Acyclovir – Background


Acyclovir is a nucleotide analog antiviral used to treat herpes simplex, Varicella zoster, herpes zoster, herpes labialis, and acute herpetic keratitis. Acyclovir is generally used first line in the treatment of these viruses and some products are indicated for patients as young as 6 years old. Acyclovir was granted FDA approval on 29 March 1982.


Brand Names

Zovirax, Sitavig, Xerese, Cymex Ultra, Virasorb


  • Store this drug at room temperature. Keep it between 59°F and 77°F (15°C and 25°C)
  • Keep it away from light
  • Don’t store this medication in moist or damp areas, such as bathrooms

Identity of Acyclovir

Type of medicine Antiviral Medication
Synonyms Aciclovir, Aciclovirum, Acycloguanosine, and Acyclovir
Molecular Formula C8H11N5O3
Molecular Weight 225.2046 g/mol
IUPAC Names 2-amino-9-[(2-hydroxyethoxy)methyl]-6,9-dihydro-3H-purin-6-one
Structural formula of main components Acyclovir Structural formula.png
Pure active ingredient Aciclovir
Appearance white blue, oblong, capsule
Melting point 255 °C
Available as injectable solution, oral suspension, tablet, tablet, and capsule
Prescription Prescription is required

What is Acyclovir (Zovirax) used for?

  • Herpes Simplex — Mucocutaneous/Immunocompetent Host
  • Herpes Simplex — Mucocutaneous/Immunocompromised Host
  • Herpes Simplex Encephalitis
  • Herpes Zoster
  • Varicella-Zoster
  • Herpes Zoster — Prophylaxis
  • Varicella-Zoster — Prophylaxis
  • Infection – Bacterial/Fungal/Protozoal/Viral
  • Herpes Zoster Iridocyclitis
  • Herpes Zoster Peripheral Neuropathy
  • Herpes Zoster Meningitis
  • Varicella Pneumonitis
  • Herpes Zoster Otitis Externa
  • Herpes Simplex Labialis
  • Herpes Simplex — Suppression
  • Herpes Zoster Myelitis
  • Herpes Simplex – Congenital

Acyclovir side effects

Get emergency medical help if you have signs of an allergic reaction to acyclovir: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have:

  • Easy bruising or bleeding, purple or red pinpoint spots under your skin;
  • Changes in behavior;
  • Confusion, hallucinations; or
  • Signs of a kidney problem little or no urinating; painful or difficult urination; swelling in your feet or ankles; feeling tired or short of breath.

Common acyclovir side effects may include:

  • Mild skin pain;
  • Rash, itching;
  • Nausea, vomiting, diarrhea;
  • Headache; or
  • Mouth pain while using an acyclovir buccal tablet.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects.

Mechanism of action

Acyclovir is becoming acyclovir monophosphate due to the action of viral thymidine kinase. Acyclovir monophosphate is converted to the diphosphate form by guanylate kinase. Acyclovir diphosphate is converted to acyclovir triphosphate by nucleoside diphosphate kinase, pyruvate kinase, creatine kinase, phosphoglycerate kinase, succinyl-CoA synthetase, phosphoenolpyruvate carboxykinase and adenylosuccinate synthetase. Acyclovir triphosphate has higher affinity for viral DNA polymerase than cellular DNA polymerase and incorporates into the DNA where the missing 2′ and 3′ carbons causes DNA chain termination. In other cases, acyclovir triphosphate competes so strongly for viral DNA polymerase that other bases cannot associate with the enzyme, inactivating it.

Drug Interactions

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor’s approval.

Some products that may interact with this drug include: other drugs that may cause kidney problems (including nonsteroidal anti-inflammatory drugs-NSAIDs such as ibuprofen, naproxen).

Acyclovir is very similar to valacyclovir. Do not use medications containing valacyclovir while using acyclovir.

How to take acyclovir?

This dosage information is for acyclovir oral tablet. All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on:

  • Your Age
  • The condition being treated
  • How severe your condition is
  • Other medical conditions you have
  • How you react to the first dose

Forms and strengths

Generic: Acyclovir

  • Form: oral tablet
  • Strengths: 400 mg, 800 mg

Dosage for shingles, genital herpes, or chickenpox

Adult dosage (ages 18–64 years)

  • Shingles typical dosage: 800 mg every 4 hours, five times per day for 7–10 days.
  • Genital herpes:
    • Typical initial dosage: 200 mg every 4 hours, five times per day, for 10 days.
    • Typical dosage for prevention of recurrent herpes: 400 mg twice per day, every day for up to 12 months. Other dosing plans may include doses ranging from 200 mg three times daily to 200 mg five times daily. Your doctor will decide how long you should take this drug to avoid a flare-up of the infection.
    • Typical dosage for reinfection (flare-up of the infection): 200 mg every 4 hours, five times per day, for 5 days. You should take this drug as soon as the first signs of a flare-up appear.
  • Chickenpox typical dosage: Take 800 mg four times per day for 5 days. Start this drug as soon as your first symptom of chickenpox appears. It isn’t known if this drug is effective if you start it more than 24 hours after the first sign of chickenpox.

Child dosage (ages 2–17 years)

  • Chickenpox typical dosage:
    • Children who weigh 40 kg (88 lbs) or less: 20 mg/kg of body weight, given four times per day for 5 days
    • Children who weigh more than 40 kg: 800 mg four times per day for 5 days

Start this drug as soon as the first symptom of chickenpox appears. It isn’t known if this drug is effective if your child starts it more than 24 hours after the first sign of chickenpox.

Child dosage (ages 0–1 years)

It hasn’t been confirmed that acyclovir is safe and effective for children younger than 2 years.

Senior dosage (ages 65 years and older)

The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stay in your body for a longer time. This increases your risk of side effects.

Your doctor may start you on a lowered dose or a different medication schedule. This can help keep levels of this drug from building up too much in your body.

What special precautions should I follow before consuming acyclovir?

Before taking acyclovir

  • tell your doctor and pharmacist if you are allergic to acyclovir, valacyclovir (Valtrex), any other medications, milk proteins, or any of the ingredients in acyclovir products. Ask your pharmacist for a list of the ingredients.
  • tell your doctor and pharmacist what prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention any of the following: amphotericin B (Fungizone); aminoglycoside antibiotics such as amikacin (Amikin), gentamicin (Garamycin), kanamycin (Kantrex), neomycin (Nes-RX, Neo-Fradin), paramomycin (Humatin), streptomycin, and tobramycin (Tobi, Nebcin); aspirin and other nonsteroidal anti-inflammatory drugs such as ibuprofen (Advil, Motrin), and naproxen (Aleve, Naprosyn); cyclosporine (Gengraf, Neoral, Sandimmune); medications to treat HIV or AIDS such as zidovudine (Retrovir, AZT); pentamidine (NebuPent); probenecid (Benemid); sulfonamides such as sulfamethoxazole and trimethoprim (Bactrim); tacrolimus (Prograf); and vancomycin. Many other medications may also interact with acyclovir, so be sure to tell your doctor about all the medications you are taking, even those that do not appear on this list. Your doctor may need to change the doses of your medications or monitor you carefully for side effects.
  • tell your doctor if there is a possibility you may be dehydrated from a recent illness or activity, or if you have or have ever had problems with your immune system; human immunodeficiency virus infection (HIV); acquired immunodeficiency syndrome (AIDS); or kidney disease.
  • tell your doctor if you are pregnant, plan to become pregnant, or are breastfeeding. If you become pregnant while taking acyclovir, call your doctor.
  • if you are taking acyclovir to treat genital herpes, you should know that genital herpes can be spread through sexual contact even if you don’t have blisters or other symptoms and possibly even if you are taking acyclovir. Talk to your doctor about ways to stop the spread of genital herpes and about whether your partner(s) should receive treatment.

Pregnancy, breastfeeding and fertility while taking or using acyclovir

Aciclovir and pregnancy

Aciclovir cream, eye ointment, tablets or liquid are not known to be harmful in pregnancy. Aciclovir can help reduce the severity of herpes and chickenpox.

Speak to your doctor or pharmacist. They will help you decide on the best treatment for you and your baby.

Aciclovir and breastfeeding

If your doctor or health visitor says that your baby is healthy, it’s OK to take aciclovir tablets or liquid while breastfeeding.

Aciclovir from the tablets or liquid passes into breast milk in very small amounts, and it is unlikely to cause any side effects in your baby.

You can also use aciclovir cream or eye ointment while breastfeeding. It is likely that only tiny amounts will get into your breast milk which would not be expected to cause any problems in your baby.

Make sure that your baby does not come into contact with areas where you’ve used the medicine.

If you notice that your baby is not feeding as well as usual, or is being sick or has diarrhoea, or you have any other concerns about your baby, talk to your health visitor, midwife, pharmacist or doctor as soon as possible.

Aciclovir and fertility

There’s no evidence that aciclovir reduces fertility in either men or women.

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