth lining and trigger sores in susceptible individuals.
• Nutritional deficiencies. Low levels of vitamin B12, iron, zinc, or folic acid have been linked to more frequent outbreaks of canker sores.
• Hormonal changes. Some patients report outbreaks that coincide with menstrual cycles or other hormonal shifts.
• Immune system factors. The ADA notes that some experts believe an immune system reaction may be involved, in which the body’s own white blood cells temporarily attack the lining of the mouth.
• Sodium lauryl sulphate (SLS). This foaming agent found in many toothpastes has been associated with the development of canker sores in some people.

“We see canker sores frequently during routine exams, and patients are often relieved to learn they are not contagious and not a sign of serious illness,” says Dr. James C.H. Ko, DDS, MSc, of Markham Smile Centre. “That said, patients who experience recurrent outbreaks should mention it during their dental checkup , because frequent sores can sometimes point to a nutritional deficiency or other underlying condition that we can help address.”

Types of Canker Sores

Not all canker sores are the same. Dental professionals classify them into three categories:

Minor aphthous ulcers are the most common type. They are small (under one centimetre), oval-shaped, and typically heal within one to two weeks without leaving a scar. Most people who get canker sores experience this type.

Major aphthous ulcers are less common and more severe. They are larger, deeper, and can take several weeks to heal. Major canker sores may leave scarring and often cause significant discomfort that interferes with eating and drinking.

Herpetiform ulcers are uncommon and, despite their name, are not caused by the herpes virus. They appear as clusters of very small, pinpoint sores that can coalesce into larger, irregular ulcers. They typically heal within one to two weeks.

What Are Cold Sores?

Cold sores, also called fever blisters, are small, fluid-filled blisters that most commonly erupt on the lips or the skin immediately surrounding them. They are caused by the herpes simplex virus (HSV), most often Type 1, though Type 2 can also be responsible.

According to the Cleveland Clinic, cold sores are extremely common, and the herpes simplex virus that causes them is present in a significant portion of the adult population. Many carriers never develop visible sores, while others experience recurrent outbreaks triggered by stress, illness, sun exposure, or fatigue.

What Do Cold Sores Look Like?

Cold sores go through a predictable cycle. Before the blister appears, many people experience a tingling, itching, or burning sensation at the site where the sore is about to form. Within about 24 hours, a cluster of small, fluid-filled blisters develops. These blisters eventually burst, ooze, and form a yellowish crust or scab. The entire cycle from first tingle to healed skin usually takes one to two weeks.

Cold sores typically appear at the lip border (where the skin meets the lip line), though they can also develop near the nostrils, on the chin, or, less commonly, on the gum tissue near the teeth.

How Cold Sores Spread

This is one of the most important differences between canker sores and cold sores: cold sores are contagious. The virus spreads through direct contact, such as kissing, sharing utensils, drinking glasses, lip balm, or razors, particularly when blisters are open and weeping.

The ADA notes that the virus can also survive on hard surfaces, which means you can spread it by touching your sore and then coming into contact with another person or handling objects they may use.

Important points to reduce the risk of spreading cold sores:

• Avoid kissing or close skin contact with others while blisters are present.
• Do not share utensils, cups, straws, or lip products.
• Wash your hands thoroughly after touching the affected area.
• Avoid touching your eyes after contact with a cold sore, as the virus can cause a serious eye infection.

Common Cold Sore Triggers

Once you are infected with HSV, the virus remains dormant in your body and can reactivate periodically. Known triggers for cold sore outbreaks include:

• Stress and fatigue
• Sun and wind exposure (UV light is a well-documented trigger)
• Fever or illness (hence the name “fever blisters”)
• Hormonal changes
• Weakened immune system
• Dental procedures or facial trauma (in rare cases, dental work near the lip area can trigger a recurrence in susceptible patients)

Canker Sores vs. Cold Sores: How to Tell the Difference

Telling the two apart is easier than most patients expect. The simplest distinction is location : canker sores always appear inside the mouth, while cold sores almost always appear outside the mouth, on or around the lips.

Here is a closer look at the key differences:

Location. Canker sores form on the soft, non-keratinized tissue inside the mouth, including the inner cheeks, gums, tongue, and soft palate. Cold sores form on or around the lips, on the keratinized skin of the face.

Appearance. Canker sores are open, shallow ulcers with a white or yellowish centre and a red border. Cold sores are clusters of small, fluid-filled blisters that burst and crust over.

Contagiousness. Canker sores are not contagious in any way. You cannot spread them through kissing, sharing food, or any other contact. Cold sores are contagious and spread through direct or indirect contact, especially when blisters are open.

Cause. The precise cause of canker sores is unknown, though triggers such as stress, trauma, and nutritional deficiencies are recognized. Cold sores are caused specifically by the herpes simplex virus (HSV-1 or HSV-2).

Recurrence pattern. Both types can recur, but the patterns differ. Canker sores may return weeks or months after a previous episode, with no clear predictability. Cold sores tend to recur in the same location and are often preceded by the telltale tingling or burning sensation.

Scarring. Minor canker sores heal without scarring. Major canker sores may scar. Cold sores usually do not leave permanent scars, though the scabbing phase can be cosmetically noticeable for several days.

Treatment and Home Care

Most minor canker sores and cold sores heal on their own within one to two weeks. However, the right home care can reduce pain, speed healing, and prevent complications.

Canker Sore Relief

For canker sores, the goal is to reduce pain and protect the sore from further irritation:

• Over-the-counter topical gels or ointments containing benzocaine or other numbing agents can provide temporary pain relief when applied directly to the sore.
• Antimicrobial mouth rinses can help keep the area clean and may reduce discomfort. If you are unsure which rinse is right for you, our guide to choosing the right mouthwash covers which ingredients to look for and which to avoid.
• Saltwater rinse. Dissolving half a teaspoon of salt in a cup of warm water and gently rinsing several times a day can soothe irritation.
• Avoid irritants. Stay away from spicy, acidic, or rough-textured foods until the sore heals.
• Soft-bristle toothbrush. Gentle brushing reduces the risk of further trauma to the affected area.

For patients with severe or frequently recurring canker sores, a dentist may prescribe a corticosteroid rinse or gel or recommend nutritional testing to check for deficiencies in B12, iron, or folate.

Cold Sore Relief

For cold sores, the goal is to shorten the outbreak, manage pain, and prevent spreading the virus:

• Over-the-counter antiviral creams containing docosanol can be applied at the first sign of tingling and may help shorten the duration of the outbreak.
• Prescription antiviral medications. Your dentist or physician can prescribe oral antiviral drugs (such as valacyclovir or acyclovir) that are most effective when taken within the first 48 to 72 hours of symptom onset.
• Cool compresses applied to the affected area can help reduce swelling and discomfort.
• Lip balm with SPF. Since sun exposure is a known trigger, applying lip balm with sun protection can help prevent future outbreaks.
• Avoid picking or peeling the scab. This delays healing and increases the risk of secondary bacterial infection.

“Patients sometimes wait too long before mentioning cold sores during a dental visit,” notes Dr. Amir Guorgui, DDS, of Markham Smile Centre. “If you know you are prone to outbreaks, let your dental team know before any procedure. We can work with your physician to arrange antiviral medication ahead of dental appointments that involve the lip area, which helps prevent a flare-up.”

When Should You See a Dentist About a Mouth Sore?

Most canker sores and cold sores are manageable at home and resolve without professional treatment. However, there are situations where a dentist or physician should evaluate the sore promptly.

Red Flags That Require Professional Attention

You should book an appointment if you notice any of the following:

• The sore has not healed within two weeks. Both canker sores and cold sores should show clear improvement by this point. A sore that lingers longer may need to be evaluated for other conditions.
• The sore is unusually large. Canker sores larger than one centimetre across, or cold sore blisters that spread significantly, warrant professional evaluation.
• You experience frequent recurrences. If you develop canker sores more than a few times a year, or if cold sore outbreaks occur every few weeks, your dentist can help investigate underlying causes and provide preventive treatment.
• You have a high fever, difficulty swallowing, or swelling that spreads beyond the immediate sore. These symptoms may indicate a secondary infection or a more serious condition.
• The sore is accompanied by severe pain that does not respond to over-the-counter treatment.
• You are unsure what type of sore you have. Your dentist can provide a definitive diagnosis, often just through a visual examination.

As the ADA advises, your dentist should examine any mouth sore that lasts a week or longer, because while most sores are harmless, some may be symptoms of other conditions that benefit from early detection.

What Your Dentist Can Do

Your dentist has several treatment tools that are not available over the counter:

• Prescription-strength topical medications (corticosteroids or antimicrobial gels) that reduce inflammation and accelerate healing for severe canker sores.
• Prescription antiviral drugs for cold sores are significantly more effective than OTC creams, especially when started early.
• Laser therapy. Some dental practices use low-level laser therapy to promote faster healing and reduce pain in recurrent oral sores.
• Screening for underlying conditions. During your regular dental checkup , your dentist may recommend blood work to check for nutritional deficiencies or immune-related factors if canker sores are recurring frequently.
• Oral cancer screening. While canker sores and cold sores are benign, your dentist uses every visit as an opportunity to screen for abnormal lesions. A sore that does not heal, changes shape, or bleeds without cause always warrants further evaluation.

How to Reduce Your Risk of Mouth Sores

While there is no guaranteed way to prevent either type of sore, several habits can reduce how often they occur and how severe they are.

Preventing Canker Sores

• Maintain consistent oral hygiene. Regular brushing with a soft-bristled toothbrush and daily flossing help keep oral tissues healthy and reduce irritation. Professional dental cleanings every six months support overall oral health and give your dental team a chance to catch early signs of any issues.
• Consider switching toothpaste. If you notice canker sores forming after using a new toothpaste, check the ingredients for sodium lauryl sulphate (SLS). Switching to an SLS-free toothpaste may help.
• Eat a balanced diet. Ensure adequate intake of B12, iron, zinc, and folic acid, particularly if you follow a restricted or plant-based diet.
• Manage stress. Easier said than done, but stress is one of the most consistently reported triggers. Physical activity, adequate sleep, and structured downtime can help.
• Protect your mouth from trauma. If braces, retainers, or a rough tooth edge is causing repeated irritation, let your dentist know so it can be adjusted.

Preventing Cold Sore Outbreaks

• Use lip balm with SPF. Sun protection is one of the simplest and most effective preventive measures.
• Avoid known triggers. If stress, illness, or fatigue consistently precede your outbreaks, addressing those factors can reduce recurrence.
• Do not share personal items that come into contact with the mouth or face.
• Talk to your dentist or physician about suppressive therapy. Patients who experience frequent cold sore outbreaks (more than six per year) may benefit from daily low-dose antiviral medication that significantly reduces the frequency and severity of episodes.

Canker Sores and Cold Sores Are Different, But Both Deserve Attention

Canker sores and cold sores are two of the most common oral conditions patients ask about, and the confusion between them is understandable. They can both be painful, both tend to recur, and both can make everyday activities like eating and talking uncomfortable.

The fundamental differences, however, matter for your health and for the people around you. Canker sores are internal, non-contagious ulcers with no single known cause. Cold sores are external, contagious blisters caused by the herpes simplex virus. Knowing which one you are dealing with helps you choose the right treatment, take appropriate precautions, and decide when professional care is needed.

Most mouth sores are harmless and heal on their own. But any sore that lasts longer than two weeks, grows unusually large, comes with fever or difficulty swallowing, or keeps recurring should be evaluated by a dental professional. At Markham Smile Centre, oral health screenings are part of every general dentistry visit, and we are always ready to help you manage oral sores with treatments that go beyond what is available over the counter.

Frequently Asked Questions

Are canker sores contagious?

No. Canker sores (aphthous ulcers) are not caused by a virus or bacteria and cannot be transmitted through kissing, sharing utensils, or any other form of contact. They are completely non-contagious.

Can you get a canker sore on your lip?

Canker sores develop on the soft tissue inside the mouth, including the inner lip, but they do not appear on the outer surface of the lip or on the skin. If you have a sore on the outside of your lip, it is more likely a cold sore.

How long can canker sores last?

Minor canker sores typically heal within one to two weeks. Major canker sores can take several weeks to fully resolve. If a sore persists beyond two weeks, see your dentist for evaluation.

How long do cold sores last?

Cold sores usually go through their full cycle (tingling, blistering, crusting, healing) within one to two weeks. Prescription antiviral medications can shorten this timeline if started early.

When should I see a dentist about a mouth sore?

See your dentist if the sore lasts longer than two weeks, is unusually large or painful, keeps recurring frequently, or is accompanied by fever, difficulty swallowing, or spreading swelling.

Can stress cause mouth sores?

Yes. Stress is a commonly reported trigger for both canker sores and cold sore outbreaks. Managing stress through sleep, exercise, and routine self-care can help reduce the frequency of episodes.

Can my dentist prescribe medication for cold sores?

Yes. Dentists can prescribe antiviral medications (such as valacyclovir) for cold sores. These are most effective when started within the first 48 to 72 hours of symptoms. For patients with frequent outbreaks, daily suppressive therapy may also be recommended.

Is a mouth sore a sign of oral cancer?

The vast majority of mouth sores, including canker sores and cold sores, are benign. However, a sore that does not heal within two weeks, changes in appearance, or bleeds without obvious cause should be evaluated by a dentist. Oral cancer screenings are part of routine dental checkups at Markham Smile Centre.