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Valacyclovir: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Dec 31, 2019.

1. How it works

  • Valacyclovir is an antiviral drug that is rapidly converted into acyclovir inside the body. Acyclovir has activity against 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 a number of different ways to prevent these viruses from replicating. Activity against HSV is greater than activity against VZV.
  • 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 chicken pox.
  • 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 only needs to be taken twice daily, compared to five times daily for acyclovir. This makes it more convenient for people to take.
  • Valacyclovir is better absorbed than acyclovir.
  • Does not appear to have any clinically significant drug-drug or drug-food interactions.
  • 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, 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 in 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.
  • The dosage of valacyclovir may need reducing in people with kidney disease.
  • May interact with a number of medicines (such as antacids, cimetidine, digoxin, probenecid and thiazide diuretics); however, in people with normal kidney function, these interactions are not clinically significant.

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 drug that helps treat herpes and varicella-zoster infections. It should be started as soon as possible after symptom onset and only needs to be taken twice a day.

5. Tips

  • Take valacyclovir exactly as directed by your doctor and for the exact time prescribed, even if your symptoms improve.
  • Valacyclovir treatment is best started within 72 hours of symptom onset (ie, rash, blisters, tingling, burning). Data suggests valacyclovir is most effective if started within 24 hours.
  • 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.
  • 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 avoid sexual contact while you have symptoms. At other times, always use a condom.
  • 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.

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.

7. Interactions

Medicines that interact with valacyclovir may either decrease its effect, affect how long it works for, 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.


Valacyclovir. Revised 07/2019.

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.

Copyright 1996-2021 Revision date: December 31, 2019.

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