Name: Acyclovir

What is the dosage for acyclovir?

Acyclovir may be taken with or without food.

  • Adult oral doses for treating genital herpes are 200 mg every 4 hours (5 times daily) for 7-10 days or 400 mg three times daily for 5-10 days.
  • Herpes Zoster (shingles) is treated with 800 mg every 4 hours (5 times daily) for 7 to 10 days.
  • The dose for treating chicken pox is 800 mg 4 times daily. The usual adult intravenous dose is 5-10 mg/kg every 8 hours for 7-10 days.

Is acyclovir safe to take if I'm pregnant or breastfeeding?

There are no adequate studies of acyclovir in pregnant women. In a patient registry of women who used acyclovir during the first trimester, the rate of birth defects was similar to the rate of birth defects in the general population.

Acyclovir is excreted in breast milk, and a significant amount may be transferred to the infant.

Cautions for Acyclovir


  • Known hypersensitivity to acyclovir or valacyclovir.403 409



Renal Effects

Increased BUN and/or Scr, anuria, and hematuria have been reported.403 409 Transient increases in BUN and/or Scr and decreases in Clcr reported in patients receiving IV acyclovir, particularly following rapid (over <10 minutes) IV infusion.409

Abnormal urinalysis (increase in formed elements in urine sediment) and pain or pressure on urination reported rarely with IV acyclovir.409

Renal failure, resulting in death, has occurred.341 403 409

Possible precipitation of acyclovir in renal tubules, resulting in renal tubular damage and acute renal failure, when the solubility of free acyclovir in the collecting duct is exceeded or following rapid IV administration.409

Risk of adverse renal effects during IV therapy depends on degree of hydration, urine output, concomitant therapy (i.e., nephrotoxic drugs), preexisting renal disease, and rate of administration (see Rate of Administration under Dosage and Administration).409

Alterations in renal function during IV acyclovir therapy can progress to acute renal failure but generally are transient and resolve spontaneously or following improved hydration and electrolyte balance, dosage adjustment, or discontinuance of the drug.409

Hematologic Effects

Potentially fatal thrombotic thrombocytopenic purpura/hemolytic uremic syndrome reported in immunocompromised patients receiving acyclovir.403 409

General Precautions

Nervous System Effects

Possible encephalopathic effects (e.g., lethargy, obtundation, tremors, confusion, hallucinations, agitation, seizures, coma) in patients receiving IV acyclovir.409

Use with caution in patients with underlying neurologic abnormalities and in those with serious renal, hepatic, or electrolyte abnormalities or substantial hypoxia.409

Local Effects

Severe local inflammatory reactions, including tissue necrosis, have occurred following infusion of acyclovir into extravascular tissues.409

Sodium Content

Sodium salt of acyclovir contains 4.2 mEq of sodium per gram of acyclovir.409

Specific Populations


Category B.403 409

CDC, AAP, and others state that oral acyclovir may be used during pregnancy to treat first episodes or severe recurrent episodes of genital herpes244 322 381 412 421 and IV acyclovir may be used during pregnancy to treat severe HSV infection (especially life-threatening disseminated infections).244 322 412 421 CDC and others also recommend acyclovir for treatment of varicella during pregnancy,412 415 particularly during the second and third trimesters.415


Distributed into milk following oral or IV administration.251 308 403 409 421 Use with caution.403 409

Women with active herpetic lesions near or on the breast should refrain from breast-feeding.322

Pediatric Use

Safety and efficacy of oral acyclovir not established in children <2 years of age.403

Geriatric Use

For treatment of herpes zoster (shingles, zoster), no substantial differences in efficacy of oral acyclovir relative to younger adults, but duration of pain after healing may be longer in geriatric patients.403

Insufficient experience in patients ≥65 years of age to determine whether geriatric patients respond differently to IV acyclovir than younger adults.409

Select dosage with caution because of age-related decreases in renal function and potential for concomitant disease and drug therapy.409 Consider monitoring renal function.409

Possible increased incidence of adverse CNS effects (coma, confusion, hallucinations, somnolence), GI effects (nausea, vomiting), or dizziness during oral acyclovir therapy compared with younger adults.403

Hepatic Impairment

Use with caution.a

Renal Impairment

Decreased acyclovir clearance.409 Increased risk of adverse renal and encephalopathic effects.409

Adjust dosage to prevent drug accumulation, decrease risk of toxicity, and maintain adequate plasma drug concentrations.316 (See Renal Impairment under Dosage and Administration.)

Common Adverse Effects

With oral therapy, nausea and/or vomiting and diarrhea.403 With IV therapy, local reactions at the injection site (inflammation, phlebitis).409

What should i avoid while taking acyclovir (zovirax)?

Herpes infections are contagious and you can infect other people, even while you are being treated with acyclovir. Avoid letting infected areas come into contact with other people. Avoid touching an infected area and then touching your eyes. Wash your hands frequently to prevent passing the infection to others.

Acyclovir will not prevent the spread of genital herpes. Avoid sexual intercourse or use a latex condom to prevent spreading the virus to others.

Side effects

Adverse Drug Reaction Overview

The most frequent adverse reactions associated with the use of ZOVIRAX® (acyclovir) are headache and nausea.

Neurological side effects have also been reported in rare instances. Elderly patients and patients with a history of renal impairment are at increased risk of developing these effects. In the reported cases, these reactions were generally reversible on discontinuation of treatment (see WARNINGS AND PRECAUTIONS and ADVERSE REACTIONS, Post-Market Adverse Drug Reactions).

Clinical Trial Adverse Drug Reactions

Because clinical trials are conducted under very specific conditions the adverse reaction rates observed in the clinical trials may not reflect the rates observed in practice and should not be compared to the rates in the clinical trials of another drug. Adverse drug reaction information from clinical trials is useful for identifying drug-related adverse events and for approximating rates.

Treatment of Herpes Simplex

Short-term administration (5-10 days): The most frequent adverse reactions reported during clinical trials of treatment of genital herpes with oral ZOVIRAX® in 298 patients are listed in Table 1.

Table 1 : Adverse Reactions Reported in Clinical Trials of Treatment of Genital Herpes with Acyclovir

Adverse Reactions Total %
Nausea and/or Vomiting 8 2.7

Suppression of Herpes Simplex

Long-term administration: The most frequent adverse events reported in a clinical trial for the prevention of recurrences with continuous administration of 400 mg (two 200 mg capsules) 2 times daily are listed in Table 2.

Table 2 : Adverse Reactions Reported in a Clinical Trial for the Prevention of Recurrences of Genital Herpes with Acyclovir

Adverse Reactions 1st Year
(n=586) %
2nd Year
(n=390) %
3rd Year
(n=329) %
Nausea 4.8    
Diarrhea 2.4    
Headache 1.9 1.5 0.9
Rash 1.7 1.3  
Paresthesia   0.8 1.2
Asthenia     1.2

Evidence so far from clinical trials suggests that the severity and frequency of adverse events is unlikely to necessitate discontinuation of therapy.

Herpes Zoster

The most frequent adverse reactions reported during three clinical trials of treatment of herpes zoster (shingles) with 800 mg of oral ZOVIRAX® 5 times daily for 7 or 10 days or placebo are listed in Table 3.

Table 3 : Adverse Reactions Reported in Clinical Trials of Treatment of Herpes Zoster

Adverse Reactions ZOVIRAX®
(n=323) %
(n=323) %
Malaise 11.5 11.1
Nausea 8.0 11.5
Headache 5.9 11.1
Vomiting 2.5 2.5
Diarrhea 1.5 0.3


The most frequent adverse events reported during three clinical trials of treatment of chickenpox with oral ZOVIRAX® or placebo are listed in Table 4.

Table 4 : Adverse Reactions Reported in Clinical Trials of Treatment of Chickenpox

Adverse Reactions ZOVIRAX®
Diarrhea 3.2 2.2

Less Common Clinical Trial Adverse Drug Reactions ( < 1%)

Other adverse reactions reported in less than 1% of patients receiving ZOVIRAX® in any clinical trial included: abdominal pain, anorexia, constipation, dizziness, edema, fatigue, flatulence, inguinal adenopathy, insomnia, leg pain, medication taste, skin rash, sore throat, spasmodic hand movement and urticaria.

Abnormal Hematologic And Clinical Chemistry Findings

No clinically significant changes in laboratory values have been observed in clinical trials for the treatment of chickenpox and zoster, and for the treatment and suppression of genital herpes with ZOVIRAX®.

Post-Market Adverse Drug Reactions

The following events have been reported voluntarily during post-market use of ZOVIRAX® in clinical practice. These events have been chosen for inclusion due to either their seriousness, frequency of reporting, potential causal connection to ZOVIRAX® or a combination of these factors. Post-market adverse events are reported spontaneously from a population of unknown size, thus estimates of frequency cannot be made.

General: Fever, headache, pain and peripheral edema.

Nervous: Dizziness, paresthesia, agitation, confusion, tremor, ataxia, dysarthria, hallucinations, psychotic symptoms, convulsions, somnolence, encephalopathy and coma have been reported. These events are generally reversible and usually reported in patients with renal impairment, or with other predisposing factors (see WARNINGS AND PRECAUTIONS). These symptoms may be marked, particularly in older adults.

Digestive: Diarrhea, gastrointestinal distress and nausea.

Hematogical and Lymphatic: Anaemia, leukopenia, lymphadenopathy and thrombocytopenia.

Hypersensitivity and Skin: Alopecia, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, rashes including photosensitivity, pruritus, urticaria, dyspnoea, angioedema and anaphylaxis.

Hepatobiliary Tract and Pancreas: Reports of reversible hyperbilirubinemia and elevated liver related enzymes. Hepatitis and jaundice.

Musculoskeletal: Myalgia.

Special Senses: Visual abnormalities.

Urogenital: Elevated blood creatinine and blood urea nitrogen (BUN). Acute renal failure, renal pain and hematuria have been reported. Renal pain may be associated with renal failure (see WARNINGS AND PRECAUTIONS).

Read the entire FDA prescribing information for Zovirax (Acyclovir)

Read More »
  • Chickenpox (Varicella)
  • Genital Herpes in Women (Symptoms, Signs, Treatment)
  • Shingles
  • STDs in Men

For the Consumer

Applies to acyclovir: capsule, powder for solution, solution, suspension, tablet

Other dosage forms:

  • buccal mucosa tablet

Along with its needed effects, acyclovir may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking acyclovir:

More common - For acyclovir injection only
  • Pain, swelling, or redness at place of injection
Less common
  • Abdominal or stomach pain
  • decreased frequency of urination or amount of urine
  • increased thirst
  • loss of appetite
  • nausea or vomiting
  • unusual tiredness or weakness
  • Black, tarry stools
  • blood in urine or stools
  • chills, fever, or sore throat
  • confusion
  • convulsions (seizures)
  • hallucinations (seeing, hearing, or feeling things that are not there)
  • hives
  • pinpoint red spots on skin
  • trembling
  • unusual bleeding or bruising
Frequency not determined
  • Bleeding or oozing from puncture sites or mucous membranes (bowel, mouth, nose, or urinary bladder), continuing
  • blistering, peeling, or loosening of skin
  • bluish coloring, especially of the hands and feet
  • blurred vision
  • bruising at the place of injection
  • changes in facial skin color
  • changes in vision
  • clumsiness
  • coughing
  • decreased consciousness
  • difficulty in breathing or swallowing
  • dizziness or feeling faint, severe
  • fast heartbeat
  • irritability
  • itching or skin rash
  • large hive-like swelling on face, eyelids, lips, tongue, throat, hands, legs, feet, sex organs
  • mood or mental changes
  • muscle cramps, pain, or weakness
  • pale skin
  • red or irritated eyes
  • sense of agitation or uneasiness
  • shakiness and unsteady walk
  • sores, ulcers, or white spots in mouth or on lips
  • swelling of eyelids, face, feet, hands, lower legs or lips
  • swollen, painful, or tender lymph nodes (glands) in neck, armpit, or groin
  • unsteadiness or other problems with muscle control or coordination
  • yellow eyes or skin

Some side effects of acyclovir may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common - Especially seen with high doses
  • General feeling of discomfort or illness
Less common - Especially seen with long-term use or high doses
  • Diarrhea
  • headache
Frequency not determined
  • Burning, prickling, or tingling sensations
  • drowsiness
  • loss of hair

Usual Pediatric Dose for Herpes Simplex - Congenital

Neonatal Herpes:
Birth to 3 months: 10 mg/kg IV every 8 hours for 10 days (manufacturer dosing)

Birth to 3 months: 20 mg/kg IV every 8 hours (CDC recommendation)
-Duration of therapy: Disease limited to the skin and mucous membranes: 14 days; Disseminated disease or disease involving the CNS: 21 days

Follow with oral suppressive therapy: 300 mg/m2 orally 3 times a day for 6 months

-Neonates born to women who acquire HSV near term should be treated due to high risk of infection; infants exposed to HSV during birth should be followed by a pediatric infectious-disease specialist.
-For neonatal HSV with CNS involvement, confirm virus is absent from cerebrospinal fluid prior to stopping therapy; CSF HSV DNA PCR should be performed on days 19 and 21 and repeated as needed.
-Following IV treatment, oral prophylaxis for 6 months should be considered in those with CNS or skin, eyes, and mouth disease as it may be associated with superior neurodevelopmental outcome and prevent cutaneous recurrences.
-CDC STD Treatment Guidelines and the Guidelines for the Prevention and Treatment of Opportunistic Infections Among HIV- Exposed and HIV-Infected Children may be consulted for additional guidance.

Use: For known or suspected neonatal HSV.

Usual Pediatric Dose for Herpes Simplex - Mucocutaneous/Immunocompetent Host

Treatment of First Episode of Genital Herpes:
Less than 12 years: 40 to 80 mg/kg/day orally in divided doses 3 to 4 times a day for 5 to 10 days
Maximum dose: 1000 mg/day

12 years or older: 200 mg orally every 4 hours 5 times a day OR 400 mg orally 3 times a day
Duration of therapy: 7 to 10 days

Severe Disease or Complications Requiring Hospitalization:
Less than 12 years: 10 mg/kg IV every 8 hours for 7 days
12 years or older: 5 mg/kg IV every 8 hours for 7 days

Recurrence of Genital HSV Infection:
Less than 12 years: 20 to 25 mg/kg orally twice a day; Maximum dose: 400 mg
12 years or older: 200 mg orally 5 times a day for 5 days OR 800 mg orally 2 times a day for 5 days OR 800 mg orally 3 times a day for 2 days

-All patients with newly acquired genital herpes should receive antiviral therapy as first episodes can cause a prolonged clinical illness, even among persons with mild clinical manifestations initially; therapy should be initiated at the earliest sign or symptom of primary infection; IV therapy is indicated for patients with severe infection.

Use: For the treatment of first episode or recurrence of mucosal and cutaneous herpes simplex (HSV-1 and HSV-2).

Usual Pediatric Dose for Varicella-Zoster

Immunocompetent host:
2 years or older (40 kg or less): 20 mg/kg orally 4 times a day for 5 days
2 years or older (over 40 kg): 800 mg orally 4 times a day for 5 days
Maximum doses: Single: 800 mg; Daily: 3200 mg/day

Immunocompromised host:
Less than 1 year: 10 mg/kg IV 3 times a day for 7 to 10 days
1 year or older: 500 mg/m2 IV 3 times a day for 7 to 10 days

HIV-exposed and HIV-infected Children
-Mild disease with no or moderate immune suppression (CDC immunologic category 1 and 2): 20 mg/kg orally 4 times a day for 7 to 10 days and until no new lesions for 48 hours
Maximum dose: 800 mg
-Severe immune suppression (CDC immunologic category 3): 10 mg/kg or 500 mg/m2 IV every 8 hours for 7 to 10 days and until no new lesions for 48 hours

HIV-Infected Adolescents:
-Uncomplicated course: 800 mg orally 5 times a day for 5 to 7 days (alternative therapy; oral valacyclovir or famciclovir are preferred therapy)
-Severe or complicated course: 10 to 15 mg/kg IV every 8 hours for 7 to 10 days; may switch to oral therapy after defervescence if no evidence of visceral involvement

-Therapy should be initiated at the earliest sign of chickenpox, no later than 24 hours after onset of rash.
-In children 1 year or older, body surface area may be used for dosing instead of body weight.
-Guidelines for the Prevention and Treatment of Opportunistic Infections Among HIV- Exposed and HIV-Infected Children and HIV- Infected Adults and Adolescents may be consulted for additional guidance.

Use: For the treatment of chickenpox (varicella).

How it works

  • Acyclovir is an antiviral drug that has activity against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV).
  • Acyclovir works in a number of different ways to prevent these viruses from replicating. Activity against HSV is greater than activity against VZV.
  • Acyclovir belongs to the class of medicines called antivirals.


  • Acyclovir treatment is best started within 72 hours of symptom onset (ie, rash, blisters, tingling, burning) and most effective if started within 48 hours of symptom onset.
  • Take acyclovir for the exact time prescribed by your doctor, even if your symptoms improve. Although acyclovir treats viruses, it is not effective against the flu or the common cold.
  • Shake acyclovir oral suspension well before using. Use a proper dosing syringe or medicine cup to correctly measure dosage, not a kitchen teaspoon.
  • If you have been prescribed buccal tablets to treat a cold sore, place the flat side of the buccal tablet against your upper gum, behind your lip and in front of your canine tooth on the same side of your mouth as the cold sore. Close your mouth and gently press on the outside of your lip to hold the tablet in place for 30 seconds. Allow the tablet to slowly dissolve; eating and drinking may continue as normal while the buccal tablet is in place, but avoid brushing your teeth, chewing gum, or wearing an upper denture.
  • If you are taking acyclovir long-term and your weight changes, let your doctor know as acyclovir dosages are based on weight.
  • Try to maintain good hydration when taking acyclovir.
  • If you have a shingles rash or a cold sore, try and keep it as clean and dry as possible; although you must be careful not to transfer the virus to others, so always use disposable tissues or a separate towel that is washed by itself. Wearing loose clothing may help prevent irritation of a shingles rash.
  • Seek urgent medical advice if you have any signs of an allergic reaction (such as hives; difficulty breathing; facial or throat swelling), easy bruising or bleeding or difficulty urinating, swelling in your feet, or shortness of breath.
  • 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 hands frequently.
  • If you are taking acyclovir for genital herpes, it will not prevent you from passing the infection to your sexual partner. Sexual intercourse should be avoided while you have lesions or the first symptoms of an outbreak. Even if you have no symptoms, transmission of genital herpes may still occur.
  • Talk to your doctor or pharmacist before buying any medication over the counter to check that it is compatible with acyclovir, including vitamins or herbal products.
  • Acyclovir may make your skin more sensitive to the sun and more likely to burn. Wear at least an SPF 30+ sunblock and sun-protective clothing when you need to go outside.

What is acyclovir?

Acyclovir is an antiviral drug. It slows the growth and spread of the herpes virus in the body. Acyclovir will not cure herpes, but it can lessen the symptoms of the infection.

Acyclovir is used to treat infections caused by herpes viruses, such as genital herpes, cold sores, shingles, and chicken pox.

Acyclovir may also be used for purposes not listed in this medication guide.