What causes cold sores?

Cold sores are small sores, usually around the mouth, caused by the herpes virus - usually HSV1.

Why do I get repeat cold sores?

After your first infection the virus continues to live in the nerves around the mouth and it occasionally reactivates causing a new cold sore infection. Reactivation can be triggered by a variety of things including other infections such as colds or 'flu', getting tired and run down, menstruation, stress, sunlight or strong wind on the affected area, and minor injuries.

photo of a cold sore

How do I know it's a cold sore?

Most people with cold sores recognise the pattern - there is usually a tingling feeling 1-2 days before the redness followed by blistering, and then a small ulcer develops. It usually lasts for 7-10 days before healing with no scar. The lesion is usually in the same place.

Is a cold sore the same as genital herpes?

They are both skin conditions caused by Herpes viruses. In the past, cold sores were mostly caused by HSV1 and genital herpes by HSV2. However, in recent years there is more overlap.

How did I 'catch' cold sores?

The virus is transmitted by direct contact, so it is passed from person to person, often by kissing.

How do I stop giving cold sores to my family and partner?

As the virus is transmitted by contact, do not let the cold sore touch another person's skin. Use tissues when drying the area. Do not use a communal towel or share a flannel. Cold sore infection can be spread by direct contact from the lips to the genital region, so do not have oral sex whilst you have a cold sore.

What treatment can I use for my cold sore?

Aciclovir is one of a group of drugs used to treat herpes infections. It is available as tablets or as a cream - both are available from Dr Fox following an online cold sore consultation.

You could also try soothing the area by using a cool wet compress and washing the area gently with salt water. Dry thoroughly but gently. A greasy moisturising cream, e.g. petroleum jelly, may also be soothing. Wash your hands thoroughly after touching the sore to prevent spreading the virus.

Can anyone take or use Aciclovir?

There are a few theoretical drug interactions, but at the doses used to treat cold sores in either cream or tablets, these are mostly unlikely to be a problem. Aciclovir may not be suitable for people with a history of seizures or kidney disease, or if you are taking theophylline or aminophylline for asthma. There is an increased risk of very rare neurological side effects in the elderly, so discuss with your own doctor if you are over 65 years. There are checks in the online consultation questions and you may need to see your regular doctor.

What are the possible side effects of Aciclovir?

The most common side effects are dizziness, headache, rash, itching skin, tiredness, nausea and sickness, diarrhoea, stomach pain, fever and skin reaction after exposure to light (photosensitivity). These are more likely if taking tablets, and at higher doses than used for cold sores.

Rarely, people can develop an allergy to Aciclovir. On very rare occasions, this could involve swelling of the lips, face, or eyelids, and shortness of breath with wheeze. If this happens stop taking tablets and seek immediate medical assistance.

The manufacturer's Aciclovir 200mg patient information leaflet has a full list of possible side effects.

I am pregnant - can I take/use Aciclovir?

The manufacturers do not recommend taking Aciclovir if you are pregnant, however studies have shown that it is usually safe. Please discuss with your regular doctor. Dr Fox does not supply aciclovir to pregnant women.

See BUMPS use of medicines in pregnancy factsheet - Aciclovir for further information.

I am breastfeeding - can I take/use Aciclovir?

If taken by mouth there is a significant amount in the breast milk, and whilst this is unlikely to cause harm to your baby, it is best avoided. Dr Fox does not supply Aciclovir to women who are breastfeeding.

What if my cold sore doesn't get better?

If you are using Aciclovir cream you can continue for up to 10 days in total.

The course of Aciclovir tablets is a standard 5 days only.

If the sore has not resolved at the end of the treatment, it may be that you started the treatment too late to alter the course of the cold sore.

Or it may not have been a cold sore. It could be something else eg Impetigo - spreading, golden, crusty sores, which require antibiotic (not antiviral) treatment. If you are unsure then see your regular doctor.

Can I take aciclovir to reduce the number of outbreaks I have?

If you are having recurrent outbreaks of cold sores then you could try taking aciclovir 400mg twice a day for 6-12 months which may reduce or stop the number of outbreaks you are experiencing. It is advised to stop treatment after 6 or 12 months to reassess the frequency of outbreaks you are having.

Can I use Zovirax/aciclovir cream for genital herpes on my penis/vulva?

Zovirax/aciclovir cream is not as effective for genital herpes as taking aciclovir tablets and there is no added benefit to using it as well as tablets. So, it is not recommended to use Zovirax/aciclovir cream in the genital area. For further information read national guideline for the management of anogenital herpes.

Can taking daily Lysine help to prevent cold sores?

Taking Lysine has in the past been suggested as a preventative for cold sores. Research studies looked at this in the 1970s and 1980s. A more recent review of those studies, from 2017 suggested a need for further trials as the evidence for prevention of cold sores was poor and mixed, especially at the low doses usually sold. So, at present there is no scientific evidence that Lysine will help. A preventative dose of aciclovir 400mg twice a day does reduce the number of outbreaks in many cases.

Cold sore treatment
Dr Amanda Wood

Authored 08 September 2020 by Dr A. Wood
Last updated 22 October 2021

Reviewed by Dr Tony Steele, Dr B. Babor, Dr P. Hunt
Last reviewed 17 October 2020