Acyclovir: uses, side effects, interactions, and dosage

Acyclovir is a prescription antiviral medication used to treat certain viruses, especially those in the herpes family. The brand name is Zovirax, but acyclovir is also available as a generic medication. Valacyclovir (brand name Valtrex) is a related antiviral medication that gets broken down into acyclovir.

Like other antivirals, acyclovir acts against viruses—especially the herpes viruses. Acyclovir works by shoving itself into the viral DNA, thus preventing the virus particles from reproducing (Taylor, 2020). This way, acyclovir prevents the virus from spreading throughout your body and gives your immune system a chance to fight the symptoms. However, it does not kill the virus, which can live in your body for years. Sometimes, the virus pops up again months to years after the initial infection.

Some people find that they have frequent flare-ups of their herpes infections. Acyclovir can help these people prevent (or suppress) future episodes through suppressive therapy (Leung, 2000).

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Acyclovir is an antiviral drug that treats viruses belonging to the Herpesviridae family, which include:

• Genital herpes (caused by herpes simplex virus 1 & 2)

• Cold sores (caused by herpes simplex virus 1 & 2)

• Chickenpox (caused by varicella-zoster virus)

• Shingles (a.k.a herpes zoster, caused by varicella-zoster virus

Some people find that they have frequent flare-ups of their herpes infections. Acyclovir can help these people prevent (or suppress) future episodes through suppressive therapy (Leung, 2000).

While effective against outbreaks of certain viruses, acyclovir is not without its side effects. Fortunately, most adverse effects are mild and well-tolerated.

Common side effects include (DailyMed, 2013):

• Malaise (not feeling well)

• Nausea/vomiting

• Diarrhea

Rare side effects include (DailyMed, 2013):

• Behavior changes, like aggression, confusion, agitation, delirium, sleepiness, loss of consciousness

• Anaphylaxis or severe allergic reaction (rash, hives, trouble breathing, etc.)

• Blood cell changes (low levels of red blood cells, white blood cells, or platelets) 

• Liver problems or abnormal liver lab tests

• Muscle pain (myalgia)

• Severe skin rash, like Stevens-Johnson syndrome or toxic epidermal necrolysis

• Kidney failure

Seek medical attention at the first sign of any serious side effects.

This list does not include all possible side effects and others may exist. Check with your pharmacist or healthcare provider for more information.

Before starting acyclovir, talk to your healthcare provider about other medications you are taking to avoid any potential drug interactions. Acyclovir may have the following drug interactions (UpToDate, 2013):

• Cladribine: Cladribine is a chemotherapy medicine used to treat leukemia. It loses its effectiveness if taken with acyclovir, so avoid using the two together.

• Foscarnet: Foscarnet is an antiviral drug also used to treat herpes viruses. Taking it with acyclovir increases the risk of toxicity to the kidney. 

• Probenecid: Probenecid is used to treat gout and gouty arthritis. It decreases the removal of acyclovir by the kidney. Taking the two medicines together may lead to higher levels of acyclovir and increase the risk of adverse effects.

Getting varicella or zoster vaccines while taking acyclovir may interfere with the vaccine's effectiveness (UpToDate, n.d.). Avoid taking acyclovir 24 hours before and 14 days after getting the vaccine.

This list does not include all possible drug interactions with acyclovir and others may exist. Check with your pharmacist or healthcare provider for more information.

Certain groups are at higher risk of side effects and should avoid using acyclovir or use it with caution and careful monitoring. These groups include (UpToDate, n.d.):

• People with kidney disease or kidney problems: The kidneys are responsible for removing acyclovir from the body—people with decreased kidney function need a lower dose of acyclovir. Otherwise, too much of the drug can accumulate, leading to kidney failure.

• Older adults: Older adults often have reduced kidney function due to their age and may need a lower acyclovir dose. Also, older people are more likely to experience side effects like agitation, hallucinations, confusion, seizures, coma, etc. (DailyMed, 2013). Lastly, this group has a higher risk of having nausea, vomiting, and dizziness while on acyclovir.

• People with low immune system function (e.g., those with HIV/AIDS or bone marrow transplants): This group is at a higher risk for developing problems with their blood cells, including thrombotic thrombocytopenic purpura (TTP) or hemolytic uremic syndrome (HUS). These may be life-threatening conditions.

• Pregnant or breastfeeding women: Acyclovir is considered Pregnancy Category B by the FDA, meaning that there are no well-controlled clinical studies of acyclovir in pregnant women. It does cross the placenta, and pregnant women should consult with their healthcare provider before starting acyclovir. Women should use acyclovir during pregnancy only if the potential benefit justifies the potential risk to the fetus. Since it enters breast milk, breastfeeding women should use caution and seek medical advice before taking acyclovir.

This list does not include all at-risk groups. Talk to your healthcare provider for more information.

For the most effective result, you should take acyclovir as soon as possible after you notice the symptoms of genital herpes, cold sores, chickenpox, or shingles. Most people take acyclovir in its pill (tablet or capsule) form or as an oral suspension (for those who can't swallow pills). In some cases, it may be given intravenously. However, acyclovir is also available in other forms (UpToDate, n.d.):

• 5% topical cream for cold sores (brand name Zovirax cream)

• 5% topical ointment for either cold sores or genital herpes (brand name Zovirax ointment)

• 50 mg buccal tablet that you put between your upper lip and gum (brand name Sitavig)

Talk to your healthcare provider to see which dose of acyclovir is right for you.

Acyclovir pills are usually taken several times throughout the day, up to five times daily. The tablets or capsules are available in 200 mg, 400 mg, or 800 mg strengths. If you miss a dose, try to take it as soon as you remember. However, if it is getting close to the time for your next dose, skip it. You don't want to double up on the medication to make up for a missed dose.

Most insurance plans cover acyclovir, but it may depend on the dose and the form of the medicine; the price also varies. The cost of acyclovir pills ranges from $8 to $18 for a 30-day supply, depending on the strength. The topical forms, however, can cost anywhere from $25 to almost $100 per tube. Talk to your pharmacy about your options. Lastly, the newest version of acyclovir, the buccal tablet, costs around $950 for one dose pack (GoodRx.com).

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Current version

August 21, 2020

Written by

Chimene Richa, MD

Fact checked by

Mike Bohl, MD, MPH, ALM

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About the medical reviewer

Mike Bohl, MD, MPH, ALM

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.