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Valacyclovir: 7 things you should know

Medically reviewed by Carmen Pope, BPharm. Last updated on April 5, 2023.

1. How it works

  • Valacyclovir is an antiviral prodrug that is rapidly converted into acyclovir inside the body. Acyclovir has activity against several different viruses including herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV). These viruses cause illnesses such as cold sores, genital herpes, and shingles.
  • Valacyclovir works in several different ways to prevent these viruses from replicating. Inhibits viral DNA replication by inhibiting viral DNA polymerase, incorporation, and termination of the growing viral DNA chain, and inactivation of the viral DNA polymerase.
  • Highly selective for thymidine kinase encoded by HSV and VZV viruses which converts acyclovir to acyclovir monophosphate, which is then converted to the diphosphate, then to the active triphosphate. Activity against HSV is greater than activity against VZV. Also exhibits activity against other viruses that do not code for this enzyme.
  • Valacyclovir belongs to the group of medicines known as antivirals.

2. Upsides

  • Valacyclovir is used to treat infections caused by herpes and varicella-zoster viruses, such as genital herpes, cold sores, shingles, and chickenpox.
  • Active against herpes viridae including herpes simplex virus types 1 (HSV-1), 2 (HSV-2), varicella-zoster virus (VZV), and cytomegalovirus.
  • Approved to treat cold sores in children aged older than 12 and for the treatment of chickenpox in children aged two to 18 years.
  • Valacyclovir does not cure genital herpes or completely rid the body of herpes or varicella-zoster virus.
  • Valacyclovir is an oral tablet that only needs to be taken twice daily, compared to five times daily for acyclovir. This makes it more convenient for people to take. Treatment may last from one to 14 days depending on the condition being treated and the immunocompetency of the person being treated. Treatment for cold sores should not exceed one day.
  • May be taken with or without food.
  • Valacyclovir is better absorbed than acyclovir. This results in concentrations in the body that are comparable to those achieved with IV acyclovir.
  • Does not appear to have any clinically significant drug-drug or drug-food interactions.
  • Not metabolized by CYP enzymes.
  • Generic valacyclovir is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Headaches, nausea, vomiting, and abdominal pain are the most commonly reported side effects.
  • Rarely, may affect the kidneys or cause a bleeding disorder. In children aged less than 12, diarrhea, fever, dehydration, cold sores, and a runny nose are more commonly reported.
  • May not be suitable for everybody, including people with advanced HIV, allogenic bone marrow transplant, kidney problems, taking certain medications, or those who are dehydrated. Elderly people may be more sensitive to some of the side effects of valacyclovir.
  • Rarely, may cause side effects such as agitation, hallucinations, confusion, or seizures. The risk is higher in people with kidney disease and those receiving higher than the recommended dosage of valacyclovir.
  • Valacyclovir is not a cure for genital herpes or herpes labialis (cold sores).
  • The dosage of valacyclovir needs to be reduced in people with kidney disease.
  • Resistant strains of HSV and VZV have been reported.
  • Adequate hydration must be maintained otherwise acyclovir can precipitate in the renal tubules if its solubility is exceeded in the intratubular fluid.
  • Weigh up the risks versus benefits of treatment because acyclovir (the active form of valacyclovir) crosses the placenta into the unborn child. Acyclovir also passes into breast milk.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Valacyclovir is an antiviral prodrug that is rapidly converted into acyclovir in the body. It should be started at the first sign of genital herpes or cold sore symptoms but is not a cure for these conditions. Valacyclovir only needs to be taken twice a day compared with acyclovir which needs to be taken five times a day.

5. Tips

  • Take valacyclovir exactly as directed by your doctor and for the exact time prescribed, even if your symptoms improve.
  • May be taken with or without food.
  • Valacyclovir treatment is best started within 24 hours of symptom onset (ie, rash, blisters, tingling, burning). Data suggests valacyclovir is most effective if started within this time.
  • Keep valacyclovir suspension in the refrigerator and shake well before use. Discard any unused portion after 28 days.
  • Drink plenty of water while taking valacyclovir and do not allow yourself to become dehydrated.
  • Although valacyclovir treats viruses, it is not effective against the flu or the common cold.
  • If you have a shingles rash or a cold sore, try and keep it as clean and dry as possible. Be careful not to transfer the virus to others; ensure you do not share linen, such as towels. Wearing loose clothing may help prevent a shingles rash from becoming irritated.
  • If you are taking valacyclovir for genital herpes or cold sores avoid sexual contact until you have finished the course of treatment. At other times, always use a condom, because herpes can be transmitted in the absence of symptoms.
  • Seek urgent medical advice if you have any signs of an allergic reaction (such as hives; difficulty breathing; facial or throat swelling).
  • Herpes infections are highly contagious and you should cover your rash and avoid intimate contact. Also, avoid touching the rash and then your eyes. Wash your hands frequently. If you have treated yourself for genital herpes and it recurs, then seek retreatment as soon as possible.
  • If you have recurring cold sores, initiate treatment as soon as possible following the onset of symptoms (tingling, burning, or itching).
  • Talk to your doctor if you are pregnant or planning a pregnancy because acyclovir (the active form of valacyclovir) crosses the placenta into the unborn child. Acyclovir also passes into breast milk.

6. Response and effectiveness

  • Valacyclovir is a prodrug that is rapidly converted to acyclovir after oral dosing. Peak concentrations of acyclovir are reached within one to three hours.
  • It may take up to three days before a reduction in symptoms occurs. Even if symptoms fully abate, valacyclovir should be taken until the prescribed course has been completed. Best started within 24-48 hours of symptom onset.
  • For recurrent episodes of genital herpes, therapy should be initiated within 24 hours of the onset of signs or symptoms; efficacy has not been proven if started after this time.

7. Interactions

Medicines that interact with valacyclovir may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with valacyclovir. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with valacyclovir include:

  • cimetidine
  • clopidogrel
  • clozapine
  • hepatitis B medications, such as entecavir
  • some vaccinations, such as varicella virus or zoster virus vaccine
  • NSAIDs, such as ibuprofen, diclofenac, or naproxen
  • other antivirals, such as acyclovir
  • probenecid
  • tenofovir
  • theophylline
  • warfarin, an anticoagulant (blood thinner)
  • zidovudine.

Alcohol should be avoided while taking valacyclovir because it weakens the immune system and may exacerbate the side effects of valacyclovir.

Note that this list is not all-inclusive and includes only common medications that may interact with valacyclovir. You should refer to the prescribing information for valacyclovir for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use valacyclovir only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: April 5, 2023.