Cold sores are usually caused by the herpes simplex virus type 1 (HSV-1).

In most cases, the virus is passed on in early childhood – for example, when a child is kissed by a family member or friend with a cold sore.

The virus passes through the skin and travels up the nerves, where it lies dormant (inactive) until it is triggered at a later date.

Cold sore triggers

Factors thought to trigger outbreaks of cold sores include:

• having another infection, such as a respiratory tract infection
• having a high temperature (fever)
• emotional upset or psychological stress
• tiredness and fatigue
• an injury to the affected area
• menstruation (periods)
• strong sunlight

However, in many cases there is no obvious trigger for an outbreak.

Occasionally, cold sores can be caused by the herpes simplex virus type 2 (HSV-2).

This can happen after having oral sex with a man or woman who has genital herpes, which is usually caused by HSV-2.

In genital herpes, painful blisters develop on your genitals and the surrounding area.

Infections caused by the cold sore virus are often mild and usually disappear without treatment. However, in rare cases, they can cause complications.

People particularly at risk are those with weak immune systems, such as people who are HIV positive or those having chemotherapy treatment.

Some of the possible complications that can develop as a result of the herpes simplex virus are outlined below.

Dehydration

Dehydration is a lack of water in the body. It can sometimes occur as a result of the pain caused by a cold sore.

It can be easy to not drink enough fluid if your mouth is painful. Young children with cold sores are particularly at risk of becoming dehydrated.

Read more about dehydration.

Secondary infection

It is possible for the cold sore virus to spread to other parts of your body. This is known as a secondary infection. Secondary infections can usually be easily treated with antiviral medication.

Examples of secondary infection include:

• skin infections – these often occur if the virus comes into contact with broken skin, such as a cut or graze, or a skin condition such as eczema
• herpetic whitlow (whitlow finger) – this causes painful sores and blisters to appear on and around your fingers
• herpetic keratoconjunctivitis – this causes inflammation (swelling and irritation) of your eye area and sores to develop on your eyelids

Left untreated, herpetic keratoconjunctivitis can cause the cornea (the transparent layer at the front of your eye) to become infected, which can eventually lead to blindness.

It is therefore important not to touch your eyes if you have an unhealed cold sore. If you must touch your eyes – for example, to remove contact lenses – wash your hands thoroughly first.

Encephalitis

Encephalitis is a condition where the brain becomes inflamed and swollen. This can be very serious and can cause brain damage and even death.

In very rare cases, encephalitis can be caused by the cold sore virus spreading to the brain. It can be treated with intravenous injections of antiviral medications, such as aciclovir.

Read more about encephalitis.

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.

It is not possible to prevent becoming infected with the herpes simplex virus or to prevent outbreaks of cold sores, but you can take steps to minimise the spread of infection.

Cold sores are at their most contagious when they rupture (burst) and remain contagious until they have completely healed.

Other people should avoid direct contact with your cold sore and the surrounding area until it has completely healed and disappeared.

However, if you or your child have a cold sore, there is no need for you to stay away from work or for your child to miss school.

Minimising the spread of infection

You can help minimise the risk of the cold sore virus spreading and cold sores recurring by following the advice below.

• avoid touching cold sores unless you are applying cold sore cream – creams should be dabbed on gently rather than rubbed in as this can damage your skin further
• always wash your hands before and after applying cold sore cream and after touching the affected area
• do not share cold sore creams or medication with other people as this can cause the infection to spread
• do not share items that come into contact with the affected area, such as lipsticks or cutlery
• avoid kissing and oral sex until your cold sores have completely healed
• be particularly careful around newborn babies, pregnant women and people with a low immune system, such as those with HIV or those having chemotherapy

Preventing cold sore outbreaks

If you know what usually triggers your cold sores, try to avoid the triggers. For example, a sun block lip balm (SPF 15 or higher) may help prevent cold sores that are triggered by bright sunlight.

Use an antiviral cold sore cream as soon as you feel the tingling sensation of a developing cold sore.

However, there is no benefit in using an antiviral cream continuously to try to prevent future cold sores. They cannot cure the virus and are only effective once the virus has been triggered.

You will not usually have any symptoms when you first become infected with the herpes simplex virus (the primary infection).

An outbreak of cold sores may occur some time later and keep coming back (recurrent infection).

However, if the primary infection does cause symptoms, they can be quite severe.

Herpes simplex virus primary infection

In children

Symptoms of the primary infection are most likely to develop in children who are younger than five years old. Symptoms include:

• swollen and irritated gums with small, painful sores in and around the mouth – this is known as herpes simplex gingivostomatitis
• sore throat and swollen glands
• producing more saliva than normal
• high temperature (fever) of 38ºC (100.4ºF) or above
• dehydration (a lack of water in the body)
• nausea (feeling sick)
• headaches 

Herpes simplex gingivostomatitis usually affects young children, but adults can also develop it. It can last between 7 and 14 days, with the sores taking up to three weeks to heal. However, gingivostomatitis does not usually recur after the primary infection.

In adults

Primary herpes simplex viruses are rare in adults, but the symptoms are similar to those experienced by children.

You will usually have a sore throat with or without swollen glands. You may also have bad breath (halitosis) and painful sores in and around your mouth. These can develop into ulcers with grey or yellow centres.

If you develop the herpes simplex virus at an early age, it may be triggered periodically in later life and can cause recurring bouts of cold sores. After the primary infection, the symptoms are usually reduced to just the cold sores themselves.

Recurrent infections (cold sores)

Recurrent infections usually last for less time and are less severe than the primary infection. The only symptom is an outbreak of cold sores, although you may also have swollen glands.

An outbreak of cold sores will usually start with a tingling, itching or burning sensation around your mouth. Small fluid-filled sores will then develop, usually on the edges of your lower lip.

If you have frequent recurrent infections, you may develop cold sores in the same place every time. They may grow in size and cause irritation and pain. Initially, they may ooze before crusting or scabbing over within 48 hours of the initial tingling sensation.

Most cold sores disappear within 7 to 10 days without treatment and usually heal without scarring.

Cold sores usually clear up without treatment within 7 to 10 days. Antiviral tablets or cream can be used to ease your symptoms and speed up the healing time.

Antiviral creams and tablets

Antiviral creams such as aciclovir or penciclovir (also known as Fenistil) may speed up the healing time of a recurrent cold sore infection if used correctly.

Cold sore creams are widely available over the counter from pharmacies without a prescription. They are only effective if you apply them as soon as the first signs of a cold sore appear, when the herpes simplex virus is spreading and replicating. Using an antiviral cream after this initial period is unlikely to have much effect.

If you have frequently recurring bouts of cold sores, use antiviral medication as soon as you feel the tingling sensation that indicates that a cold sore is about to develop. You will need to apply the cream up to five times a day for four to five days.

Antiviral creams can only help to heal a current outbreak of cold sores. They do not get rid of the herpes simplex virus or prevent future outbreaks of cold sores occurring.

Antiviral tablets are generally more effective than creams at treating cold sores, but are usually only prescribed for more severe cases.

Cold sore patches

Cold sore patches that contain a special gel called hydrocolloid are also available. They are an effective treatment for skin wounds and are placed over the cold sore to hide the sore area while it heals.

Non-antiviral treatments

Several non-antiviral creams are also available over the counter without a prescription from pharmacies.

These creams are not specifically designed to treat cold sores and will not help them heal faster, but they may help ease any pain or irritation. Ask your pharmacist to recommend a suitable treatment for you.

Pain can also be treated with painkillers such as ibuprofen or paracetamol (both are available in liquid form for young children).

However, don’t take ibuprofen if you have asthma or stomach ulcers, or you have had them in the past.

Children under the age of 16 should not take aspirin.

Speak to your GP if you have cold sores and you are pregnant.

Treating complications

If your cold sores are particularly severe or your immune system is damaged (for example, if you have HIV or you are having chemotherapy treatment), you may be at risk of developing serious complications.

For example, your risk of developing encephalitis (brain tissue inflammation) or the infection spreading to other parts of your body, such as your eyes, is increased.

Visit your GP if you are at risk. They may prescribe antiviral tablets and refer you for specialist treatment. The type of treatment recommended will depend on the severity of your cold sore symptoms and the complication that is causing problems.

For example, if you develop herpetic keratoconjunctivitis (a secondary eye infection), you may need to see an ophthalmologist (a specialist eye doctor).

Read more about the complications of cold sores.

Gingivostomatitis

Also visit your GP if you or your child develops gingivostomatitis (swollen, painful gums) as a result of the primary herpes simplex infection. They will be able to suggest treatments to help ease your symptoms.

If the infection is painful, your GP may suggest using a preparation that contains benzydamine (available as an oral rinse or oral spray) to help relieve any pain in your mouth or throat.

Brushing your teeth may also be painful because of the inflammation (swelling) of your gums. Your GP may suggest using an antiseptic mouthwash. This will help prevent secondary infections and will also control a build-up of plaque if you cannot brush your teeth effectively.

As with the treatment of cold sores, any pain or fever can be treated using ibuprofen or paracetamol. Again, do not take ibuprofen if you have asthma or if you have stomach problems, such as stomach ulcers. Children under the age of 16 should not take aspirin.

In rare cases of gingivostomatitis, it is possible for your lips to become stuck together in places. Using a lip barrier cream, available from your local pharmacist, will help prevent this happening.

If you or your child has gingivostomatitis, it is important to drink plenty of fluids to avoid becoming dehydrated. Young children are particularly at risk as they may refuse to eat or drink because of the pain in their mouth.

It is important to watch out for signs of dehydration, such as:

• headaches
• tiredness
• irritability
• lightheadedness
• low urine output

Most cases of gingivostomatitis will get better in 7 to 14 days, although it may take up to three weeks for the sores to heal completely.

Specialist treatment

If you or your child still has symptoms of gingivostomatitis after two weeks or the infection is severe, go back to your GP, who may refer you for specialist treatment.

Specialist referral may also be needed for gingivostomatitis if you are pregnant or have a weakened immune system.

Also visit your GP if you have a newborn baby who develops gingivostomatitis. They may also need to be referred for specialist treatment.