Introduction 

Cold sores are small blisters that develop on the lips or around the mouth. They are caused by the herpes simplex virus and usually clear up without treatment within 7 to 10 days.

You will not have any symptoms when you first become infected with the herpes simplex virus. An outbreak of cold sores may happen some time later.

Cold sores often start with a tingling, itching or burning sensation around your mouth. Small fluid-filled sores will then appear, most commonly on the edges of your lower lip.

Read more about the symptoms of cold sores.

What causes cold sores?

The strain of herpes simplex virus usually responsible for cold sores is known as HSV-1.

In rare cases, cold sores can also be caused by the herpes simplex virus type 2 (HSV-2). This can be the result of having oral sex with someone who has genital herpes (where painful blisters develop on and around the genitals).

Read more about the causes of cold sores.

Herpes simplex virus

The herpes simplex virus – or "cold sore virus" – is highly contagious and can be easily passed from person to person by close direct contact. After someone has contracted the virus, it remains dormant (inactive) for most of the time.

However, every so often the virus can be activated by certain triggers, resulting in an outbreak of cold sores. These triggers vary from person to person but can include fatigue, an injury to the affected area, and, in women, their period.

Some people have frequently recurring cold sores (around two or three times a year), while others have one cold sore and never have another. Some people never get cold sores at all because the virus never becomes active.

Treating cold sores

Cold sores usually clear up by themselves without treatment within 7 to 10 days.

However, antiviral creams are available over the counter from pharmacies without a prescription. If used correctly, these can help ease your symptoms and speed up the healing time.

To be effective, these treatments should be applied as soon as the first signs of a cold sore appear (when you feel a tingling, itching or burning sensation around your mouth). Using an antiviral cream after this initial period is unlikely to have much of an effect.

Cold sore patches are also available that contain hydrocolloid gel, an effective treatment for skin wounds. The patch is placed over the cold sore while it heals.

Antiviral tablets may be prescribed for severe cases.

Read more about treating cold sores.

Complications of cold sores

Cold sores are usually mild. However, in rare cases, they can cause complications.

Dehydration sometimes occurs if drinking fluids becomes painful. Young children are particularly at risk of becoming dehydrated.

The herpes simplex virus can also spread to other parts of your body. This is known as a secondary infection. It can sometimes cause painful sores called whitlows to develop on your fingers.

Read more about the complications of the herpes simplex virus.

Preventing infection

It is not possible to completely prevent becoming infected with the herpes simplex virus or to prevent outbreaks of cold sores.

Cold sores are at their most contagious when they burst (rupture), but remain contagious until they are completely healed. You should therefore avoid close contact with others until your cold sore has completely healed and disappeared.

If you have unhealed cold sores, it is particularly important to avoid close contact with people with weakened immune systems because they are more vulnerable to infections.

For example, you should avoid close contact with:

  • newborn babies (never kiss a newborn baby if you have a cold sore)
  • people with HIV
  • people receiving treatments that are known to weaken the immune system, such as chemotherapy

Read more about preventing a herpes simplex infection.

Cold sores are small blisters on the lips or around the mouth that are caused by the herpes simplex virus  

When to visit your GP

If you have had outbreaks of cold sores before, it's likely that you will know what they are if they return.

You only need to visit your GP if you think you or your child has become infected with the herpes simplex virus for the first time, or if the cold sores have not healed after 7 to 10 days.

Page last reviewed: 10/04/2014

Next review due: 10/04/2016