Acyclovir for Herpes Labialis (Cold Sores)

Acyclovir, the first of the nucleoside analogues, was discovered by a team led by Gertrude B. Elion in the late 1970s. Acyclovir is a synthetic purine analogue active against herpes simplex viruses HSV-1 and HSV-2, and varicella-zoster.

Acyclovir is used to treat genital herpes simplex virus, herpes labialis (cold sores), herpes zoster (shingles), HSV encephalitis, neonatal HSV, mucocutaneous HSV in immunocompromised patients, and varicella-zoster (chickenpox).

Acyclovir is available as a topical cream, as oral capsules, tablets, and suspension, and as a powder for intravenous administration.

HSV-1 Infection: Cold Sores

Herpes labialis (also known as cold sores or fever blisters) is caused mainly by herpes simplex virus type 1 (HSV-1). Infection occurs when virus penetrates through abraded skin or mucosal surfaces. HSV-1 is transmitted via direct contact with contaminated saliva or secretions. After primary infection the virus travels through sensory nerves and establishes chronic, lifelong, latent infection within neurons in sensory ganglia.

Symptoms of initial HSV-1 infection usually develop within two weeks after infection and include painful, itchy lesions on the lips and skin around the mouth. The first episode of cold sores may also be accompanied by fever, malaise, and muscle pain.

Severe herpes infection in which the lesions extend into the oral cavity, is called herpetic gingivostomatitis.

Recurrent outbreaks of cold sores are often preceded by a prodrome characterized by burning, tenderness, and tingling at the site of HSV-1 reactivation. The lesions of recurrent infections are rarely as severe as those of initial infections.

Treatment of Cold Sores

Acyclovir is effective in shortening the duration of symptoms and lesions of HSV-1 infections. The beneficial effects of antiviral therapy are more pronounced if the medication is started early after the onset of infection (during prodrome or erythema lesion stage).

Benefits of treatment

  • Shortened the episode duration and lesions healing time by about 1 day15.
  • Reduced duration of pain
  • Reduced viral shedding

Topical acyclovir provides less benefits then oral.

Acyclovir dosage for episodic treatment14

  • Primary episode of cold sores: 200-400 mg 4-5 times per day for 10 days.
  • Recurrent episodes of cold sores: 200-400 mg 5 times daily for 5 days.

Prevention of Cold Sores

Acyclovir is also used to prevent recurrences of herpes infection. Prophylactic daily use of acyclovir reduces reactivation of HSV17, but does not eliminate asymptomatic viral shedding.

Indications for prophylactic acyclovir therapy:

  • Frequent recurrent herpes labialis (more than six per year)
  • History of HSV-associated erythema multiforme
  • Persons anticipating intense sun exposure (e.g. skiers, outdoor work)
  • Facial dermabrasion, laser resurfacing, chemical peels, facial surgical procedures or manipulations
  • Severe immunosuppression, immunocompromised patients
  • Persons whose appearance is very important

Benefits of prophylactic therapy:

  • 53% reduction in frequency of herpes recurrences13
  • Reduced severity of outbreaks
  • Suppression of asymptomatic viral shedding 9 -- acyclovir can reduce asymptomatic viral shedding by 80%
  • Reduced risk of serious complications associated with HSV infection

Acyclovir dosage for prophylaxis14: 400 mg 2 times per day.
Surgical prophylaxis: 800 mg 2 times per day.

How long can you take Acyclovir?

Acyclovir has been shown to prevent recurrent herpes outbreaks even after more than 5 years of continuous suppressive therapy16.

Important facts

1. Acyclovir does NOT cure the herpes infection or free the body of latent HSV.

2. Treatment of primary or recurrent cold sores does NOT reduces the frequency of future recurrences.

3. Acyclovir acts only during active viral replication, which largely precedes visible blisters. Therefore the drug is effective if started during prodromal stage.

Pharmacological characteristics

  • Elimination half-life: 2.5 to 3.3 hr
  • Bioavailability: 10-30% and decreases with increasing dose
  • Metabolism: Converted by viral enzymes to acyclovir monophosphate, and further converted to diphosphate then triphosphate (active form) by cellular enzymes
  • Excretion: Urine (62% to 90% as unchanged drug and metabolite)

Mechanism of action

Acyclovir, an acyclic guanine nucleoside analog, prevents the growth and multiplication of the herpes virus. Acyclovir can insert itself into virus DNA during virus replication. However, the drug does not prevent death of the infected cells.

Acyclovir is selectively phosphorylated only within virus-infected cells by viral thymidine kinase (TK). Further phosphorylation by cellular enzymes leads to the production of acyclovir triphosphate, which competes with the natural nucleotide, dGTP, resulting in the selective inhibition of viral DNA polymerase. Incorporation of the analogue triphosphate into the growing DNA chain prevents continued extension of the DNA chain.

HSV type 1 is the most sensitive virus to acyclovir followed by HSV type 2, varicella-zoster virus is even less sensitive.

User Reviews, Ratings, Discussions


  • 1. Girard M. Safety of acyclovir in general practice: a review of the literature. Pharmacoepidemiol Drug Saf. 1996 Sep;5(5):325-32.
  • 5. Tyring SK, Douglas JM, Corey L, Spruance SL, Esmann J. A randomized, placebo-controlled comparison of oral valacyclovir and acyclovir in immunocompetent patients with recurrent genital herpes infections. Arch Dermatol. 1998 Feb;134(2):185-91. PubMed
  • 9. Wald A, Zeh J, Barnum G, Davis LG, Corey L. Suppression of subclinical shedding of herpes simplex virus type 2 with acyclovir. Annals Internal Medicine. 1996 Jan 1;124(1 Pt 1):8-15 PubMed
  • 13. Rooney JF, Straus SE, Mannix ML, Wohlenberg CR, Alling DW, Dumois JA, Notkins AL. Oral acyclovir to suppress frequently recurrent herpes labialis. A double-blind, placebo-controlled trial. Ann Intern Med. 1993 Feb 15;118(4):268-72. PubMed
  • 14. D. L. Kasper and A. S. Fauci, Harrison's Infectious Diseases 1st Edition; McGraw-Hill, New York, 2010; pp. 735-740
  • 15. Spruance SL, Stewart JC, Rowe NH, McKeough MB, Wenerstrom G, Freeman DJ. Treatment of recurrent herpes simplex labialis with oral acyclovir. J Infect Dis. 1990 Feb;161(2):185-90. PubMed
  • 16. Goldberg LH, Kaufman R, Kurtz TO, et al. Long-term suppression of recurrent genital herpes with acyclovir: a 5-year benchmark. Acyclovir Study Group. Arch Dermatol 1993;129:582–587.
  • 17. Rahimi H, Mara T, Costella J, Speechley M, Bohay R. Effectiveness of antiviral agents for the prevention of recurrent herpes labialis: a systematic review and meta-analysis. Oral Surg Oral Med Oral Pathol Oral Radiol. 2012 May;113(5):618-27.

By HealthyStock Research Group, September 2009
Medical resources reviewed: August 2018

Interesting facts

Acyclovir tablets

  • Generic name: Acyclovir (Acycloguanosine)
  • Trade names: Zovirax
  • Pharmacologic category: Antiviral agent, Nucleoside analogue
  • FDA approved: 1982
  • Pregnancy risk factor: B
  • Acyclovir is the only antiviral medication available for intravenous administration.

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