Is It a Canker Sore or a Cold Sore?–Differences, Causes, and Treatment

Many of us use the terms "cold sores" and "canker sores" interchangeably. After all, they're both pesky, painful mouth sores, right? But, it turns out there are more differences than similarities between the two conditions. Here's how to tell if you have a canker sore versus a cold sore—and what to do about them.

What Is a Canker Sore?

Canker sores—also called aphthous ulcers—show up as open sores on the inside of your mouth, usually on the inner parts of your cheeks or lips or on your tongue, said Paras Bharatkumar Patel, DDS, assistant professor of oral and maxillofacial pathology at Texas A&M College of Dentistry in Dallas. "You wouldn't find them on the gingival [gum] tissue or the hard palate," he said, although they can appear at the very base of your gums.


There are many symptoms of canker sores, and they follow a general progression. The typical symptoms are:

  • A small bump inside the mouth that bursts a day or two later
  • A whitish or yellowish area with a red border once the bump bursts
  • Typically appear in clusters
  • Range from ⅛ of an inch to 1¼ inches in diameter


No one knows exactly what causes or triggers canker sores—unlike cold sores—they aren't contagious. However, causes may include:

  • A virus
  • An allergy to food or medicine
  • Stress
  • Smoking
  • A nutritional deficiency like low vitamin B12, folic acid, or iron
  • Hormones—which may account for why twice as many females as men get canker sores
  • Spicy or acidic foods, like tomatoes and citrus fruit
  • A family history of canker sores
  • People who have dry mouths—which can also be due to some medications

Canker sores may also be due to a physical injury. "At times, I see them show up a few days after some traumatic event: contact sports, a frayed brush, a sharp potato chip, or even poorly fitting oral devices," said Tyrone Rodriguez, DDS, a former spokesperson for the American Dental Association.

Many canker sores remain a mystery. "Not all cases have a single known cause, as there are [many] possibilities that may not all be able to be explored," said Patel.


In most cases, you can let canker sores run their course. Most go away within a couple of weeks. For pain—which usually only lasts three or four days—you can try acetaminophen (Tylenol) or use an oral anesthetic gel like benzocaine. Stay away from nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen since they may cause canker sores in some people.

Some at-home remedies also work, like gargling with a solution of salt and baking soda in water. Avoid toothpaste and mouthwash with an ingredient called sodium lauryl sulfate, which may contribute to canker sores.

In some cases, canker sores can be a sign that something more serious is going on, including celiac disease or another autoimmune condition. "People who have recurrent canker sores should probably see a doctor who really understands these other disease processes," said Jordan Josephson, MD, an ear, nose, and throat specialist at Lenox Hill Hospital in New York City.

Also, see a healthcare provider if your canker sores make it hard to eat or drink or if they last longer than two weeks.

What Is a Cold Sore?

Unlike canker sores, cold sores appear outside your mouth, often on the corner of your lip. Also called fever blisters, they are, in fact, little blisters.


Symptoms of cold sores include:

  • Tingling in the area a day or two before where the cold sore appears
  • A blister that breaks quickly, leaks fluid, and scabs over
  • Most severe pain in the first 24 hours after the cold sore appears


There's no mystery about what causes cold sores: It's the herpes simplex virus 1 or HSV-1. Once you have HSV-1, it never goes away. Herpes viruses are extremely contagious and very common.

After the initial infection, HSV-1 travels up the nerve, staying out of sight until something triggers it. Then, it migrates back down the nerve to the mouth. "Once it establishes itself, any time there's trauma or any sort of transient lapse in immunity, the virus is going to manifest itself," said Patel.

Triggers of cold sores can include:

  • Stress
  • Sun exposure
  • Fatigue
  • Your period
  • Other infections (like a cold)

If you know your triggers, you may be able to prevent an attack. For example, by reducing stress or putting zinc on your lips before going outside.


Most cold sores go away in their own time, usually seven to 10 days after they appear. The pain often reaches a peak in the first 24 hours. In the meantime, there's little you can do except address the pain. You can try icing the blisters or taking over-the-counter pain relievers.

Topical antiviral creams or ointments may lessen pain or quicken healing, but only if applied at the very beginning of the outbreak or, even better, during the tingling phase.

Cold sores generally aren't harmful unless your immune system is weakened, potentially because of another condition like HIV/AIDS or cancer treatment. In these cases, there is a greater risk of the virus affecting the brain or spinal cord, and, said Patel, "you're going to have to treat them a little more aggressively." That could mean taking an oral antiviral, like acyclovir, on a long-term basis, Patel said.

Cold sores also spread like wildfire, so it's important to take steps to prevent them. "You've got to be careful not to share straws, lip balm, or lipstick, or drink from the same glass," said Rodriguez.

If you happen to touch a cold sore, wash your hands right after, and make sure you don't touch any other part of your body, especially the eyes, because you can transmit HSV-1 to yourself. "Just ignoring cold sores isn't necessarily the best way to take care of them, as they can spread to other body parts," said Rodriguez. Don't scratch or pick at the blisters, either.

Canker Sore Vs. Cold Sore: What's the Difference?

Illustration of a canker and cold sore

Illustration by Paige McLaughlin for Health

One of the main differences between cold sores and canker sores is location. Canker sores usually appear inside your mouth, while cold sores appear on the outside.

The two sores are also caused by different things. "One [cold sores] is clearly caused by viruses, and the other could be different things for different people," said Patel.

Cold sores are also extremely contagious, while canker sores are not.

While canker sores tend to last a little longer, both types are harmless unless you have an underlying immune system problem.

A Quick Review

While both canker sores and cold sores are common, the cause of canker sores is unknown, and the cause of cold sores is the HSV-1 virus. While both can be painful, canker and cold sores generally need to run their course. If you experience either of these, home remedies are often helpful.

However, if you experience regular or prolonged canker sores, reach out to a healthcare provider to rule out any conditions causing your canker sores. And, if you experience cold sores and have an underlying health condition that affects your immune system, consider asking a healthcare provider about oral antiviral treatments.

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  1. MedlinePlus. Canker sores.

  2. MedlinePlus. Cold sores.

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