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

You may 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 then appear, usually on the edges of your lower lip.

Read more about the symptoms of cold sores.

When to get professional advice

Cold sores aren’t usually serious and can be treated by a pharmacist. Your pharmacist may recommend that you contact your GP practice if required.

If you’ve had outbreaks of cold sores before, it’s likely that you’ll know what they are if they return.

You only need to visit your pharmacist if you’re unsure whether it’s a cold sore or if it’s severe and spreading further than just the lip. See your pharmacist if a cold sore hasn’t healed after 7 to 10 days.

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.

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 inactive (dormant) 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 sunlight, 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, which is 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, but may cause complications in rare cases. People with weak immune systems caused by illness or treatments such as chemotherapy are particularly at risk of 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. Examples of when this can occur 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 swelling and irritation (inflammation) 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’s 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.

In very rare cases, encephalitis, a condition where the brain becomes inflamed and swollen, 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.

Preventing infection

It’s not possible to prevent infection with the herpes simplex virus or 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 burst (rupture), but remain contagious until they’re completely healed. Avoid close contact with others until your cold sore has completely healed and disappeared.

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

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’re 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
• don’t share cold sore creams or medication with other people as this can cause the infection to spread
• don’t 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
• 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 triggered by bright sunlight