Acyclovir facts
Valtrex facts

Acyclovir (Zovirax)

Generic Name: Acyclovir

Brand Name: Zovirax

Oral: capsules 200 mg; tablets 400 mg, 800 mg; oral suspension (200 mg per 5-mL spoonful);
Topical: ointment 5%; cream 5%.


Acyclovir (Zovirax) is prescribed to treat genital herpes, herpes labialis (cold sores on the lips), herpes zoster, and varicella-zoster (chickenpox). Acyclovir reduces the duration of herpes episode and makes the lesions clear faster. Suppressive therapy decreases the frequency and severity of recurrences, and lessens the viral shedding.

Acyclovir is not curative, because it does not rid the body of dormant viruses.

Topical acyclovir (Zovirax) cream is applied to cold sores only. Topical formulation does not prevent the recurrence of sores but relieves pain and itching, and accelerates the curing of cold sores if applied promptly after onset of earliest symptoms.


Genital herpes:

Initial episode (unlabeled use): 40-80 mg/kg/day divided into 3-4 doses for 5-10 days (maximum: 1 g/day).
Chronic suppression (unlabeled use; limited data): 80 mg/kg/day in 3 divided doses (maximum: 1 g/day), re-evaluate after 12 months.

Initial episode: 200 mg every 4 hours while awake (5 times/day) for 10 days (per manufacturer's labeling); 400 mg 3 times/day for 5-10 days has also been reported.
Recurrence: 200 mg every 4 hours while awake (5 times/day) for 5 days (per manufacturer's labeling; begin at earliest signs of herpes); 400 mg 3 times/day for 5 days has also been reported.
Chronic suppression: 400 mg twice daily or 200 mg 3-5 times/day, for up to 12 months followed by re-evaluation (per manufacturer's labeling); 400-1200 mg/day in 2-3 divided doses has also been reported.

Cold sores:

Topical acyclovir 5% cream is applied 5 times a day. It is flexible and can be applied at any stage of cold sores to decrease pain and the duration of hard crust, even if you have full-blown sores.

Herpes zoster:

Adults (immunocompetent): 800 mg every 4 hours (5 times/day) for 7-10 days.

Varicella-zoster (chickenpox):

Chickenpox is generally a mild, self-limiting viral disease. However, varicella is extremely contagious and majority of children catch this virus before adolescence.

When started promptly, acyclovir accelerates the recovery from chickenpox and shortens the period when an ailing child or adolescent is contagious.

Children >= 2 years and <= 40 kg (immunocompetent): 20 mg/kg/dose (up to 800 mg/dose) 4 times/day for 5 days.

Adults and Children >40 kg (immunocompetent): 800 mg/dose 4 times a day for 5 days.

Warnings & Precautions

  • Nephrotoxicity: Renal failure (sometimes fatal) has been reported. Dehydration, pre-existing renal disease and nephrotoxic medications increase risk.
  • Thrombocytopenic purpura/hemolytic uremic syndrome has been reported.
  • Elderly: Use with caution in the elderly; higher risk for CNS and renal adverse events.
  • Children: Safety and efficacy of oral formulations have not been established in children < 2 years of age.
  • Adequate hydration: Maintain adequate hydration during oral or intravenous acyclovir therapy.

Side Effects


  • Central nervous system: malaise (12%); headache (2%)
  • Gastrointestinal: nausea (2-5%), vomiting (3%), diarrhea (2- 3%)

Topical (cream, ointment)

  • Dermatologic: mild burning or stinging (30%); pruritus (4%), itching


  • Dermatologic: hives (2%), itching (2%), rash (2%)
  • Gastrointestinal: nausea/vomiting (7%)
  • Hepatic: liver function tests increased (1-2%)
  • Local: Inflammation at injection site or phlebitis (9%)
  • Renal: BUN increased (5-10%), creatinine increased (5-10%), acute renal failure

Pregnancy & Lactation

Pregnancy Risk Factor: B
Pregnancy Implications. Teratogenic effects were not observed in animal studies. Acyclovir has been shown to cross the human placenta. There are no adequate and well-controlled studies in pregnant women. Results from a pregnancy registry, established in 1984 and closed in 1999, did not find an increase in the number of birth defects with exposure to acyclovir when compared to those expected in the general population. However, due to the small size of the registry and lack of long-term data, the manufacturer recommends using during pregnancy with caution and only when clearly needed.

Lactation. Acyclovir enters breast milk.


  • Probenecid: IV acyclovir plasma levels may be increased, and the duration of action prolonged, while the urinary excretion and renal Cl may be reduced.
  • Zidovudine: Increased propensity for lethargy.
  • Incompatibility: Precipitation may occur with bacteriostatic water. Do not add acyclovir to biologic or colloidal fluids.


Symptoms of overdose include agitation, confusion, elevated serum creatinine, and renal failure. In the event of overdose, sufficient urine flow must be maintained to avoid acyclovir precipitation within renal tubules. Hemodialysis has resulted in up to 60% reduction in serum acyclovir levels.

Effect of Acyclovir on Primary Genital Herpes [2]:

Symptom/sign Duration of symptoms/signs (days)
Without acyclovir With acyclovir*
Viral shedding 9 2
Time until all lesions are crusted 10 7
Time until all lesions are healed 16 10
Local pain 7 5
Constitutional symptoms 6 3

*--Dosage = 200 mg five times daily for 10 days, given within six days of lesion onset. Information from Mertz GJ, Critchlow CW, Benedetti J, Reichman RC, Dolin R, Connor J. Double-blind placebo-controlled trial of oral acyclovir in first-episode genital herpes simplex virus. JAMA 1984;252:1147-51.


Last modified:February, 2015

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