Overview
Both fever blisters and canker sores are common across all ages and can be painful. However, they are separate conditions with different causes. Read further to learn how to tell them apart and find out which treatments might help ease the pain.
Fever Blisters (also known as cold sores) are painful fluid-filled sores that form on the outside of the mouth around the lips. Fever blisters are very contagious.
Canker Sores are painful white or yellow sores that only form inside the mouth, usually on the insides of the cheeks or lips or on the tongue. Canker sores (also called aphthous ulcers) are not contagious.
Causes
Fever Blisters are caused by an infection with the herpes simplex virus, usually type 1, or HSV-1. It is estimated that nearly 90 percent of U.S. adults have been infected with the virus, although many people with the infection do not ever show symptoms. The virus generally resides in the nerve cells where it may periodically reactivate, traveling to the skin and causing recurring fever blisters.
Canker Sores do not have a known cause. They may be triggered by an injury, stress, smoking, or deficiencies in folic acid, iron, or vitamin B12.
Symptoms
Symptoms of fever blisters:
- Fluid-filled blisters on the outside of the mouth around the lips.
- Burning or tingling sensation at the site where the blisters will appear.
- And sometimes, fever, fatigue, or swelling of the lymph nodes, similar to other viral infections.
Symptoms of canker sores:
- A round white or yellow sore with a red border inside the mouth, usually on the inside of the lips or cheeks or on the tongue.
- A burning or tingling sensation may occur in the mouth prior to the formation of the sore.
Diagnosis
The main way to tell the difference between a fever blister and a canker sore is by location. Fever blisters occur outside the mouth, generally around the border of the lips. Canker sores occur inside the mouth.
They also differ in appearance: Fever blisters are patches of several small fluid-filled blisters, while canker sores are usually single round white or yellow sores with a red border.
Treatment
Typically, both types of sores go away on their own and treatment is not usually required. There is no known treatment that can permanently cure fever blisters or canker sores and prevent them from recurring. Treatment focuses on minimizing the pain and speeding recovery time.
Fever blisters can be treated with prescription antiviral medications either applied as topical creams or taken orally. See a doctor if your cold sores persist longer than a few weeks, you develop a high fever, or you notice eye irritation.
Canker Sores can be treated with over-the-counter (non-prescription) gels to numb the pain. Avoiding spicy or abrasive foods may also help. Rinsing the mouth with over-the-counter antiseptic medications may help prevent infection of the sore and speed healing.
See a doctor if you have frequently recurring canker sores or if the sores are severe enough to make eating or drinking difficult. If you have recurring canker sores as a result of a vitamin deficiency, treating the deficiency may reduce their frequency.
Helpful Tips
Both fever blisters and canker sores can have triggers that produce flare ups. Try to identify and then avoid triggers so your flare ups become less frequent.
Tips for Fever Blisters
- Sun exposure may sometimes trigger outbreaks of fever blisters. If you have recurring blisters, using sunscreen may help reduce their frequency.
- Fever blisters are contagious. Avoid kissing, sharing eating utensils, cups, water bottles, or other items if you are experiencing symptoms.
- Be aware that in young children, fever blisters can cause loss of appetite, drooling, and fever that can last for several days.
- Talk to your doctor or your child’s pediatrician if symptoms are severe or recur frequently.
Tips for Canker Sores
- If you regularly get canker sores, talk to your doctor about testing for food allergies, as allergic reactions can sometimes trigger canker sore outbreaks.
Additional Resources
- MedlinePlus (Fever Blisters)
Information about oral herpes (the infection that causes cold sores) from the NIH National Library of Medicine. - MedlinePlus (Canker Sores)
Information about canker sores from the NIH National Library of Medicine.
Cold sores are common and usually clear up on their own within 10 days. But there are things you can do to help ease the pain. A cold sore usually starts with a tingling, itching or burning feeling. Over the next 48 hours: Cold sores should start to heal within 10 days, but are contagious
and may be irritating or painful while they heal. Some people find that certain things trigger a cold sore, such as another illness, sunshine or menstrual periods.
Other possible causes of your symptomsCheck if it's a cold sore
SymptomsPossible cause Painful red spot on the face filled with pus
Spot or boil
Blister on the inside of the lip or mouth
Mouth ulcer
Red sores or blisters on the face that become crusty, golden-brown patches
Impetigo
How long cold sores are contagious
Cold sores are contagious from the moment you first feel tingling or other signs of a cold sore coming on to when the cold sore has completely healed.
A pharmacist can help with cold sores
A pharmacist can recommend:
- creams to ease pain and irritation
- antiviral creams to speed up healing time
- cold sore patches to protect the skin while it heals
You can buy electronic devices from pharmacies that treat cold sores with light or lasers.
Some people find these helpful, but there have not been many studies to confirm if they work.
Information:
If you regularly get cold sores, use antiviral creams as soon as you recognise the early tingling feeling. They do not always work after blisters appear.
Find a pharmacy
Things you can do yourself
Cold sores take time to heal and they're very contagious, especially when the blisters burst.
Important: Important
Kissing a baby if you have a cold sore can lead to neonatal herpes, which is very dangerous to newborn babies.
Do
eat cool, soft foods
wash your hands with soap and water before and after applying cream
avoid anything that triggers your cold sores
use sunblock lip balm (SPF 15 or above) if you're outside in the sun
take paracetamol or ibuprofen to ease pain and swelling (liquid paracetamol is available for children) – do not give aspirin to children under 16
drink plenty of fluids to avoid dehydration
Don’t
do not kiss anyone while you have a cold sore
do not have oral sex until your cold sore completely heals as you could give your partner genital herpes
do not touch your cold sore (apart from applying cream) – if you do wash your hands
do not rub cream into the cold sore – dab it on instead
do not eat acidic or salty food if it makes your cold sore feel worse
Non-urgent advice: See a GP if:
- the cold sore has not started to heal within 10 days
- you're worried about a cold sore or think it's something else
- the cold sore is very large or painful
- you or your child also have swollen, painful gums and sores in the mouth (gingivostomatitis)
- you have a weakened immune system – for example, because of chemotherapy or diabetes
Treatment from a GP
A GP may prescribe antiviral tablets if your cold sores are very large, painful or keep coming back.
Newborn babies, pregnant women and people with a weakened immune system may be referred to hospital for advice or treatment.
Why cold sores come back
Cold sores are caused by a virus called herpes simplex.
Once you have the virus, it stays in your skin for the rest of your life. Sometimes it causes a cold sore.
Most people are exposed to the virus when they're young after close skin to skin contact, such as kissing, with someone who has a cold sore.
It does not usually cause any symptoms until you're older. You will not know if it's in your skin unless you get a cold sore.
Page last reviewed: 20 July 2020
Next review due: 20 July 2023