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Aciclovir is the medical name for the active ingredient of the tablets. Aciclovir is an antiviral medicine which stops herpes viruses multiplying and spreading. For essential information about cold sores and their treatment read our medical information page.

Aciclovir tablets come in various strengths, 200mg, 400mg, and 800mg. Different strengths and different lengths of courses are recommended to treat different types of herpes virus infections. Cold sores are generally caused by herpes simplex virus type 1 (HSV1). A 5-day course of 200mg strength tablets taken 5 times a day is the recommended course of treatment of a cold sore outbreak. Various manufacturers/brands of tablets are available, produced by different drug companies, including Boots, GSK (Zovirax brand), Aurobindo, Actavis, Milpharm, Ranbaxy, Wockhardt, Teva. All have the same active ingredient. The shelf life of the medication is up to 3 years, which means that people who are prone to cold sores can keep a stand-by supply at home for use when needed.

If you get repeated cold sore outbreaks (more than 6 a year) you may want to consider taking treatment to try and suppress these outbreaks by taking a 400mg tablet twice daily for 6-12 months.

How to take aciclovir tablets for outbreaks

Take one aciclovir 200mg tablet 5 times a day at 4 hourly intervals for 5 days (miss out a 4 hourly dose in the middle of the night), swallowed whole, with or without food. Alternatively dissolve the tablet in a glass of water and stir before drinking. Start tablets as soon as possible after the first signs of a cold sore. It is best to start at the stage when there is tingling and burning, even before the sore is visible. Tablets will still have an effect if started later, but results are best with early treatment. Continue for the full 5-day course. If a tablet is missed, take the next as soon as remembered. Do not take extra tablets to make up for missed doses. Avoid dehydration and drink plenty of water during treatment.

How to take aciclovir tablets for cold sore suppression

Take one aciclovir 400mg tablet twice a day for 6-12 months, swallowed whole or dissolved in water. Remember to drink plenty of water to avoid dehydration. 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.

When to consult a doctor

Most people who suffer from repeated cold sores are familiar with their symptoms and can safely self-treat. People who have not previously had sores on the lips or have sores that have not healed after 10 days should consult a doctor. Cold sores are often painful and are sometimes associated with mild symptoms of unwellness. They do not spread out across the face or cause fever and other symptoms of infections.

Cautions taking aciclovir tablets

Most people can take aciclovir tablets, even with other prescribed medication. Suitability checks are carried out during the online consultation.

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 your regular doctor before using aciclovir if also taking any of the following medications:

• Mycophenolate mofetil (prevents transplant rejection).
• Theophylline or aminophylline (used in asthma).
• Any medication which mentions urine or kidney problems as a side effect in the patient information leaflet.

Discuss using aciclovir with your regular doctor 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.

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.

Aciclovir tablets side effects

Not everybody gets side effects. The most common, occurring in less than 1 in 10, are dizziness, headache, irritation, rash, itching skin, tiredness, nausea and sickness, diarrhoea, stomach pain, and fever and skin reaction after exposure to light (photosensitivity). See the manufacturer's leaflet for a full list of side effects. If there is a very rare allergic type reaction with swelling of the lips, face, or eyelids, or shortness of breath with wheeze, stop taking tablets and seek immediate medical assistance.

Pregnancy and breastfeeding

The manufacturers of aciclovir cream and tablets advise caution with use 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.

See BUMPS - Aciclovir factsheet.

What else can I do to prevent outbreaks?

Sunlight is often a trigger for cold sores. Wearing a sun hat and using a strong sunscreen and a sunblock lip balm (SPF >15) will help to reduce outbreaks. A healthy diet, plenty of sleep, and not getting run down also helps. Do not use sun beds.

If you are getting more than 6 outbreaks a year, then aciclovir 400mg tablets can be taken twice daily to reduce the frequency of outbreaks. This is called suppression treatment.

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.
• Drink plenty of water to prevent dehydration.

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 their manufacturers' trials to be effective at reducing symptoms and overall healing times.

Delay dental treatment until lesions have healed.

Other cold sore treatments

Cold sores can also be treated by taking valaciclovir tablets. Valaciclovir is converted into aciclovir in the body. Some other antiviral medications may also be prescribed by specialist doctors, in certain specific situations.

Aciclovir cold sore cream is also available. The cream is applied to the cold sore 5 times a day. It is probably slightly less effective than aciclovir tablets at treating cold sores.

There are many other cold sore creams available without prescription. Most contain ingredients which soothe cold sores but do not contain an active antiviral. Some cold sore creams contain docosanol, which may have a small direct effect on the herpes virus, but is not as effective as aciclovir.

Further information

• NHS - Cold sores
• British Association of Dermatologists - Herpes simplex

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