Valaciclovir (Valtrex)

How does it work?

Valtrex tablets contain the active ingredient valaciclovir. Valaciclovir is known as a pro-drug. Once inside the body it is broken down into the active ingredient aciclovir, which is an antiviral medicine used to treat infections with herpes viruses. These 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.

Valaciclovir is taken by mouth to treat herpes infections that cause shingles (herpes zoster). It is also taken to treat herpes simplex infections of the skin and mucous membranes, including genital herpes. Valaciclovir can be used to treat recurrent flare-ups of herpes simplex infection. It is most effective at preventing the sores from developing if treatment is started as soon as the warning signs appear.

If you suffer from frequent herpes simplex flare-ups your doctor may prescribe valaciclovir for you to take regularly, to suppress the virus and prevent recurrences. Valaciclovir taken regularly can also reduce the risk of passing the genital herpes to a sexual partner, provided safe sex practices such as condoms are used as well. However, you should still avoid sexual contact when the sores are present to avoid passing on the virus.

Valaciclovir has a further use in preventing infection with cytomegalovirus (CMV) in people who have had an organ transplant such as a kidney transplant. CMV is another member of the herpes group of viruses. In healthy people this common virus normally produces symptoms milder than the common cold, but in people whose immune systems are compromised it can cause serious infection because the immune system cannot fight it.

People who have had an organ transplant are given powerful medicines that suppress the immune system in order to stop it attacking the transplanted organ. However, because they suppress the immune system, these medicines also suppress the ability of the body to fight infection. Valaciclovir is therefore given to people who have received an organ transplant, in order to help the body kill off any CMV virus.

What is it used for?

  • Treating shingles (herpes zoster).
  • Treating genital herpes (herpes simplex infection).
  • Treating herpes simplex infections of the skin, eg cold sores.
  • Treating herpes simplex infections of the eye.
  • Preventing recurrence of herpes simplex infections, eg recurrent cold sores, genital herpes or eye infections.
  • Reducing the risk of spreading genital herpes to a sexual partner (when used with safe sex practices such as using condoms).
  • Preventing cytomegalovirus (CMV) infection in people who have had an organ transplant.

How do I take it?

  • Valtrex tablets can be taken either with or without food.
  • The dose prescribed and how often to take the medicine depends on the condition being treated. It is important to follow the instructions given by your doctor. These will also be printed on the dispensing label that your pharmacist has put on the packet of medicine.
  • You should take the tablets at regular intervals and always complete the course prescribed, even if you think the infection has cleared up.
  • If you forget to take a dose take it as soon as you remember, unless it is nearly time to take your next dose. In this case skip the missed dose and continue as usual – don’t take a double dose to make up for a missed dose.


  • If you are taking this medicine to treat or prevent genital herpes you should be aware that it does not completely eliminate the risk of passing the virus to your sexual partner. You should continue to practice safe sex, particularly by using condoms, even while you are taking this medicine. You should also avoid sexual contact while the sores are present.
  • It is important to drink plenty of water while taking this medicine to avoid getting dehydrated, particularly if you are over 65 years of age or have kidney problems. This will help avoid potential side effects of the medicine on the kidneys or nervous system.
  • Tell your doctor if you feel very drowsy, confused or agitated, or if you experience disturbed thoughts, hallucinations, tremors or fits while taking this medicine. These rare nervous system side effects are more likely to occur in elderly people, people with kidney problems, or people taking high doses of this medicine after an organ transplant. They usually get better when Valtrex is stopped or the dose reduced.

Use with caution in

  • Elderly people.
  • People with decreased kidney function.
  • People with liver disease or a liver transplant.
  • People with an underactive immune system (your doctor will want to make sure that this medicine is controlling the infection - you may need treatment by injection if it is not).

Not to be used in

  • People who are allergic 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 in 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.

Very common (affect more than 1 in 10 people)

  • Headache.

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

  • Feeling sick.
  • Vomiting.
  • Diarrhoea.
  • Dizziness.
  • Skin reactions such as rash, itching, or increased sensitivity of the skin to sunlight (photosensitivity).

Uncommon (affect between 1 in 100 and 1 in 1000 people)

  • Abdominal discomfort.
  • Nettle-type rash (hives or urticaria).
  • Shortness of breath.
  • Drowsiness.
  • Confusion.
  • False perceptions of things that are not really there (hallucinations).
  • Tremor.
  • Agitation.
  • Kidney pain (felt in the lower back) or blood in the urine. Tell your doctor if you experience this as it could be a sign of a problem with your kidney function.
  • Disturbances in liver function.
  • Decrease in the number of platelets in the blood (thrombocytopenia).
  • Decrease in the number of white blood cells in the blood (leucopenia – mainly in people with a weak immune system).

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

  • Shaky movements and unsteady walk (ataxia).
  • Speech problems.
  • Convulsions.
  • Psychotic symptoms.
  • Delirium.
  • Decreased kidney function or acute kidney failure.
  • Swelling of the face, throat and tongue (angioedema). See a doctor straight away if you experience this allergic reaction.

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?

You should tell your doctor or pharmacist what medicines you are using, particularly those listed below, before you start treatment with this medicine. This includes those bought without a prescription and herbal medicines. Likewise, check with your doctor or pharmacist before starting any new medicines while you are taking this medicine, to make sure that the combination is safe.

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

  • aminoglycoside antibiotics such as neomycin
  • ciclosporin
  • foscarnet
  • methotrexate
  • pentamidine
  • platinum compounds such as cisplatin
  • tacrolimus.

The blood level of this medicine may be increased by the following medicines:

  • cimetidine
  • mycophenolate
  • probenecid
  • tenofovir.

This is not normally clinically important. However, where high doses of valaciclovir are being used to prevent CMV infection, eg following an organ transplant, your doctor may consider using alternatives to these medicines, or simply monitor you more closely.

Other medicines containing the same active ingredient

Valaciclovir tablets are also available without a brand name, ie as the generic medicine.