Cold sores

Medical information

Written and reviewed by a team of doctors. Dr Fox is regulated by the CQC & GPhC.

What are cold sores?

Cold sores are small lumpy blisters and ulcers on the lips caused by a herpes virus - usually HSV1 (herpes simplex virus type 1), but occasionally HSV2 (herpes simplex virus type 2) which more commonly causes genital herpes. Aciclovir tablets and cream are antiviral treatments which can reduce the severity and duration of cold sores. Tablets may be a more convenient treatment than cream.

Is it a cold sore?

People who have had cold sores before usually recognise the symptoms, mainly because the sores come up in exactly the same place each time. There is often a tingling sensation for 24 to 48 hours before the redness, then blistering, then an ulcer develops.

Most cold sores usually last 7-10 days before completely healing, usually without scarring. If symptoms are not improving after a few days, or are worsening, or there is a high fever and generalised flu-like symptoms, consult a doctor.

People who have not had cold sores before, and have symptoms for the first time, should consult a doctor. The first episode may be more severe, including a sore throat or mouth and swollen lymph glands as well as the classic blistering sores on the lips.

Sores that are yellow, sticky, and crusty - and spread out over a period of a few days - are likely to be impetigo, not cold sores. Impetigo is a bacterial infection and it should be treated with antibiotics or hydrogen peroxide, not aciclovir, which is an antiviral.

Below is a photograph of a typical cold sore.

Aciclovir cream and tablets

Aciclovir cream and aciclovir 200mg tablets are supplied by Dr Fox pharmacy to treat recurrent cold sores on the lips. Some people have cold sores which come up time and again in the same place on the lips. This is because after the first infection the virus stays living in the body and so new cold sores can break out - usually at times of stress or in response to triggers such as fatigue, cold weather, strong sun or wind exposure, or menstruation. In some cases cold sores come up without an obvious trigger. Early treatment with either aciclovir cream or tablets will usually reduce the severity and the duration of the sore.

Treatment with either tablets or cream should be started as soon as symptoms begin, preferably at the tingling stage, before a sore breaks out on the skin surface. If treatment is not started within a few days of symptoms starting, it is not likely to be effective. Aciclovir cream is also available from pharmacies and many supermarkets.

Dose of aciclovir

Treatment dose

Apply aciclovir cream or take one aciclovir 200mg tablet 5 times daily for 5 days (every 4 hours during the day time, leaving out a dose overnight). Tablets can be swallowed whole or dissolved in water - there are 25 tablets total in one course. Cream is usually continued for about the same length of time, but can be used for up to 10 days, until the sore has almost completely healed. The tablets are usually more effective than the cream.

Suppressive dose

Some patients with severe and frequent cold sores take daily aciclovir to help prevent outbreaks of cold sores. Frequent episodes are usually defined as 6 or more episodes in a year. To suppress outbreaks it is recommended to take aciclovir 400mg tablets twice a day for 6-12 months. This may reduce the number of outbreaks you have. If you continue to have outbreaks on suppressive treatment you should arrange a review at your usual GP surgery. Do not take outbreak treatment whilst on suppressive treatment. After 12 months, stop taking the suppressive dose of aciclovir and reassess how often the cold sore outbreaks occur. If outbreaks continue again frequently, the suppression can be re-started for another few months, before trying another break. Research has shown that aciclovir can be safely continued for many years if necessary.

Aciclovir cautions

Most people can take aciclovir tablets, even with other prescribed medication.

Do not use aciclovir if you have a known hypersensitivity/allergy to aciclovir or valaciclovir. Consult your regular doctor before using aciclovir if you have a known hypersensitivity/allergy to other antiviral drugs of the same class, e.g. cidofovir, entecavir, famciclovir, ganciclovir, ribavirin, valganciclovir.

Consult a doctor before using aciclovir if taking:

• Mycophenolate mofetil, used to stop your body rejecting transplanted organs.
• Theophylline or aminophylline (used in asthma and other breathing problems).
• Any medication which mentions urine or kidney problems as side effects in the patient information leaflet.

People with significantly reduced kidney function may require reduced doses of aciclovir tablets. Whilst taking aciclovir tablets, it is important to drink plenty of water to prevent dehydration.

Consult your regular doctor before taking aciclovir if you have any of the following conditions:

• Reduced immunity e.g. after a bone marrow transplant, low white blood cell count, HIV.
• Neurological illness.
• Long-standing low oxygen levels.
• Abnormal liver blood tests.
• Blood chemistry abnormalities.
• Age over 65.

Pregnancy and breastfeeding: The manufacturers of aciclovir cream and tablets advise to use with caution, as with any medication, in pregnancy and breastfeeding. This is precautionary advice, and there is no evidence of harm from aciclovir to developing pregnancies or breastfed infants. Aciclovir has been shown to pass into breast milk.

Active cold sores can pass the herpes virus to newborn infants so consult your GP or midwife if you develop cold sores in late pregnancy or whilst breastfeeding.

Side effects of aciclovir

Most people do not experience significant side effects. Of those who experience side effects the most common (affecting between 1 in 10 and 1 in 100) are dizziness and headache or nausea, vomiting, diarrhoea and abdominal pain, fatigue and fever, skin reactions, or photosensitivity.

People who are affected by drowsiness should not drive or operate machinery.

A full list of cautions and side effects can be found in the manufacturer's patient information leaflets:

• Aciclovir 5% cream
• Aciclovir 200mg tablets (5-day course)
• Aciclovir 400mg tablets (6-12 month suppression)

Other measures to help with cold sores

• Pain can be helped by paracetamol or ibuprofen.
• Mouthwash, and anaesthetic gels may help with pain from lesions inside the mouth.
• Lip barrier gels may be soothing.
• Cold sore patches may help with healing, disguise the sore and also provide a barrier to reduce risk of spread.

If cold sores are not improving after 5-7 days or you are seriously unwell with them, consult your regular doctor.

There are devices on the market which offer heat or near-infra-red light therapy for cold sores. Both types of product have been shown in manufacturers' trials to be effective at reducing symptoms and overall healing times.

How to prevent passing the virus to others

Active cold sores shed herpes virus. They are sometimes called a 'kissing disease' as the virus from an active cold sore is easily passed on by any close contact with other people. When you have a cold sore, it is therefore very important to avoid intimate contact and also to avoid sharing towels, flannels, and other items such as lipstick, lip balm, and lip gloss. Wash your hands with soap and water after touching the cold sore and be careful not to transfer the virus to your eyes especially if you wear contact lenses.

Delay dental treatment until lesions have healed.

Cold sores and genital herpes

Cold sores and genital herpes infections are caused by similar types of herpes viruses - HSV1 and HSV2. Because of this, a cold sore infection can spread by direct contact from the lips to the genital region, or vice versa. The cold sore virus can also be spread from the lip region of one person to the lip region of another and from the genital region of one person to the genital region of another. Kissing and any oral sex should be avoided when there is an active cold sore.

Dr Fox supplies cold sore medication on prescription - you are required to answer a short medical questionnaire before your order can be completed.

Authored 18 February 2015 by Dr Tony Steele
MB ChB Sheffield University 1983. Former hospital doctor and GP. GMC no. 2825328

Reviewed by Dr A. Wood, Dr C. Pugh, Dr B. Babor
Last reviewed 06 September 2023
Last updated 8 January 2025
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