Canker Sore vs Cold Sore: How to Tell the Difference

What is a canker sore?

Canker sores (aphthous ulcers) are small, shallow ulcers that form on the soft tissues inside the mouth — the inner cheeks, inner lips, tongue, floor of the mouth, or soft palate. They are never on the outer lip or skin.

What they look like: Oval or round, white or yellow center with a red border. They can be quite painful, especially when eating acidic, salty, or physically abrasive foods.

What causes them: The exact cause is not fully understood, but contributing factors include minor mouth trauma (biting the cheek, rough brushing), stress, certain foods (citrus, tomatoes, spicy foods), nutritional deficiencies (iron, B12, folate), and hormonal changes 1Ref 1American Dental Association (2013).American Dental Association Statement on Regular Dental Visits.Dental evaluation for persistent or unusual oral sores as part of routine oral health monitoring.. They are not caused by a virus and are not contagious.

How long they last: Most heal on their own within 1–2 weeks without treatment.

Treatment: Most canker sores do not require treatment beyond comfort measures — over-the-counter topical anesthetics (benzocaine gels), avoiding triggers, and rinsing with salt water. Prescription corticosteroid gels can shorten healing time for larger or more painful ulcers 1Ref 1American Dental Association (2013).American Dental Association Statement on Regular Dental Visits.Dental evaluation for persistent or unusual oral sores as part of routine oral health monitoring..

What is a cold sore?

Cold sores (fever blisters, herpes labialis) are caused by herpes simplex virus type 1 (HSV-1), and occasionally HSV-2. They appear on the outer lip, the skin around the mouth, or sometimes on the chin or nose — never inside the mouth on the soft tissues.

What they look like: Cold sores begin as tingling, itching, or burning at the site — this 'prodrome' phase precedes any visible change. Within a day or two, a cluster of small fluid-filled blisters appears, which then breaks open and crusts over. The cycle typically takes 7–10 days.

How they spread: HSV-1 is transmitted through direct contact with the sore or virus-containing saliva — kissing, sharing utensils, towels, or lip products 2Ref 2Centers for Disease Control and Prevention (2021).Herpes Simplex Virus — STI Treatment Guidelines 2021.HSV-1/2 transmission, clinical course (prodrome, blistering), and antiviral treatment options for herpes labialis (cold sores).. The virus remains in the body permanently and can reactivate during stress, illness, sun exposure, or immune changes. HSV-1 prevalence in the US was approximately 47.8% among persons aged 14–49 in 2015–2016, higher among females (50.9%) than males (45.2%) 3Ref 3McQuillan G, Kruszon-Moran D, Flagg EW, Paulose-Ram R (2018).Prevalence of Herpes Simplex Virus Type 1 and Type 2 in Persons Aged 14–49: United States, 2015–2016.US population prevalence of HSV-1 (47.8% in persons 14–49, higher in women), supporting the commonality of cold sores..

Treatment: Cold sores are self-limited but can be treated with antiviral medications (such as acyclovir or valacyclovir) to shorten duration and severity 2Ref 2Centers for Disease Control and Prevention (2021).Herpes Simplex Virus — STI Treatment Guidelines 2021.HSV-1/2 transmission, clinical course (prodrome, blistering), and antiviral treatment options for herpes labialis (cold sores).. These are most effective when started at the first sign of tingling.

How do I tell them apart at a glance?

| Feature | Canker sore | Cold sore | |---|---|---| | Location | Inside the mouth only | Outside the mouth (lip, skin) | | Appears as | Flat ulcer with white/yellow center | Clustered blisters that crust | | Contagious | No | Yes | | Cause | Not fully understood; not viral | Herpes simplex virus (HSV-1/2) | | Recurs | Sometimes | Yes; can recur repeatedly | | Warning signs | May have none | Tingling or burning before blister |

If you are uncertain about a sore in your mouth — especially one that has not healed in more than two weeks, or that is growing, bleeding, or painless — see a dentist. Persistent oral sores should be evaluated to rule out other causes.

When should I see a doctor or dentist for a mouth sore?

Most canker sores and cold sores resolve without medical care. See a dentist or clinician if:

• A sore has not healed after two to three weeks
• Sores are unusually large (larger than about 1 cm), extremely painful, or occurring frequently
• You have a fever or difficulty eating or drinking alongside mouth sores
• You are immunocompromised — recurrent herpes outbreaks may need suppressive antiviral therapy
• You have concerns about an area that looks unusual — irregular edges, white patches, red patches, or firmness under the surface

While canker sores and cold sores are not serious in most people, a persistent non-healing oral ulcer is one of the warning signs that prompts dentists and oral medicine specialists to screen for oral cancer. This is especially true in adults over 40 who use tobacco or alcohol.

Can I prevent canker sores or cold sores?

For canker sores: Identify your personal triggers (certain foods, stress, minor trauma) and minimize them where possible. Using a soft toothbrush and a toothpaste without sodium lauryl sulfate (SLS) may reduce frequency for some people. Nutritional deficiencies — particularly B12, folate, and iron — are associated with recurrent aphthous ulcers and worth checking if outbreaks are frequent.

For cold sores: Avoid direct contact with active sores and do not share items that touch the mouth during an outbreak. High sun exposure to the lips can trigger outbreaks in some people; SPF lip balm reduces this. People with frequent cold sores may benefit from daily suppressive antiviral medication — a conversation to have with a primary care clinician or dermatologist.