Cold sores and canker sores are the two most common mouth sores, and they're constantly confused with each other. That confusion matters. The two conditions have completely different causes, and they need completely different treatments.
The fastest way to tell them apart: a cold sore is a contagious viral infection that appears on the outside of the lips, while a canker sore is a non-contagious ulcer that forms inside the mouth. They look different, feel different, and respond to different treatments.
Here's how to tell which one you have — and what to do about it.
Cold Sore vs. Canker Sore: The Key Differences at a Glance
Location and contagiousness are the two differences that matter most. Cold sores sit outside the lips and spread easily from person to person; canker sores form inside the mouth and can't be passed on at all (Cleveland Clinic – Cold Sores; Cleveland Clinic – Canker Sores). The table below covers every distinguishing feature.
| Feature | Cold Sore (Fever Blister) | Canker Sore (Mouth Ulcer) |
|---|---|---|
| Cause | Herpes simplex virus, most commonly HSV-1 (occasionally HSV-2) | Not caused by a virus and not an infection; the exact cause is unknown |
| Contagious? | Yes — highly contagious | No — not contagious at all |
| Typical location | Outside the lips, around the mouth, sometimes on the nose or chin | Inside the mouth — tongue, inner cheeks, gums, inner lips, or roof of the mouth |
| Appearance | Clusters of small, fluid-filled blisters that crust over | Open, shallow oval or round ulcers with a white or yellow centre and a red border |
| Common triggers | Illness or fever, sun exposure (UV light), stress, hormonal changes, fatigue or weakened immunity | Not fully understood; may follow minor injury, stress, or nutritional factors |
| Is it herpes? | Yes — it is a herpes simplex virus infection | No — despite the name, canker sores are not related to the herpes virus |
| Also known as | Fever blister | Aphthous ulcer / mouth ulcer |
What Is a Cold Sore (Fever Blister)?
A cold sore is a contagious sore on or around the lips caused by the herpes simplex virus, most commonly HSV-1 (Cleveland Clinic). You'll also hear it called a fever blister — the two names describe exactly the same condition. There's no medical difference between a "cold sore" and a "fever blister," and both names come from the same observation: outbreaks often flare up when your body is run down by a cold, a fever, or another illness.
Once you're infected — usually through skin-to-skin contact like kissing — the virus stays in your body permanently, lying dormant in nerve cells. It doesn't cause symptoms most of the time. Then something wakes it up.
Common reactivation triggers include:
- Illness or fever (hence "fever blister")
- Sun exposure (UV light)
- Stress
- Hormonal changes
- Fatigue or weakened immunity
What they look like: Cold sores usually announce themselves with a tingling or burning sensation on the lip border a day or two before anything is visible. A cluster of small fluid-filled blisters then forms, breaks, oozes, and crusts over into a scab. The whole cycle takes about 7 to 10 days.
Important: Cold sores are contagious. They spread through kissing, sharing utensils or drinks, or touching the sore and then touching someone else. They're most contagious while the blisters are open and oozing, but transmission can occur even when no sore is visible (Cleveland Clinic).
What Is a Canker Sore?
A canker sore (aphthous ulcer) is a non-contagious open sore that forms on the soft tissue inside the mouth. Unlike cold sores, canker sores are not caused by a virus — they're not an infection at all, and they're not related to herpes despite the similar-sounding name (Cleveland Clinic).
The exact cause of canker sores isn't fully understood, but known triggers include:
- Physical trauma (biting your cheek, dental work, hard-bristled toothbrush)
- Stress and fatigue
- Certain foods (citrus, spicy food, tomatoes)
- Nutritional deficiencies (B12, iron, folate, zinc)
- Sodium lauryl sulfate (SLS) in toothpaste
- Hormonal changes
For a complete breakdown, read what causes canker sores.
What they look like: Canker sores are small, shallow, oval-shaped ulcers with a white or yellowish centre and a red inflamed border. They appear on the inside of the lips, cheeks, tongue, gums, or roof of the mouth — never on the outside of the lips.
Types of canker sores:
- Minor (most common) — small, oval, heal within about two weeks without scarring
- Major — larger than 1 cm, deeper, extremely painful, slower to heal and may scar
- Herpetiform — clusters of tiny (1-2 mm) ulcers that can merge into larger irregular sores. Despite the name, these are not related to the herpes virus
The Location Test — The Fastest Way to Tell
The single most reliable way to distinguish between a cold sore and a canker sore:
- Outside the lips or on the lip border — almost certainly a cold sore
- Inside the mouth (tongue, cheek, gum, inner lip) — almost certainly a canker sore
This rule isn't 100% absolute. Cold sores occasionally appear on the hard palate inside the mouth, and in rare cases on other oral surfaces. But for the vast majority of people, location is the definitive indicator.
Searched for "cold sore inside lip" or "cold sore inside mouth"? What you're seeing is most likely a canker sore. Canker sores frequently appear on the inner lip, which is exactly why the confusion happens.
Can You Have Both?
Yes. Some people get both cold sores and canker sores, sometimes in the same period. If you're unsure which you're dealing with, ask yourself:
- Where is it? Outside = cold sore. Inside = canker sore.
- Did it start with fluid-filled blisters? Blisters = cold sore. Open ulcer from the start = canker sore.
- Was there tingling before it appeared? Tingling prodrome = cold sore. No warning = canker sore.
- Does anyone near you have a similar sore? Possible transmission = cold sore. No transmission pattern = canker sore.
Still unsure after these questions? See your doctor or dentist. They can usually diagnose by visual examination alone.
Treatment — Why It Matters Which One You Have
Cold sores and canker sores respond to completely different treatments. A viral infection outside the lips needs antiviral therapy; a non-viral ulcer inside the mouth needs pain relief and wound protection (Cleveland Clinic). Using the wrong treatment on the wrong sore wastes time and money — and won't help you heal.
How to Treat a Cold Sore
Cold sores are caused by a virus, so they respond to antiviral medication:
- Prescription antivirals — valacyclovir (Valtrex) or acyclovir can shorten outbreaks and reduce severity, especially when taken at the first sign of tingling
- OTC antiviral cream — docosanol (Abreva) may reduce healing time when applied early
- Do not use canker sore treatments on cold sores. Canker sore products — including chemical cautery treatments — are designed for ulcers inside the mouth. They are not designed for cold sores and will not help. Speak with your pharmacist about antiviral options instead
How to Treat a Canker Sore
Canker sores are not viral, so antivirals won't help. Effective treatments focus on pain relief and protecting the ulcer so it can heal:
- Chemical cautery — ORALMEDIC seals the canker sore in a single application. According to the manufacturer, this provides immediate pain relief and promotes healing in 3 to 5 days; for comparison, untreated minor canker sores typically heal on their own within about two weeks (Cleveland Clinic)
- Benzocaine gels — Orajel and Anbesol temporarily numb pain for 1-2 hours (reapply up to 4x daily)
- Salt water rinse — helps keep the area clean and reduces swelling
- Anti-inflammatory rinses — benzydamine (Tantum) reduces inflammation
For a full comparison of canker sore treatments, read how to get rid of a canker sore fast.
Dealing with a canker sore (inside the mouth)? A single-application treatment seals the ulcer and relieves pain on contact.
Shop the mouth ulcer treatment
For canker sores only — not for use on cold sores or any sore outside the lips.
Frequently Asked Questions
Is a fever blister the same as a cold sore?
Yes. "Fever blister" is simply another name for a cold sore — the two terms refer to the same condition. Both describe clusters of small, fluid-filled blisters caused by the herpes simplex virus, usually HSV-1, that form on or around the lips and crust over as they heal. They are contagious (Cleveland Clinic).
Are canker sores contagious?
No. Canker sores are not caused by a virus or bacteria and cannot spread from person to person (Cleveland Clinic). You cannot "catch" a canker sore from kissing, sharing utensils, or any other form of contact.
Are cold sores a sign of an STI?
Cold sores are most commonly caused by HSV-1, which is typically transmitted through non-sexual contact such as kissing or sharing drinks — and HSV-1 is extremely common in the general population (Cleveland Clinic). While HSV-2 (which more commonly causes genital herpes) can occasionally cause oral sores, most cold sores are HSV-1 and are not considered an STI.
Can a canker sore turn into a cold sore (or vice versa)?
No. They are fundamentally different conditions. A canker sore cannot become a cold sore, and a cold sore cannot become a canker sore. If a sore changes in appearance or behaviour, see your healthcare provider.
Can ORALMEDIC treat cold sores?
No. ORALMEDIC is designed specifically for canker sores (aphthous ulcers) inside the mouth. It should not be used on cold sores, fluid-filled blisters, or any sore on the outside of the lips. If you have a cold sore, speak with your pharmacist about antiviral options.
This article is for general information only and is not a substitute for professional dental or medical advice. If a mouth ulcer lasts longer than three weeks, is unusually large or painful, or keeps recurring, see a dentist or physician. ORALMEDIC is a Class I medical device; always follow the product instructions.