Badge field

What are cold sores and fever blisters of the mouth?

Published date field


Cold sores and fever blisters are caused by herpes simplex virus type 1 (HSV-1). The virus is passed from person to person by saliva (either directly, or by drinking from the same glass or cup) or by skin contact. Cold sores usually appear as clusters of tiny blisters on your lip. The surprising thing about cold sores is they’re incredibly common. About 8 out of 10 people have the virus that causes cold sores, and most people are first infected before they’re 10 years old.

After your first infection, the virus remains dormant (inactive) in the nerves of your face. In some people, the virus becomes active again from time to time and, when this happens, cold sores appear. HSV-1 can become active again because of a cold or fever. Stress also can lead to a cold sore outbreak. This includes mental and emotional stress, as well as dental treatment, illness, trauma to the lips or sun exposure. HSV-1 also can infect the eyes, the skin of the fingers and the genitals. However, most genital herpes infections are caused by herpes simplex type 2 (HSV-2). HSV-1 can cause serious illness in people who have other health problems. The virus also can cause serious illness in people whose immune systems are weakened by either illness or medicines they’re taking.

If you’re infected with HSV-1 for the first time you may experience a fever, headache, nausea and vomiting. You may also notice the main symptoms as painful swelling and open sores around the mouth. Some people can also experience a sore throat. These symptoms usually begin about a week after you’re exposed to HSV-1.

Cold sores can also appear when HSV-1 is reactivated later in life. They may occur after a period of illness or stress, poor nutrition or sunlight exposure. Sometimes there's no known reason. Dental procedures that stretch the lip may occasionally trigger the virus too.

The border of the lip is the most common place where these sores appear. They may occasionally occur inside the mouth, when this happens it’s known as a canker sore and is more likely in people who have a weakened immune system or other medical problems.

The first sign of a cold sore is a tingling, burning or itching, quickly followed by swelling and redness. Within 24 to 48 hours, one or more tiny blisters (also called "fever blisters") can appear. These blisters pop and form painful cold sores. Over time, you’ll notice the sores are eventually covered by crusts, which look like scabs. The crusts are shed and then form again while the sore heals.


Your dentist or physician can usually diagnose cold sores by asking you about your medical history and examining you. If you have other medical conditions, your doctor may do other tests to diagnose cold sores. These tests are usually not necessary in healthy people.

Expected Duration and Stages of a Cold Sore

When you’re first infected with HSV-1, your symptoms are likely to last for 7 to 14 days. Cold sores usually crust within 4 days and heal completely within 8 to 10 days.

To help prevent a first herpes infection in children, do not let them be kissed by anyone who has cold sores, fever blisters or signs of a first herpes infection. However, HSV-1 is very common, and most children will be infected by the time they reach adulthood. Several different vaccines are being developed against HSV (types 1 and 2), but these appear to protect only people who have never been infected.

There is evidence that using sunscreen on your lips will prevent cold sores caused by sun exposure. Antiviral medicines may prevent cold sores from forming. In certain situations, your dentist or physician may prescribe these medicines. If you expect to encounter a known trigger, medicine taken in advance can decrease the chance of a cold sore.

What Triggers Cold Sores?

Once you have the oral herpes virus or HSV-1, you may notice that certain things cause you to have a cold sore outbreak. Some common triggers that can cause the fever blisters to appear include illness like the cold or flu or a weakened immune system. The weather can be another trigger, for example, exposure to hot, sunny climates or cold, windy days can cause an outbreak. Stress is also a big trigger for cold sores. [kw1] Unfortunately, once you have the virus, it’s hard to avoid outbreaks as there’s not always an obvious reason for them, but even knowing what might trigger the sores or noticing patterns in your outbreaks can help you to minimise them in the future.

Treatment Options

Some medicines can help cold sores heal faster. They also relieve pain and discomfort. The medicines are acyclovir (Zovirax), famciclovir (Famvir) and valacyclovir (Valtrex). These drugs can’t get rid of the virus from your system, but they can help the severity of your outbreaks if you act fast. Take them as soon as you feel the tingling or burning sensation that signals the start of a cold sore. Once you have blisters on your lip, the medicines will not help much.

These drugs also can stop cold sores from popping up in the first place. Some people take them when they know they’ll be under stress.

Keep the area clean and apply lip balm. Try not to touch the area. Do not pick at the crusts over the sores. Avoid kissing anyone while you have blisters and sores. Cold sores can spread through kissing and by sharing things that touch the lips and the skin around them, such as spoons, forks, glasses and towels.

When to Call a Professional

Cold sores are common and usually, they’re not dangerous. If you have a weakened immune system because of a disease or because of medicines you take, HSV-1 can cause a serious illness. Call your dentist or physician right away if:

  • Lip or mouth sores persist longer than one week
  • The sores make it hard for you to talk or swallow
  • You develop a fever
  • You have a second outbreak of blisters in quick succession to the first


Cold sores can be unpleasant, but be assured that they’re incredibly common! Read on to find the answers to some of the most popular questions relating to cold sores and fever blisters.

What’s the difference between cold sores and fever blisters?

A fever blister is just another name for a cold sore. Both are fluid-filled blisters usually found on the outside of your mouth, around the lips, and both are caused by the Herpes Simplex Virus (HSV-1). These shouldn’t be confused with canker sores, which are usually found inside the mouth.

What can trigger a fever blister?

Fever blisters can be triggered by the same factors as cold sores. Often there’s no obvious trigger but they can include:

  • A weakened immune system caused by another infection or virus such as a cold, flu or respiratory infection.
  • Having a high temperature or fever.
  • Emotional upset or stress.
  • Tiredness and fatigue.
  • An injury to the affected area.
  • Menstruation or hormones.
  • Strong sunlight or very cold temperatures. [kw2]

How do you get rid of a fever blister fast?

The most effective way to get rid of fever blisters is to use antiviral medication, either in pill or gel form, as soon as you feel the tingling signs of the start of a cold sore. These antiviral drugs, such as Zovirax, can reduce the healing time of your fluid-filled blisters by 1 to 2 days. [kw3]

There are also many home treatment options you can try that could help with the inflammation or pain including:

  • Hold ice wrapped in a clean towel on the blister for 5-15 minutes.
  • Creams, ointments or lip balm containing lemon balm.
  • Taking a zinc supplement or applying a zinc oxide cream until your sores heal.
  • Diluted oregano oil, tea tree oil, witch hazel, apple cider vinegar or liquorice extract applied to the area on a cotton pad. [kw4]

Does a fever blister make you sick?

If you have an outbreak of fever blisters, you’ll usually find the first time is the worst. In a first-time outbreak, you might also notice you have a fever, sore throat, headache, muscle ache or swollen lymph nodes. Outbreaks after the first will usually be less severe and you shouldn’t feel ill if you’re otherwise healthy. However, if you have another health condition that causes a weakened immune system, you may experience more severe symptoms or get breakouts more often. If this is the case, consult your doctor for advice. [kw5]







Want more tips and offers sent directly to your inbox?

Sign up now

This article is intended to promote understanding of and knowledge about general oral health topics. It is not intended to be a substitute for professional advice, diagnosis or treatment. Always seek the advice of your dentist or other qualified healthcare provider with any questions you may have regarding a medical condition or treatment.