Cold sores and canker sores are the two most common mouth sores, and they're constantly confused with each other. That confusion matters because they have completely different causes and need completely different treatments.
The short version: a cold sore is a contagious viral infection that appears on the outside of the lips. 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.
The Key Differences at a Glance
Cold sores appear outside the lips, around the mouth, sometimes on the nose or chin. Canker sores appear inside the mouth — on the tongue, inner cheeks, gums, inner lips, or roof of the mouth. Cold sores are clusters of small fluid-filled blisters that crust over; canker sores are open, shallow oval or round ulcers with a white or yellow centre and red border. Cold sores are caused by the herpes simplex virus (HSV-1, occasionally HSV-2) and are highly contagious. Canker sores are not contagious at all. Cold sores often begin with a tingling, burning, or itching prodrome 1-2 days before blisters appear; canker sores have no warning — the ulcer just appears. Cold sores heal in 7-10 days; minor canker sores in 7-14 days, major ones up to 6 weeks. Cold sores are treated with antiviral medication (acyclovir, valacyclovir); canker sores with OTC gels, rinses, or chemical cautery (ORALMEDIC).
What Is a Cold Sore?
A cold sore (fever blister) is caused by the herpes simplex virus, most commonly HSV-1. Once you're infected — usually through skin-to-skin contact like kissing — the virus remains in your body permanently, living dormant in nerve cells near the base of the brain.
Cold sores reactivate periodically, triggered by:
- Illness or fever
- Sun exposure (UV light)
- Stress
- Hormonal changes
- Fatigue or weakened immunity
What they look like: Cold sores start as a tingling or burning sensation on the lip border, then develop into a cluster of small fluid-filled blisters. The blisters eventually break, ooze, and form a crusty scab. The whole cycle takes about 7 to 10 days.
Important: Cold sores are contagious. They can spread through kissing, sharing utensils, or touching the sore and then touching someone else. They're most contagious when blisters are open and oozing, but transmission can occur even when no sore is visible.
What Is a Canker Sore?
A canker sore (aphthous ulcer) is a non-contagious open wound 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.
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 1-2 weeks without scarring
- Major — larger than 1 cm, deeper, extremely painful, can take up to 6 weeks 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 very rare cases on other oral surfaces. But for the vast majority of people, location is the definitive indicator.
If you've 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 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.
If you're 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. Using the wrong one wastes time and money.
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 by about a day when applied early
- Do not use canker sore treatments on cold sores — products like ORALMEDIC are not designed for cold sores and will not help
How to Treat a Canker Sore
Canker sores are not viral, so antivirals won't help. Effective treatments focus on pain relief and accelerating healing:
- Chemical cautery — ORALMEDIC seals the canker sore in a single application, providing immediate pain relief and promoting healing in 3 to 5 days
- 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.
Frequently Asked Questions
Are canker sores contagious?
No. Canker sores are not caused by a virus or bacteria and cannot spread from person to person. 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 (kissing, sharing drinks). HSV-1 is extremely common — an estimated two-thirds of people under 50 worldwide carry it. 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). 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.