Everyday Health

Cold Sores vs. Canker Sores: What’s the Difference?

Originally published October 2, 2019

Last reviewed May 3, 2022

Reading Time: 3 minutes

These two mouth irritants aren’t the same thing. Find out how to tell which one you might have and, better yet, what to do about them.

Although sores in and around the mouth may be embarrassing, they shouldn’t be, because they’re very common. At least half the population gets canker sores, and about the same amount gets the virus that can cause cold sores. But the two types of sores are completely different, so here’s what you need to know to tell them apart.

1. Known versus unknown causes

Cold sores, also called fever blisters or oral herpes, are most often caused by the herpes simplex virus type 1, which people usually pick up in childhood. Although the virus then settles into bundles of nerve cells in the body and may never bother you again, it stays with you for the rest of your life and can pop up from time to time.

The causes of canker sores, or aphthous ulcers, are less known. It may be that the immune system mistakenly attacks the mucous cells of the mouth. Canker sores are also linked to Crohn’s disease and other systemic conditions, including nutritional deficiencies; they may also be caused by stress, hormonal shifts or allergic reactions to certain ingredients in food or toothpaste.

2. Oozing blister outside the mouth versus painful ulcer inside

The way cold sores and canker sores appear is very different, too. Cold sores occur most often outside your mouth, in the corners or on your lip. If you find one inside, it’s usually on harder tissue, such as your gums or the roof of your mouth. Cold sores form blisters that then break open, ooze and crust over. You may feel a tingling or burning sensation before a blister appears.

Canker sores are small and round or oval and develop in the soft tissue inside your mouth. You might get one behind your lip, on the inside of your cheek or under your tongue. They’re red, flat and often have a whitish top. If they become large or painful, they may prevent you from comfortably eating and drinking.

3. Contagious versus not contagious

Perhaps the most important thing to know is whether these conditions can be passed to others. Cold sores are highly contagious, especially when you have an active one — but you can transmit the virus even when you don’t. It’s spread by saliva or skin contact. If you touch your cold sore, wash your hands right away to avoid spreading the virus to other parts of your skin.

Canker sores, on the other hand, aren’t contagious, so you don’t have to worry about giving them to anyone else.

If they do show up, what’s the best treatment?

Most of the time, both types of sores will heal by themselves within a week or two, but medications are available to ease their effects.

For cold sores, taking an antiviral medication at the earliest sign may reduce pain and shorten the outbreak, according to Sharon Orrange, MD, an internal medicine physician at Keck Medicine of USC and clinical associate professor of medicine at the Keck School of Medicine of USC. The type of medication you choose, however, can have an impact.

“Topical medications for cold sores don’t work that well, are expensive and usually require frequent daily applications,” Orrange says. “But evidence shows that drugs such as valacyclovir effectively reduce both healing time and the duration of pain.”

For small, painful canker sores, she suggests using an over-the-counter numbing medication. But if you have large painful canker sores that are making it difficult to eat, you should ask your physician for help.

“Prescription pastes and gels are available that can help speed your healing,” Orrange says. “Also, see your doctor if your canker sores recur, because the recurrence could indicate celiac disease, inflammatory bowel disease or HIV.”

Although they’re annoying, sometimes painful and can’t be cured, there’s good news about canker and cold sores: In the majority of cases, both are relatively harmless and don’t have lasting consequences.


canker sores
celiac disease
cold sores
Crohn’s disease
Dr. Sharon Orrange
Tina Donvito
Tina Donvito is a freelance writer covering health, culture, travel and parenting.