Boots WebMD Partners in Health
Return To Boots

Medicines & treatments centre

Terms Of Use


Brand Name(s) : Zovirax, Virasorb, Boots Avert Cold Sore Cream
Side Effects


Aciclovir should be used with caution in:

It should not be used in:

  • People who are hypersensitive to aciclovir, any of the ingredients in the medicine or to valaciclovir,.

Also see list of precautions and interactions


Store below 25°C.


What is it used for?

  • Aciclovir is used to treat herpes infections.
  • It is one of the most commonly-used antiviral drugs (medicines that treat infections caused by viruses). It works by stopping the virus from multiplying.
  • In general this drug is used to treat infections caused by herpes virus.

Listed below are the typical uses of aciclovir:

On occasion your doctor may prescribe this medicine to treat a condition not on the above list.


How often do I take it?

  • This medication is available as tablets, dispersible tablets, oral suspension, cream, liquid for intravenous injection, or as an eye ointment.
  • The tablets should be taken several times per day, with a glass of water, with or without food.
  • The cream should be applied several times per day, at approximately four-hourly intervals, to sores or areas likely to develop sores.
  • Other formulations may need to be taken more or less frequently, as indicated by your doctor or pharmacist
  • Use this medication for the duration of the prescription in order to get the most benefit from it.
  • Remember to use it at the same time each day - unless specifically told otherwise by your doctor.
  • It may take up to 4 or 5 days (depending on which form of medication is used) before the full benefit of this drug takes effect, but in severe infections this period may be longer.
  • Certain medical conditions may require different dosage instructions as directed by your doctor.

What dose?

  • Dosage is based on your age, gender, medical condition, response to therapy, and use of certain interacting medicines.

Do I need to avoid anything?

  • There have been occasional reports of brain disorders, such as confusion, hallucinations (seeing or hearing things that are not there) and sleepiness. If affected in these ways avoid driving or operating machinery.
  • Consult your doctor or pharmacist for more details.

When can I stop?

  • Always complete the full course as prescribed by your doctor.


Aciclovir eye ointment:

  • Irritation
  • Swollen, runny eyes
  • Stining sensation in the eye straight after using the ointment

Other aciclovir formulations:

If any of these persist or you consider them severe then inform your doctor.

Tell your doctor immediately if you develop any of the following symptoms:

  • Feeling weak, dizzy or short of breath (this could be due to anaemia, which is a lack of red blood cells)
  • Liver problems (hepatitis) or yellowing or the skin or whites of the eyes (jaundice)
  • Feeling dizzy, confused or sleepy or seeing or hearing things that are not there (hallucinations), fits or loss of consciousness.

Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

A serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction include: rash, itching/swelling (especially of the face/tongue/throat), dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine (which includes vaccines, herbals and over the counter medicines) that you are taking. It is run by the medicines safety watchdog called the Medicines and Healthcare products Regulatory agency (MHRA). Please report any suspected side effect on the Yellow Card Scheme website.


Before taking aciclovir, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies.

This medication should not be used if you have certain medical conditions. Before using this medicine, consult your doctor or pharmacist if you have: an allergy (hypersensitive) to aciclovir or valaciclovir.

Before using this medication, tell your doctor or pharmacist your medical history, especially any of the following: any kidney problems, any nervous disorder, at risk of dehydration.

Before having surgery, tell your doctor or dentist that you are taking this medication.

Does alcohol intake affect this drug?

  • Whilst there is no specific guidance regarding drinking alcohol and taking aciclovir, there have been occasional reports of some side effects, such as confusion, hallucinations and somnolence (drowsiness), which may be enhanced by alcohol. Thus, is may be sensible to avoid or moderate alcohol intake whilst taking this drug.

The elderly: aciclovir should be used with caution in the elderly as they are likely to have a reduced kidney function compared to younger patients. The dose may need to be lowered to compensate for this and they will need to drink plenty of water.

Pregnancy and breast-feeding - please ensure you read the detailed information below


The safety of aciclovir has not been established during pregnancy. Ask your doctor or pharmacist if you have any doubts or questions about this.

It is sensible to limit use of medication during pregnancy whenever possible. However, your doctor may decide that the benefits outweigh the risks in individual circumstances and after a careful assessment of your specific health situation.

If you have any doubts or concerns you are advised to discuss the medicine with your doctor or pharmacist.


Aciclovir is suitable to take if you are breastfeeding. However, it has been shown to pass into breast milk and therefore caution is advised.

It is sensible to limit use of medication during breastfeeding whenever possible. However, your doctor may decide that the benefits outweigh the risks in individual circumstances and after a careful assessment of your specific health situation.

If you have any doubts or concerns you are advised to discuss the medicine with your doctor or pharmacist.


Before using this medicine, tell your prescriber of all the medicines you are taking including prescription medicines and medicines you have bought over the counter without a prescription. Tell your prescriber if you are taking vitamins or complementary remedies such as herbal products, as these can also interact with medicines.

If you are taking more than one medicine, these may interact with each other. Your prescriber may decide to use medicines which interact with each other if the benefit outweighs the risks. In these cases, the dose of your medicines may need to be adjusted or you may be monitored more closely.

The following medicines may interact with aciclovir:

  • Probenecid, used for treating gout
  • Cimetidine, used to heal stomach ulcers
  • Mycophenolate mofetil, used after transplant operations
  • Tacrolimus, an immunosuppressant drug
  • Ciclosporin, an immunosuppressant drug
  • Zidovudine, used in HIV infections
  • Theophylline, used in asthma and other breathing problems

This information does not contain all possible interactions. Therefore, tell your prescriber of all the products you are using before taking this medicine.

If you have a question or want to discuss anything about your medicine, speak to your local pharmacist.


An overdose of aciclovir may cause: nausea (feeling sick), vomiting, headache and confusion.

If you think you, or someone you care for, might have accidentally taken more than the recommended dose of aciclovir or intentional overdose is suspected, contact your local hospital, GP or if in England call 111. In Scotland call NHS 24. In Wales, call NHS Direct Wales. In the case of medical emergencies, always dial 999.


If you miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose and resume your usual dosing schedule. Do not double the dose to catch up.

Find a medication

Search by medication name for information on over-the-counter or prescription medications including side effects and interactions.
indicates detailed medicines information

Source: Selected from data included with permission and copyrighted by Boots UK Limited. This copyrighted material is sourced from a licensed data provider and is not for distribution, except as may be authorised by the applicable terms of use.
CONDITIONS OF USE: The information in this database is intended to supplement, not substitute for, the expertise and judgement of healthcare professionals. The information is not intended to cover all possible uses, directions, precautions, drug interactions or adverse events, nor should it be construed to indicate that use of a particular medicine is safe, appropriate or effective for you or anyone else. A healthcare professional should be consulted before taking any medicine, changing any diet, or discontinuing any course of treatment.

Search for a medicine or treatment

Search by medicine name or treatment for information including side effects and interactions.

Ex. Simvastatin, Ibuprofen, Amitriptyline Hydrochlorine

Popular slideshows & tools on BootsWebMD

How to help headache pain
rash on skin
Top eczema triggers to avoid
Causes of fatigue & how to fight it
Tips to support digestive health
woman looking at pregnancy test
Is your body ready for pregnancy?
woman sleeping
Sleep better tonight
Treating your child's cold or fever
fifth disease
Illnesses every parent should know
spoonfull of sugar
Surprising things that harm your liver
woman holding stomach
Understand this common condition
What your nails say about your health