Acyclovir dosage, forms, and strengths

Acyclovir, known by the brand name Zovirax, is a generic prescription antiviral drug that treats herpes viral infections, including chickenpox, shingles, genital herpes, cold sores, herpes simplex encephalitis, neonatal herpes infections, and varicella-zoster infections in people with weakened immune systems. These are uses of acyclovir that are approved by the Food and Drug Administration (FDA), but healthcare professionals may prescribe acyclovir off-label to treat herpes eye infections. With all these uses, acyclovir is available in several oral, topical, and injected formats. Dosages depend on the condition being treated. Common side effects include nausea, vomiting, headache, and malaise. Serious side effects include psychosis, allergic reactions, skin hypersensitivity, and hemolytic uremic syndrome.

Acyclovir forms and strengths

Acyclovir treats herpes infections of many types, so it comes in many formats appropriate for its particular uses: 

• Tablets: 400 milligrams (mg), 800 mg

• Capsules: 200 mg

• Oral suspension: 200 mg/5 mL

• Cream (Zovirax): 5% (50 mg/g)

• Ointment: 5% (50 mg/g)

• Injection (intravenous infusion): must be diluted before use

Acyclovir dosage for adults

The FDA has approved acyclovir tablets, capsules, and oral liquids to treat shingles, chickenpox, and genital herpes. Injections are used for herpes simplex or varicella-zoster infections in people with weakened immune systems, herpes infections of the brain (herpes simplex encephalitis), and initial episodes of genital herpes. Acyclovir cream is approved to treat cold sores in immunocompetent adults. Finally, acyclovir ointment is approved to treat genital herpes or herpes infections of the skin or mucus membranes.

Acyclovir dosage for shingles

People with shingles will take acyclovir orally every four hours for seven to 10 days. 

Acyclovir dosage for genital herpes

Acyclovir treatment for genital herpes can be complex. People with a healthy immune system usually take oral acyclovir or use a topical ointment. Initial episodes in people with weakened immune systems may be treated by acyclovir ointment. Severe initial episodes may require infusions. Acyclovir infusions are time-consuming. Each infusion lasts for one hour, so having an infusion every eight hours will require three hours to complete.

• Standard adult dosage for genital herpes: 200 mg by mouth every four hours for 10 days, ointment applied every three hours six times per day for seven days (initial episode), or 5 mg/kg infused every eight hours for five days (severe initial episode)

Acyclovir dosage for chickenpox

Normally, a childhood disease, chickenpox, can be contracted by anyone who has never had the infection. For adults and adolescents who are 12 years of age and older, the standard regimen is 800 mg by mouth (tablets, capsules, or oral suspension) four times daily for five days. 

Acyclovir dosage for herpes simplex encephalitis

A herpes infection of the brain or tissues surrounding the brain, herpes simplex encephalitis, is a serious and life-threatening medical emergency. Emergency treatment and hospitalization is required. The mortality rate is 70%, even with treatment. Acyclovir infusions are the treatment of choice. The FDA advises 10 days of infusions, but treatment is more likely to last from 14 to 21 days. Those with a weakened immune system may require higher doses for longer.

• Standard adult dosage for herpes simplex encephalitis: 10 mg/kg infused every eight hours for 10 days

• Maximum adult dosage for herpes simplex encephalitis: 20 mg/kg infused every eight hours 

Acyclovir dosage for immunocompromised patients

Herpes infections are difficult to treat in people with weakened immune systems, so healthcare professionals are more likely to treat them with acyclovir infusions than oral acyclovir. The standard infusion dosage for herpes infections (HSV-1 and HSV-2) of the skin or mucus membranes in immunocompromised patients is 5 mg/kg infused every eight hours for seven days.

Acyclovir dosage for children

Healthcare professionals commonly use acyclovir in children as young as newborns. Chickenpox and cold sores are two common conditions treatable by acyclovir, but it is also used in neonates born with herpes infections. Additionally, the FDA has approved acyclovir to treat children with genital herpes infections. Finally, like adults, children can get herpes encephalitis, most commonly treated with acyclovir. Standard pediatric dosages for acyclovir vary by indication:

• Chickenpox in children ages 2 years and older: 20 mg/kg taken by mouth four times daily for five days OR 800 mg taken by mouth four times daily for five days for children weighing more than 40 kg (88 lbs)

• Herpes cold sore outbreaks in children ages 12 years and older: 5% cream applied to the cold sores five times daily for four days

• Mucosal or skin herpes simplex virus infections in immune-compromised children ages three months to 12 years: 10 mg/kg infused every eight hours for seven days

• Mucosal or skin herpes simplex virus infections in immunocompromised children ages 12 years and older: 5 mg/kg infused every eight hours for seven days

• Severe initial episodes of genital herpes in children ages 12 years and older: 5 mg/kg infused every eight hours for five days

• Herpes simplex encephalitis in children ages three months to 12 years: 20 mg/kg infused every eight hours for 10 days

• Herpes simplex encephalitis in children ages 12 years and older: 10 mg/kg infused every eight hours for 10 days

• Neonatal herpes infections in newborns younger than 34 postmenstrual weeks of age: 20 mg/kg infused every twelve hours for 21 days

• Neonatal herpes infections in newborns 34 postmenstrual weeks of age and older: 20 mg/kg infused every eight hours for 21 days

Acyclovir dosage restrictions

No acyclovir dosage reductions are necessary in people with hepatic impairment (liver disease). However, people with kidney problems may require lower doses. Renally impaired patients (creatinine clearance <10 mL/min) will take reduced oral doses or take doses further apart. For renally impaired patients getting acyclovir infusions, healthcare professionals will give doses further apart based on renal function. For severe renal impairment (creatinine clearance ≤10 mL/min), healthcare professionals will also reduce the dose by 50%.

Acyclovir dosage for pets 

Veterinarians commonly use acyclovir in pet birds to treat Pacheco’s disease, a life-threatening herpes infection. In dogs, vet professionals use acyclovir to prevent parvovirus infection in puppies or to treat canine herpes virus (CHV-1) infections in newborn puppies. Acyclovir can be lethal to cats, so veterinarians usually avoid it. Dosages are based on body weight.

• Standard newborn puppy dosage for CHV-1 infections: 10 mg/kg by mouth every six hours for five days

• Standard puppy dosage for parvovirus prevention: 20 mg/kg infused every eight hours for five days

How to take acyclovir

Take acyclovir as instructed. Do not take more medicine than prescribed. Drink plenty of fluids when taking acyclovir because adequate hydration is necessary to preserve kidney function. Taking acyclovir capsules or tablets is relatively simple, but creams and ointments may involve a few more complex steps. Infusions are usually administered by a healthcare professional, but a doctor may allow a patient to self-administer infusions at home.

Tablets and capsules

• This medicine can be taken with or without food.

• Store the medicine at room temperature in a dry place.

Oral suspension

• This medicine can be taken with or without food.

• Shake the bottle well before measuring out a dose.

• Use a metered oral syringe or other special measuring device. Do not use kitchen measuring tools or spoons.

• Store the medicine at room temperature in an area protected from light.

Cream

• Use this medicine at the first sign of cold sores.

• Use only on the face and lips.

• Clean the face and lips and allow them to fully dry before applying the cream.

• Wash your hands before applying the cream.

• Cover the entire cold sore or lesion, including the outer margin.

• Wash hands thoroughly after you’re finished.

• Store at room temperature.

Ointment

• Use this medicine at the first sign of genital herpes lesions.

• Use a glove or finger cot when applying the ointment.

• Cover all the blisters or lesions with ointment.

• Wash hands thoroughly when done.

• Store at room temperature in a dry place.

Infusions

• A specialty infusion nurse or other healthcare professional will give you or a caretaker hands-on training in the use of supplies, medicine storage, and the technique of administering infusions at home. 

• Do not self-administer infusions unless you fully understand how to do it.

Acyclovir dosage FAQs

Acyclovir vs. acyclovir XR: What’s the difference in doses?

Sitavig, the extended-release buccal tablet version of acyclovir, is no longer sold in the United States, leaving immediate-release acyclovir as the only option.

What happens if I miss a dose of acyclovir?

Take a missed dose as soon as it’s remembered unless it’s almost time for the next dose. In that case, take the next dose as scheduled and skip the missed dose. Because acyclovir doses are closely spaced, ask the prescriber or a pharmacist for medical advice if you’re uncertain when to take a missed dose.

Can you overdose on acyclovir?

If too much acyclovir is taken by mouth, call a poison control center or visit an emergency department. An acyclovir overdose can bring on neurological problems, such as agitation, lethargy, seizures, and coma. Overdosing on acyclovir cream or ointment is unlikely since little medicine is absorbed into the body.

What interacts with acyclovir?

Acyclovir is generally safe to take with other drugs. The only drugs that are contraindicated to take with acyclovir are cidofovir, another antiviral drug, and Veozah (fezolinetant), a new treatment for menopausal hot flashes. Regarding potentially serious drug interactions, healthcare professionals avoid combining acyclovir with nearly identical antiherpes medications, like Valtrex (valacyclovir) and Famvir (famciclovir). These drugs have nearly identical adverse effects, so the combination increases the risk of experiencing serious adverse reactions. 

Is it safe to take acyclovir during pregnancy?

Pregnant women can take acyclovir without known risk of birth defects or pregnancy problems. Acyclovir is present in breast milk, so healthcare professionals are cautious about prescribing oral or intravenous acyclovir to breastfeeding women.

Related resources for acyclovir dosages:

• Acyclovir 200 mg/5 mL (5 mL) oral suspension, Kaiser Permanente (2023)

• Acyclovir capsule prescribing information, National Library of Medicine (2023)

• Acyclovir cream prescribing information, National Library of Medicine (2021)

• Acyclovir drug summary, Prescriber’s Digital Reference (2023)

• Acyclovir sodium injection prescribing information, National Library of Medicine (2020)

• Zovirax acyclovir ointment prescribing information, National Library of Medicine (2020)