What Causes Cold Sores On Lips?

Kristin Hall

Medically reviewed by Kristin Hall, FNP

Written by Our Editorial Team

Last updated 8/10/2022

There’s nothing like a cold sore to make you want to hide indoors for a week. They’re ugly, they hurt and they’re not easy to hide. In a nutshell, they’re a big bummer.

But what causes them? 

We’re going to get straight to the point here: the herpes virus causes cold sores in or around your mouth and lips. Yes, herpes. Similar to genital herpes, but not quite the same. 

Before you panic about oral herpes, read on to learn why you shouldn’t be ashamed of having a cold sore, more about the causes them and what you can do to prevent and treat them. 

Cold sores are common, and part of life for many people all over the world. In fact, according to the World Health Organization, about 3.7 billion people worldwide under the age of 50 have the HSV-1 virus — the type of herpes virus that most commonly causes cold sores.

What Are Cold Sores? 

Cold sores are a symptom of a herpes simplex virus called HSV-1. HSV-1 is the most common cause of oral herpes. Less often, however, it can also be the cause of genital herpes. 

The opposite is true for HSV-2. It is the most common cause of genital herpes but, less often, is the cause of oral herpes. Cold sores are also sometimes called fever blisters and can affect the lips, inside of the mouth and gums. Cold sores are fluid-filled blisters, often found in clusters. When the blisters rupture, a crust or scab eventually forms over the them.

Now, before you freak out about your cold sore, you should know that HSV-1 is common. 

Some studies indicate that more than half (57 percent) of people between the ages of 14 and 49 in the U.S. have HSV-1. This means, in theory, more than half of your friends have it, likely one of your parents and probably two of your grandparents. So, there’s really nothing to be ashamed about. 

Cold sores caused by HSV-1 may be confused with pimples or canker sores, but canker sores occur inside of the mouth and are not caused by the herpes virus. And we all know what pimples are.

Cold sores are also characterized by the symptoms that often precede them, known as a "prodrome." In a sense, this period is a warning sign that a herpes outbreak is coming. It’s the first sign of a blister and is characterized by burning, itching, pain and/or tingling a day or two before the lesion(s) appear.

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Your outbreak is no match against an Rx option.

What Causes Herpes Simplex Virus 1 (HSV-1)? 

Herpes Simplex Virus 1, also called HSV-1, herpes labialis or oral herpes, is the main virus that causes cold sores. Oral herpes can also be caused by HSV-2, but this is less common.

HSV-1 is caused by transmission of the virus from an infected person. Oral herpes is called “oral” because the lesions associated with it are mainly found in and around the mouth.

The virus is most often transmitted from close contact with the someone who has an infection. This can occur by kissing, sharing a drinking vessel, makeup, razor, towels, etc. 

Oral herpes can also be transmitted to the genitals of an uninfected partner through oral sex, so avoidance of this activity is important while experiencing any symptoms of an outbreak. 

Like many illnesses, viruses and diseases, people can carry HSV-1 without showing signs. Herpes goes dormant within the body in between outbreaks. The virus is most contagious during an outbreak, but it can also be spread when the host is asymptomatic (showing no symptoms) through what is called “viral shedding.” 

What Triggers Cold Sore Recurrence? 

When you’re first infected with HSV-1, you’ll likely experience your first cold sores. But even after they subside, the oral herpes virus is still dormant in your body, waiting to show up again.

Any number of things can cause the virus to return to an active state, including things like other viral infections, stress, fatigue, exposure to harsh weather and changes in immune function. Generally, symptoms during a flare-up are milder than your first outbreak, though everyone’s response to the virus is unique.

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Your outbreak is no match against an Rx option.

Cold Sore Treatments 

There is no cure for oral herpes. But there are treatments that can speed recovery from a cold sore outbreak. Several of those treatments are prescription antiviral medications. These are often taken as a pill, and include: acyclovir, famciclovir and valacyclovir.

You can also treat the discomfort of cold sores by applying cool or warm compresses; using lip balm; avoiding spicy, salty or citrus foods; applying topical over-the-counter creams and liquids and taking pain relievers.

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment. Learn more about our editorial standards here.