Mouth ulcers and cold sores: What’s the difference?

27th February, 2020 • 5 min read

When a painful sore appears in or around your mouth, there’s a good chance you may have a mouth ulcer.

Or could it be a cold sore?

It’s not always easy to tell. Both appear on the same part of the body and can be triggered by similar factors, like stress or illness.

But knowing how to spot the difference is worthwhile because both conditions have different causes and treatments. Most importantly, cold sores are highly infectious and can be passed to others if you don’t realise you have one.

What’s the difference between a mouth ulcer and cold sore?

There are 4 main differences between a mouth ulcer (otherwise known as a canker sore) and a cold sore:

  • how they look
  • where they appear
  • what causes them
  • if they are contagious

What does a cold sore or mouth ulcer look like?


mouth ulcer
is a round or oval sore that can be red, grey, white or yellow in colour. You may have more than 1 at a time, and the sore can swell up and change size.

While mouth ulcers can feel painful, they usually heal on their own within 2 weeks.

Mouth ulcers tend to cause discomfort only when the sore has formed, but if you have a

cold sore
you will often feel it before you see it. That’s because cold sores typically begin as a tingle, burn or an itch a few hours or a day before 1 or more small, fluid-filled blisters appear.

These blisters often feel sore and may burst or weep.

It can take a few days for the sore to form a scab, and up to a week for the scab to disappear.

Where do cold sores and mouth ulcers appear?

Mouth ulcers appear inside the mouth. This may be on the tongue, cheek or inner surface of the lips.

In contrast, cold sores are usually found on the lips or around the outside of the mouth. However, they can appear anywhere on the face.

What causes cold sores and mouth ulcers?

It’s not always clear what causes a mouth ulcer. However, physical damage to your mouth can play a role. This includes:

  • biting the inside of your cheek
  • badly-fitting dentures or braces
  • a sharp tooth or rough fillings
  • cuts or burns while eating or drinking
  • damage caused by a toothbrush or irritating toothpaste

Mouth ulcers can also appear when you’re tired, stressed or anxious, or if you’ve just stopped smoking.

Furthermore, hormonal changes (such as being pregnant), medical conditions, certain medicines and low levels of

vitamin B12
can also trigger mouth ulcers.

While mouth ulcers don’t always have a specific cause, cold sores do.

Cold sores are caused by the herpes simplex virus (HSV), and once you have the virus, it stays with you for life. In most cases, the virus remains asleep in the body, but certain triggers can activate it and cause a cold sore.

These triggers include:

  • being ill
  • having your monthly period
  • exposure to strong, direct sunlight
  • stress or being ‘run down’

Are cold sores and mouth ulcers contagious?

Mouth ulcers aren’t contagious - you can’t pass them on to other people. However, the opposite is true for cold sores, which are infectious and typically passed through direct skin contact, such as kissing.

Cold sores are contagious while you have symptoms, but they are most infectious when the blister bursts. If you have a cold sore, avoid kissing anyone, especially newborn babies or people with a weakened immune system, until the sore is completely healed.

You should also take care to prevent contact between the cold sore and anyone else’s skin. This means that you should avoid activities, such as oral sex and sharing lipsticks, cold sore creams, cutlery and other items that may have touched the sore.

When to see a doctor for a mouth ulcer or cold sore

Mouth ulcers and cold sores usually clear up on their own within 2 weeks. However, you should see a doctor if you have a mouth ulcer that:

  • doesn’t clear up within 3 weeks
  • keeps coming back
  • becomes painful and red (this may be a sign that further treatment is needed)

If you have a cold sore, see a doctor if:

  • it doesn’t heal within 10 days
  • it’s large or painful
  • you have swollen and painful gums
  • you’re pregnant
  • you have a condition that can weaken your immune system, such as cancer or

Cold sores and mouth ulcers can be painful while they heal. However, there are things you can do yourself to help speed up the healing process.

Find out more about

treating mouth ulcers
treating cold sores


Mouth ulcers [Internet]. 2020 [cited 21 January 2020]. Available


Cold sores [Internet]. 2020 [cited 21 January 2020]. Available


Knott D. Cold Sores | Herpes Simplex Virus (HSV) | Symptoms and Treatment [Internet]. 2020 [cited 21 January 2020]. Available

Mouth ulcers - Your.MD [Internet]. Your.MD. 2020 [cited 6 February 2020]. Available

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.