Are Canker Sores Contagious?

I see this question constantly on forums and answer sites all over the internet. “Are canker sores contagious?!”

It’s an understandable question, especially when you’re young and dating. Either you’re the one with the canker sore and want to know what to tell people when they ask, or you’re dating someone with a canker sore and want to make sure you don’t get it yourself.

There’s a short answer and a long answer to this.


Are canker sores contagious?

The short answer: No.

The long answer: This is an extremely common misconception about canker sores. So much so, it’s often considered common knowledge. But like most common knowledge, it’s not true.

This comes from the prevailing but unfortunate confusion between canker sores and herpes. Anybody who has spent any time on this site knows I spend about 90% of my time debunking this myth. So much so that I actually got a herpes test just to prove the myth wrong. You can see the results here.

Long story short… I don’t have herpes. And I’ve gotten canker sores my whole life.

Canker sores are an immune system dysfunction, and are not caused by a bacterial or viral infection. Now, a viral or bacterial infection in your body can trigger your immune system response, which in turn can bring on canker sores, but it’s not a firsthand cause.

I’ve dated many women in my life and I’ve never given a single one of them a canker sore. Not one. Ever.


How do you know if it’s a canker sore or herpes?

There are three telltale signs:

1. Location. Cold sores appear mostly on the lips, and canker sores appear mostly inside the mouth. There are exceptions in both cases, but the overwhelming majority of the time, this is the case.

open mouth contagious


2. Blistering. Cold sores form as fluid-filled blisters. Canker sores do not.

Blister contagiousCrater not contagious


3. Clustering. This one is most important. Herpes lesions, which only rarely appear inside the mouth (known as intraoral lesions), always appear in clusters of ulcers, usually 4 or more. Canker sores very rarely appear this way, less than 10% of canker sores will cluster like this.

grape cluster contagious


single grape not contagious





I wrote extensively on this topic in my last post.

So… Are canker sores contagious?

If you or your partner have a single canker sore, or multiple canker sores in different parts of your mouth, kiss away. You’re not going to give it to someone else or catch it. If what you have appears on your lips in clusters of blisters – you might want to just hold hands for a few days. And see your doctor. There are some medicines you can take to reduce the symptoms and make them go away faster.

Now, spread the word about this. We’ve gotta bust this myth somehow.


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