What causes cold sores?

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

Why do I keep getting 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 simplex 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.

Pain can be helped by using paracetamol or ibuprofen.

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. Mouthwash, and anaesthetic gels may help with pain from lesions inside the mouth. A greasy moisturising cream, e.g. petroleum jelly, a cold sore patch or a lip barrier gel may also be soothing.

There are devices available 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.

Wash your hands thoroughly after touching the sore to prevent spreading the virus.

Can anyone take or use aciclovir?

Most people can take aciclovir tablets or use aciclovir cream, even when taking other prescribed medication. You should not use it if you have a known hypersensitivity/allergy to aciclovir or valaciclovir.

Aciclovir may not be suitable for people with a known hypersensitivity/allergy to other antiviral drugs of the same class, e.g. cidofovir, entecavir, famciclovir, ganciclovir, ribavirin, valganciclovir.

If taking any of the following medication you may still be able to use aciclovir but there is a possibility of interactions:

  • Mycophenolate mofetil (prevent transplant rejection).
  • Theophylline or aminophylline (asthma and other breathing problems).
  • Any medication which mentions urine or kidney problems as side effects in the patient information leaflet.

Aciclovir may not be suitable for you if you have any of the following conditions:

  • Reduced kidney function.
  • 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.
  • Elderly (increased risk of rare neurological side effects).

There are checks in the online consultation questions and you may need to consult 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?

A significant amount of aciclovir from aciclovir tablets will pass into breast milk. This is unlikely to harm your baby, but the manufacturers advise caution. 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 e.g. Impetigo - spreading, golden, crusty sores, which require antibiotic (not antiviral) treatment. If you are unsure then consult 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 a suppression dose of 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 aciclovir cream for genital herpes on my penis/vulva?

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 aciclovir cream in the genital area.

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.

Do aciclovir tablets contain lactose?

Some brands of aciclovir tablets do contain lactose. If you react to lactose please message Dr Fox when you place an order, to request a lactose-free brand.

Further information

See NHS - Common questions about aciclovir.

Cold sore treatment
Dr Amanda Wood

Authored 08 September 2020 by Dr A. Wood
MB ChB Manchester University 1984. NHS GP in Bristol. GMC no. 2855422

Reviewed by Dr C. Pugh, Dr B. Babor
Last reviewed 06 September 2023
Last updated 19 January 2024
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