How long does it take for Acyclovir to work?

The time it takes for antivirals such as acyclovir to work depends on a couple of factors...


Acyclovir (aciclovir) is an antiviral drug used to treat infections caused by the herpes simplex virus (HSV). It is used for both herpes type 1 (HSV-1) and herpes type 2 (HSV-2) infections.

The prescription medication is available as an oral tablet (200 mg) for the treatment of genital herpes outbreaks or in the form of a cream (e.g. Zovirax) to treat cold sores (or fever blisters). There are also creams containing acyclovir for genital herpes but they are not as effective in treating blisters compared to oral pills when used alone.

Antivirals are not a cure for herpes simplex. The medications are used to relieve symptoms and promote faster recovery during outbbreaks. They can also be used for long-term suppressive therapy for patients who experience frequent herpes outbreaks.  

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Acyclovir is used for the treatment of cold sores (fever blisters), genital herpes, and Bell’s palsy. It is not used to remedy canker sores because they are not caused by HSV.


How long does it take for acyclovir to work?

The time it takes for acyclovir to work depends on a few different factors including the severity of your symptoms, the dosage, how long you waited before starting treatment, whether you are using a cream or taking oral acyclovir, and if it’s your first or a recurrent breakout.


Acyclovir for first-time herpes outbreaks

If you’ve never had cold sores or genital herpes before and this is your first HSV outbreak, you should try to get treatment as fast as possible.


Early symptoms of both genital herpes and mouth sores include tingling and itching sensations around the respective areas of your body. A good indication that a cold sore may be about to form is a reddening of the skin around your mouth or on your lip.


Adults are usually prescribed 200 mg of acyclovir 5 times a day, at 4-hour intervals, for up to 5 days. Your healthcare provider may suggest you take the medication for longer if symptoms persist.


It can take from 10 days to 2 weeks for blisters to heal completely. But the infection may linger longer if your immune system is very weak.


Make sure you follow your doctor’s instructions when taking prescription antiviral drugs.


Acyclovir for recurrent herpes outbreaks

There is no cure for herpes and therefore treatment with acyclovir does not mean that you will be protected from future outbreaks.


Recurrence of blisters varies from person to person. Genital herpes caused by HSV-2 usually returns multiple times a year. A study of 457 patients conducted by the University of Washington found that more than 35% of patients had at least 6 outbreaks of genital blisters in a single year whilst 20% had 10+ outbreaks in a year. Men were more prone to recurring outbreaks than women.


Genital herpes caused by HSV-1 has a much lower recurrence rate of around once per year.


Cold sores appear on average once a year, but you could get them more frequently if your immune system is weak.


Acyclovir for recurrent genital herpes usually provides faster relief. But lesions may still take some time to heal completely.


One study found that a higher dose of 800 mg taken orally 3 times per day for 2 days provided quick relief and lower viral shedding for recurrent genital herpes. This may be a more convenient option for patients than taking the standard dose 5 times a day.


How long after starting acyclovir are you contagious?

HSV is always contagious. Remember that acyclovir does not cure HSV-1 or HSV-2 and therefore the infection can be spread to other people via contact with the skin, genitals, lips or body fluids at all times.


However, carriers are much more contagious during times when they are having active breakouts (oral or genital herpes). During those times, you must take extra precaution not to spread your viral infection to others. Avoid sexual intercourse, oral sex, kissing, sharing toothbrushes or cutlery with others for at least 3 days after your blisters have completely healed.


How long does it take to get rid of a cold sore with Zovirax?

Zovirax is a cream used to treat cold sores. It contains 50 mg of acyclovir as its active ingredient and can be applied 2 to 5 times per day. 


Although researchers have shown that Zovirax does promote faster recovery from fever blisters, the exact duration it takes for sores to heal is not known.


For cold sores to go away completely it will usually take 10 days. If you apply treatment very early, you may be able to get rid of them a little quicker.


How long does acyclovir stay in your system?

When taken as an oral tablet, acyclovir has a half-life of 2.5 to 3.3 hours in healthy adults. In newborns and the elderly, the medicine usually stays in the system for longer because their metabolism is slower.



  1. Spruance, S. L., Nett, R., Marbury, T., Wolff, R., Johnson, J., & Spaulding, T. (2002). Acyclovir cream for treatment of herpes simplex labialis: results of two randomized, double-blind, vehicle-controlled, multicenter clinical trials. Antimicrobial agents and chemotherapy, 46(7), 2238–2243.
  2. Cernik, C. (2008). The Treatment of Herpes Simplex Infections<subtitle>An Evidence-Based Review</subtitle>. Archives of Internal Medicine, 168(11), 1137.
  3. WHO. (2017, January 31). Herpes simplex virus. Retrieved from website:
  4. Benedetti, J. (1994). Recurrence Rates in Genital Herpes after Symptomatic First-Episode Infection. Annals of Internal Medicine, 121(11), 847.
  5. Wald, A., Carrell, D., Remington, M., Kexel, E., Zeh, J., & Corey, L. (2002). Two?Day Regimen of Acyclovir for Treatment of Recurrent Genital Herpes Simplex Virus Type 2 Infection. Clinical Infectious Diseases, 34(7), 944–948.
  6. Spruance, S. L., Nett, R., Marbury, T., Wolff, R., Johnson, J., & Spaulding, T. (2002). Acyclovir Cream for Treatment of Herpes Simplex Labialis: Results of Two Randomized, Double-Blind, Vehicle-Controlled, Multicenter Clinical Trials. Antimicrobial Agents and Chemotherapy, 46(7), 2238–2243.


Reviewed by Dr Roy Kedem, MD

Information last reviewed 07/12/21

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