Aciclovir (Zovirax tablets/suspension)

How does it work?

Zovirax dispersible tablets and Zovirax suspension both contain the active ingredient aciclovir, which is an antiviral medicine used to treat infections with herpes viruses. (NB. Aciclovir is also available without a brand name, ie as the generic medicine.)

The herpes viruses include the viruses that cause cold sores and genital herpes (herpes simplex 1 and 2) and the virus that causes chickenpox and shingles (varicella zoster or herpes varicellae). Aciclovir works by stopping these herpes viruses from reproducing and infecting more cells in the body.

Aciclovir is converted to a more active form inside cells of the body that are infected with herpes viruses. The activated aciclovir then works by blocking the action of a viral enzyme called DNA polymerase.

The herpes viruses need the DNA polymerase enzyme to copy their genetic material from RNA to DNA. This process is necessary for the viruses to multiply and continue to survive. By blocking the action of DNA polymerase, aciclovir prevents the herpes viruses from multiplying. This controls the infection and helps the immune system to deal with it.

Aciclovir is taken by mouth to treat herpes infections that cause chickenpox and shingles. It is also taken by mouth to treat genital herpes and other herpes simplex infections of the skin and mucous membranes, and to prevent these infections coming back in people who suffer recurrent infections. Oral aciclovir is also valuable for preventing herpes simplex infections in people whose immune systems are supressed and cannot fight off infection, either due to disease, eg HIV and AIDS, or medical treatment, eg chemotherapy, steroids, bone marrow transplant.

Aciclovir is also available as creams specifically for treating genital herpes and cold sores, and as an eye ointment for treating herpes simplex infections of the eye. See the factsheets linked at the end of this page for further information about these preparations.

What is it used for?

  • Chickenpox (varicella zoster infection).
  • Shingles (herpes zoster).
  • Genital herpes (herpes simplex infection).
  • Herpes simplex infections of the skin, eg cold sores.
  • Preventing recurrence of herpes simplex infections, eg recurrent cold sores or genital herpes.
  • Preventing herpes simplex infections in people with an underactive immune system, eg due to disease or treatment.


  • You should make sure that you drink plenty of fluids while you are taking this medicine.
  • Take this medicine at regular intervals as prescribed. Complete the prescribed course unless otherwise directed by your doctor.
  • Genital herpes is a sexually transmitted disease. The risk of passing the infection on to your sexual partner is greatest when you have active symptoms of the infection. There is still a risk of passing the infection on if you use condoms, and even if you are taking this medicine to help control the infection. It is therefore important that you avoid having sex while you have symptoms, even if you are taking this medicine to treat the infection.

Use with caution in

  • Elderly people.
  • Decreased kidney function.

Not to be used in

  • Allergy to aciclovir or valaciclovir.

This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.If you feel you have experienced an allergic reaction, stop using this medicine and inform your doctor or pharmacist immediately.

Pregnancy and breastfeeding

Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.

  • This medicine should be used with caution during pregnancy, and only if the expected benefit to the mother is greater than any possible risk to the developing baby. Seek medical advice from your doctor.
  • This medicine passes into breast milk. It should be used with caution by breastfeeding mothers and only if the expected benefit to the mother is greater than any possible risk to the nursing infant. Seek medical advice from your doctor.

Label warnings

  • Take at regular intervals. Complete the prescribed course unless otherwise directed.

Side effects

Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Just because a side effect is stated here, it does not mean that all people using this medicine will experience that or any side effect.

Common (affect between 1 in 10 and 1 in 100 people)

  • Headache.
  • Dizziness.
  • Nausea and vomiting.
  • Diarrhoea.
  • Abdominal pain.
  • Skin reactions such as rash and itch.
  • Abnormal reaction of the skin to light, usually a rash (photosensitivity).
  • Fatigue.
  • Fever.

Rare (affect between 1 in 1000 and 1 in 10,000 people)

  • Shortness of breath.
  • Alteration in results of liver function tests.
  • Allergic reactions such as anaphylaxis or swelling of the lips, tongue and throat (angioedema).

Very rare (affect less than 1 in 10,000 people)

  • Decrease in the number of white blood cells (leukopenia), red blood cells (anaemia) or platelets (thrombocytopenia) in the blood.
  • Confusion.
  • Agitation.
  • Tremor.
  • Shaky movements and unsteady walk (ataxia).
  • Speech problems.
  • Convulsions.
  • Sleepiness.
  • False perceptions of things that are not really there (hallucinations).
  • Psychotic symptoms.
  • Yellowing of the skin and whites of the eyes (jaundice).
  • Inflammation of the liver (hepatitis).
  • Kidney pain.
  • Kidney failure.

The side effects listed above may not include all of the side effects reported by the medicine's manufacturer. For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.

How can this medicine affect other medicines?

It is important to tell your doctor or pharmacist what medicines you are already taking, including those bought without a prescription and herbal medicines, before you start treatment with this medicine. Similarly, check with your doctor or pharmacist before taking any new medicines while taking this one, to make sure that the combination is safe.

There may possibly be an increased risk of side effects on the kidneys if aciclovir is used in combination with the following medicines, and your doctor may want to monitor your kidney function if you are taking aciclovir with either of these:

  • ciclosporin
  • tacrolimus.

The following medicines may decrease the removal of aciclovir from the body by the kidneys, but this does not usually require any dose adjustments:

  • cimetidine
  • mycophenolate
  • probenecid.

Other medicines containing the same active ingredient

Cymex ultra Lypsyl cold sore cream Soothelip
Virasorb cold sore creamZovirax cold sore cream Zovirax cream
Zovirax eye ointment

Aciclovir tablets, dispersible tablets, suspension, intravenous infusion, cream and cold sore cream are also available as the generic medicine, ie without a brand name.