They can look surprisingly similar at first glance, but canker sores and cold sores are two completely different conditions, with different causes, different treatments, and one very important difference: whether they’re contagious.
The Quickest Way to Tell Them Apart
Location is your best clue. Canker sores form inside the mouth, on the inner cheeks, gums, tongue, or the soft tissue near the base of your gums. Cold sores form outside the mouth, typically on or right around the lips, though they can occasionally appear just inside.
If you can see the sore in a mirror without pulling your lip or cheek away from your teeth, it’s very likely a cold sore.
Canker Sores: The Basics
Canker sores (also called aphthous ulcers) are small, shallow lesions with a white or yellowish center and a red border. They’re not caused by a virus and are not contagious.
Common triggers include:
- Minor mouth injuries, like accidentally biting your cheek or a rough spot from braces
- Acidic or spicy foods
- Stress
- Hormonal changes
- Certain toothpaste ingredients, particularly sodium lauryl sulfate (SLS)
- Vitamin deficiencies, especially B12, folate, iron, or zinc
Canker sores are more common in women, teens, and young adults, and they tend to run in families. Most heal on their own within one to two weeks without leaving a scar. Larger canker sores (over about a centimeter) can be more painful and take longer to heal.
Cold Sores: The Basics
Cold sores (also called fever blisters) are caused by the herpes simplex virus, almost always HSV-1. They start as small, fluid-filled blisters that cluster together, then break open, ooze, and crust over as they heal, usually within 10 to 14 days.
Cold sores are highly contagious, spread through direct contact like kissing, sharing utensils or lip balm, or even close contact when a sore isn’t visibly present (something called asymptomatic shedding).
Common triggers for an outbreak include:
- Stress
- Sun or wind exposure
- Fever or illness
- Hormonal changes
- A weakened immune system
Once you’ve contracted HSV-1, the virus stays in your body permanently and can reactivate periodically throughout your life, even years apart.
Side-by-Side Comparison
| Canker Sore | Cold Sore | |
|---|---|---|
| Location | Inside the mouth | On or around the lips |
| Contagious | No | Yes |
| Cause | Not fully understood; multiple triggers | Herpes simplex virus (HSV-1) |
| Appearance | Flat ulcer, white/yellow center, red border | Fluid-filled blister, clusters, crusts over |
| Healing time | 1 to 2 weeks | 10 to 14 days |
| Recurs? | Can recur, not virus-driven | Recurs throughout life once infected |
Managing Each One
For canker sores:
- Rinse with warm salt water or a baking soda solution
- Avoid acidic and spicy foods while it heals
- Over-the-counter oral gels can numb the area temporarily
For cold sores:
- Over-the-counter antiviral creams can shorten an outbreak if used at the first sign of tingling
- Avoid touching the sore, and wash your hands if you do
- Avoid kissing or sharing utensils, cups, or lip products until it’s fully healed
- A prescription antiviral medication may help if outbreaks are frequent or severe
When to See a Dentist or Doctor
Most canker sores and cold sores resolve on their own, but it’s worth scheduling an oral health consultation if:
- A sore is unusually large or doesn’t heal within two to three weeks
- Sores are recurring frequently
- A cold sore is accompanied by a high fever or swollen glands
- You’re not sure which type you’re dealing with, especially if it’s your first outbreak
FAQ
Can I have a canker sore and a cold sore at the same time?
It’s uncommon but possible, since they’re unrelated conditions. Stress or illness can occasionally trigger both around the same time.
Are cold sores the same as genital herpes?
They’re related but distinct. Cold sores are most often caused by HSV-1, while genital herpes is more commonly caused by HSV-2, though either strain can technically appear in either location depending on how it was transmitted.
Can toothpaste cause canker sores?
Some people are sensitive to sodium lauryl sulfate (SLS), a foaming agent in many toothpastes, and switching to an SLS-free formula has been reported to reduce canker sore frequency for some people.
Is a cold sore contagious even before the blister appears?
Yes, to some degree. The tingling or itching sensation that often precedes a visible blister can already involve viral shedding, which is part of why avoiding close contact at the first sign of a cold sore is a good precaution.
Do canker sores mean I have a vitamin deficiency?
Not necessarily, but recurring canker sores can sometimes be linked to low B12, iron, folate, or zinc. If they’re frequent, it’s worth mentioning to your dentist or doctor, who may suggest checking your levels.
Get the Right Diagnosis
If you’re not sure what you’re dealing with, or a sore isn’t healing the way it should, Dr. Heidi Finkelstein and our team at My Plantation Dentist can take a look and point you toward the right treatment. Schedule an appointment today.
Dr. Heidi Finkelstein, My Plantation Dentist
