Cold Sores

How do cold sores occur?

Cold sores are caused by the herpes simplex virus (HSV). Most cold sores are caused by the type 1 strain (HSV1). The type 2 strain (HSV2) usually causes genital herpes but very occasionally can cause cold sores around the mouth. (See separate leaflet called ‘Genital Herpes’ for more information).

A first cold sore usually occurs in childhood. The virus infects through the moist inner skin that lines the mouth. It is commonly passed on by skin contact such as kisses from a family member who has a cold sore. The first infection can cause no symptoms. For details on primary cold sore infection, see separate leaflet called ‘Cold Sores – Primary Infection’.

After the first infection, the virus settles in a nearby nerve sheath and remains there for the rest of your life. For most of the time, the virus lies dormant (inactive) and causes no symptoms. However, in some people the virus becomes active from time to time. When activated, the virus multiplies and travels down the nerve sheath to cause cold sore blisters around the mouth. Some people have cold sores often, others only now and then. It is not clear what causes the dormant virus to become active. Some things that may trigger the virus to activate and cause a cold sore include:

  • Illness. Cold sores may occur during feverish illnesses such as colds, coughs, and flu.
  • Menstruation. Cold sores are common around the time of monthly periods.
  • Stress or just being ‘run down’ for one of many reasons.
  • Sunshine. Strong, direct sunlight may trigger cold sores in some people.

What are the symptoms of a cold sore?

You may feel a tingle or itch before the blisters appear, usually around your lips or nose. This can last several hours, or up to a day or so. After the tingle, one or more blisters appear which are usually sore. The blisters contain fluid that is teeming with the herpes virus. The blisters may weep and take several days to scab. Cold sores can be very tender and painful.

Until the cold sores have scabbed over and are completely dry, the cold sore is very infectious and can be spread to other people. The scab slowly disappears over a week or so leaving no scar. Some virus will remain dormant in the nerve sheath ready to cause another cold sore sometime in the future.

Can cold sores be passed on to other people?

Yes. When you have a cold sore you should not kiss anyone or allow anyone to come into skin contact with the sore. In particular, avoid kissing newborn babies and anyone who has a poor immune system (immunocompromised). Immunocompromised people include people having chemotherapy for cancer and people with conditions such as AIDS. Avoid oral sex until the cold sores have completely healed. When you have no symptoms (when the virus is dormant), you are not usually infectious. Healthy people who already have cold sores cannot be reinfected.

Do I need any tests?

Tests are not usually needed to confirm the diagnosis of cold sores. Symptoms of tingling pain followed by the typical blisters that crust around the nose and mouth are usually enough to make the diagnosis.

Taking a sample of the blister fluid on a special viral swab can help confirm the diagnosis of HSV. A blood tests can also detect the virus more reliably than by a swab. Neither of these tests is routinely available at your GP surgery. Testing might be done in people who are immunocompromised.

Self-care advice

  • Try not to touch or pick cold sores. Only touch cold sores when applying topical creams.
  • Creams, gels and other topical treatments should be dabbed on the cold sores rather than rubbed in. This is to minimise damage to the blisters which may spread the virus around, or cause more pain by breaking the blisters or scabs.
  • Wash your hands thoroughly with soap and water after touching cold sores and after applying creams to them.
  • If you find that sunlight triggers your cold sores, try using sunscreen lip balm (SPF 15 or more) when out in bright sunlight. This has been found to prevent some bouts of cold sores in some people. Do not share lip balms with other people if you have cold sores. Pots of sunscreen lip balm may be less convenient than lipsticks, but minimise the risk of contamination of the product, if applied with a clean finger.
  • Pain can be eased by painkillers such as paracetamol or ibuprofen.
  • A pharmacist can advise a soothing cream or gel. Again, it is important not to share these treatments.
  • There is a risk of infecting the eyes with the cold sore virus if your contact lenses become contaminated. You can prevent this with careful handwashing before handling your contact lenses. If you have disposable lenses and you suspect you have contaminated them, it is probably best to throw them away. If you have any concerns it may be better to wear your glasses and seek advice from your GP or optometrist.

What are the treatments for cold sores?

Antiviral creams

Aciclovir (brand name Zovirax®) and Penciclovir (brand name Vectavir®) can be bought over-the-counter (OTC) at pharmacies, or be prescribed by a doctor. These do not kill the virus, but prevent the virus from multiplying. They have little effect on existing blisters, but may prevent them from getting worse. These creams will not prevent future episodes of cold sores. If you use an antiviral cream as soon as symptoms start then the cold sore may not last as long as usual, and may be less severe. There is debate as to how well these creams work. Aciclovir has to be applied five times per day for five days. Penciclovir is applied every two hours (during the waking hours), for four days.

Antiviral tablets

Aciclovir is also available in tablet form. There are other oral antiviral drugs too, such as Famciclovir (brand name Famvir®) and Valaciclovir (brand name Valrex®). Oral antiviral tablets are not routinely used for the treatment of cold sores. Oral antiviral tablets may be prescribed in severe cold sore infections, in newborn babies, or in immunocompromised people. As with topical antivirals, they do not prevent recurrent bouts of cold sores, nor do they have effect on existing blisters. Oral antivirals are used with the aim of stopping the virus multiplying and preventing the cold sores from becoming too severe. Severe viral infections such as a severe herpes infection can be extremely dangerous to the health of immunocompromised people.

Do I need to see my GP?

Usually you would not need to see your GP with cold sores. With recurrent bouts of cold sores you will probably come to recognise the symptoms. You should see your GP if you are unsure of the diagnosis, or the cold sores are not resolving after a week or so. If you are immunocompromised and develop possible cold sores, you should see your GP as you may need tests to confirm the virus, and/or oral antiviral drugs.

Further help and information

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