Acyclovir is the oldest antiviral medication used to treat herpes simplex viruses, including genital herpes, oral herpes (also known as herpes labialis, cold sores, or fever blisters), shingles and chickenpox. While Acyclovir does not cure herpes infections, it can substantially reduce the severity and frequency of outbreaks. In the US, Acyclovir may also be sold under the brand name Zovirax.
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Reviewed by Dr Roy Kedem, MD
Information last reviewed 11/07/19
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.
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.
The active ingredient in Acyclovir is the antiviral agent, acyclovir (400mg).
The inactive ingredients in Acyclovir are:
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
Acyclovir works best if taken at the first sign of an outbreak, which is often identifiable as 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.
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.
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.
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.
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:
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:
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:
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:
Extremely rare side effects, affecting less than 1 in every 10,000 patients, may include:
The only contraindication to Acyclovir is if you have an allergy to it, or to any component of its formulation (inactive ingredients).
The following medications may interact with Acyclovir:
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.
Alongside Acyclovir cream, the following measures may help to reduce pain or irritation caused by genital herpes:
If you think you might have genital herpes, you should get confirmation testing as soon as possible.
Resistance rates to Acyclovir in otherwise healthy individuals is relatively rare, at just 0.1 - 0.6%. Among individuals with a compromised immune system, the rate is higher, ranging from 3.5 - 10%. This is because individuals with a compromised immune system generally take Acyclovir for an extended period of time, which increases the risk of developing resistance.
Acyclovir starts to work as soon as it enters the bloodstream and works best if taken at the first sign of an outbreak. You may not be able to tell when Acyclovir is working, but it can speed up the healing process of symptoms by a few days. Always take Acyclovir for the duration prescribed by your doctor, even if your symptoms have cleared.
If you struggle to swallow the tablets whole, they can be crushed or dispersed in water for easier administration. One 400 mg tablet should be dissolved in at least 100 ml of water.
Acyclovir is not an antibiotic and is not effective against bacterial infections. It is an antiviral medication that is effective against the herpes simplex virus, which includes cold sores, genital herpes, shingles and chickenpox.
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.
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