Prevent and treat cold sore outbreaks with Acyclovir (generic Zovirax)

As the longest-serving antiviral drug against herpes viruses, Acyclovir has helped millions of people to get rid of and prevent cold sores. 

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Medicine Strength Quantity Price Stock
Acyclovir (1 episode)400mg15 tablets$18.00In Stock
Acyclovir (2 episodes)400mg30 tablets$26.00In Stock
Acyclovir (suppressive)400mg60 tablets$34.00In Stock
Acyclovir (suppressive)400mg180 tablets$41.00In Stock
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Reviewed by Dr Roy Kedem, MD

Information last reviewed 11/07/19


What is Acyclovir?

Acyclovir is an antiviral medication used to treat infection by herpes simplex, the virus responsible for cold sores and genital herpes. Acyclovir has been in use since the 1970s.  While it can suppress the herpes virus, a cure for the virus is not currently available.

How does Acyclovir work?

Acyclovir is an antiviral drug that prevents the spread of the herpes virus, and as such works best when taken at the first sign of an outbreak. Once absorbed, Acyclovir inhibits the enzyme responsible for replication of the virus, preventing it from spreading. While this does not kill the virus, which continues to live in the body between outbreaks, it reduces irritation (pain and/or itching), prevents symptoms from worsening and speeds up the healing process.


Active ingredients

The active ingredient in Acyclovir is the antiviral agent, acyclovir (400mg).

Inactive ingredients

The inactive ingredients in Acyclovir are:

  • Silicon Dioxide
  • Lactose Monohydrate
  • Magnesium Stearate
  • Cellulose, Microcrystalline
  • Povidone K25
  • Starch, Corn
  • Sodium Starch Glycolate Type A Potato
  • FD&C Blue No. 2
  • Aluminum Oxide

Avoid using this medication if you have a known allergy to Acyclovir or any of the inactive ingredients in the formulation. This medication contains lactose: if you are hypersensitive to some sugars, inform your doctor before using this medication.

Please Note: inactive ingredients may vary between different brands of generic drug

Which ingredients can cause an allergic reaction?

Acyclovir is a type of antiviral medication and has been known to cause allergic reactions in users. While the common allergic reactions are a rash, anaphylaxis is rare. However, if you notice symptoms of a severe allergic reaction like difficulty breathing, facial swelling and dizziness please call the emergency medical hotline immediately or visit your nearest emergency room as soon as possible.


How should I take Acyclovir?

Acyclovir works best if taken at the first sign of an outbreak, which is often identifiable by a tingling or burning sensation at the site of infection. Always take Acyclovir for the entire duration prescribed by your doctor, even if your symptoms improve sooner. Stopping the medication sooner than recommended may make your infection more difficult to treat.

Take each dose at around the same time each day, or at the intervals prescribed by your doctor. It is important to stay well hydrated by drinking plenty of water while taking Acyclovir, especially if you are elderly or taking a dose higher than 4 g per day.

Remember, herpes infection is contagious and can be contracted even from someone without any sores or symptoms.  It is most contagious during an outbreak and remains contagious while you are taking Acyclovir. Transmission rates can be reduced by taking medication, and by preventing the infected area from coming into contact with others.

What is the recommended dosage?

Acyclovir dosage is determined by weight. Always take the exact dosage as prescribed by your doctor. Do not take more or less than prescribed.

For an initial outbreak, a typical course of treatment is 200 mg every 4 hours (5 times daily) for 10 days.

When taken for prevention, the normal dosage of Acyclovir is 400 mg 2 times daily for up to 12 months, followed by re-evaluation. Alternative regimens include doses ranging from 200 mg 3 times daily to 200 mg 5 times daily.

After one year, continued use of Acyclovir as suppressive therapy will be reevaluated based on the frequency and severity of your outbreaks.

What if I miss a dose?

If you miss a dose, take it as soon as you remember. However, if it is nearly time for your next dose, skip the one you missed and take the next dose as scheduled. Never take a double dose to make up for a missed one.

What if I take too much?

If you believe you have overdosed Acyclovir, seek emergency medical attention immediately.  Call your doctor, a local emergency room or the Poison Help Line at 1-800-222-1222. In the rare event of an overdose, symptoms can include seizures, convulsions, agitation or loss of consciousness. 

Side Effects

What should I tell my doctor before taking Acyclovir?

Do NOT take Acyclovir if you are allergic (hypersensitive) to Acyclovir or similar antiviral medications, such as Valacyclovir. Before taking Acyclovir, tell your doctor and take particular care if:

  • You have kidney disease or reduced kidney function
  • You have a weakened or compromised immune system due to disease or certain medications
  • You are pregnant or breastfeeding. If you might be pregnant, or are planning on becoming pregnant.

Before taking Acyclovir, tell your doctor if you are taking or have recently taken any other medications, including those purchased without a prescription. In particular, tell your doctor if you are taking or have recently taken any of the following:

  • Cimetidine and probenecid, which may increase the concentration of Acyclovir
  • Mycophenolate mofetil, as blood levels of both drugs can increase if taken in combination
  • Ciclosporin, as your doctor may wish to monitor your kidney function
  • Theophylline
  • Zidovudine

What are the side effects of Acyclovir?

Like all medications, Acyclovir can cause side effects in some patients. If you experience symptoms of a severe allergic reaction, stop taking Acyclovir and seek medical attention immediately. Signs of a severe allergic reaction include swelling of the face, lips, tongue or throat, difficulty breathing or swallowing, and convulsions.

Common side effects of Acyclovir may affect up to 1in 10 people and include:

  • Dizziness or headaches
  • Nausea or vomiting
  • Diarrhea
  • Abdominal pain
  • Skin rashes and itching
  • Tiredness
  • Sensitivity to sunlight or to artificial light
  • Fever

Uncommon side effects, experienced by less that 1 in 100 people, include pale or red, raised patches on the skin, severe itching or hair loss.

Less than 1 in 1,000 users may experience the following rare side effects:

  • Increase in bilirubin and liver related enzymes
  • Increase in blood levels of urea and creatinine
  • Impaired kidney function which, in rare cases, can lead to kidney failure
  • Irregular emergence of circular red patches on the skin of the hands and arms
  • Severe skin rash, which may involve reddening, peeling and swelling of the skin resembling severe burns
  • Flushing, fever, blisters or ulcers, signs of Stevens Johnson syndrome (Blisters on your skin and the mucous membranes of your mouth, nose, eyes and genitals).  

Extremely rare side effects, affecting less than 1 in every 10,000 patients, may include:

  • Hepatitis (inflammation of the liver)
  • Jaundice (yellowing of the skin and whites of the eyes)
  • Anaemia, leucopenia or thrombocytopenia
  • Kidney failure. Inform your doctor immediately if you experience increased bruising, nosebleeds, sore throats, infections, excessive tiredness, breathlessness, abnormal paleness of the skin, or lower back pain.
  • Nervous system reactions. Inform your doctor immediately if you experience excessive drowsiness, confusion, hallucinations, sleepiness, convulsions (fits), coma, general feeling of discomfort and illness, agitation, involuntary shaking, difficulty controlling movements, or difficulty speaking.


The only contraindication to Acyclovir is if you have an allergy to it, or to any component of its formulation (inactive ingredients).

Drug interactions

The following medications may interact with Acyclovir:

  • Cladribine
  • Clozapine
  • Foscarnet
  • Mycophenylate
  • Talimogene Laherparepvec
  • Tenofovir
  • Tizanidine
  • Varicella and Zoster Vaccines (do not use Acyclovir within 14 days of vaccination, as it may reduce vaccine efficacy)
  • Zidovudine

Treatment Options

Acyclovir tablets vs Acyclovir cream

Acyclovir can be used both as a topical cream or as an oral medication, which is only available in prescription form. Acyclovir cream is usually sufficient treatment for cold sores, but should not be used inside the vagina, anus, mouth or eyes. For more serious outbreaks of herpes infection, such as genital herpes, or for preventing chronic outbreaks, your doctor may prescribe Acyclovir tablets for a period of time.

Non-Prescription Alternatives

Alongside Acyclovir cream, the following measures may help to reduce pain or irritation caused by genital herpes:

  • Gently cleanse the area with warm salt water to help prevent further infection
  • Apply an ice pack wrapped in a flannel to the area, which can help soothe the burning sensation. Never apply an ice pack directly to the area.
  • Before urinating, apply a mild local anaesthetic or vaseline to ease burning. You can also try pouring warm water over your genitals whilst urinating.
  • Wash your hands regularly with soap and hot water, particularly before and after applying Acyclovir cream to the affected area.
  • Avoid wearing tight clothing, and instead choose loose fitting clothes such as skirts and loose trousers.
  • Avoid having sex until symptoms have cleared completely.

If you think you might have genital herpes, you should get confirmation testing as soon as possible.


Disclaimer: This is not medical advice. You and your physician will determine if and how you should take any medication prescribed to you following a medical consultation.

  1. Acyclovir. 2019 Jan. Accessed 18 March 2019.
  2. Patient Information Leaflet
  3. Acyclovir. MedlinePlus. Accessed 18 March 2019.
  4. ACYCLOVIR-acyclovir tablet. DailyMed. 2014 Sept. Accessed 18 March 2019.
  5. Brannon, H. MD. Zovirax: Treatment for Cold Sores. 2018 Oct. Accessed 18 March 2019.
  6. Jiang, Y. Feng, H. Lin, Y. and Guo,X. New Strategies Against Drug Resistance to Herpes Simplex Virus. Int J Oral Csi. 2016 Mar. 8(1): 1–6. Accessed 18 March 2019.
  7. Ogbru, O. Acyclovir. Accessed 18 March 2019.

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