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Aciclovir – Antiviral Medication for Herpes

Aciclovir is an antiviral medication used to treat infections caused by herpes viruses, including cold sores, genital herpes, and shingles. It works by stopping the virus from multiplying in the body.

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Things worth knowing about "Aciclovir"

Aciclovir is an antiviral medication used to treat infections caused by herpes viruses, including cold sores, genital herpes, and shingles. It works by stopping the virus from multiplying in the body.

What is Aciclovir?

Aciclovir (also spelled Acyclovir) is an antiviral drug belonging to the class of nucleoside analogues. Developed in the 1970s, it remains one of the most important medicines for treating herpesvirus infections. Aciclovir is available on prescription, but lower-strength formulations (such as creams) can also be purchased over the counter.

Indications – When is Aciclovir Used?

Aciclovir is used for the treatment and prevention of infections caused by herpes viruses, including:

Herpes Simplex Virus Type 1 (HSV-1): Cold sores (herpes labialis)
Herpes Simplex Virus Type 2 (HSV-2): Genital herpes
Varicella-Zoster Virus (VZV): Chickenpox and shingles (herpes zoster)
Herpes Encephalitis: Brain inflammation caused by herpes viruses
Immunocompromised patients: Prophylaxis and treatment in HIV infection, organ transplantation, or chemotherapy

Mechanism of Action

Aciclovir is a so-called prodrug -- it is initially inactive and must be activated inside infected cells. This activation occurs in three steps:

Step 1: The viral enzyme thymidine kinase (present only in virus-infected cells) converts Aciclovir into Aciclovir monophosphate.
Steps 2 and 3: Cellular enzymes further convert it into Aciclovir triphosphate, the active form.
Effect: Aciclovir triphosphate inhibits the viral DNA polymerase and is incorporated into the viral DNA, causing termination of DNA synthesis and stopping viral replication.

Because activation occurs primarily in virus-infected cells, Aciclovir is highly selective and largely harmless to healthy body cells.

Dosage Forms and Dosing

Aciclovir is available in several formulations:

Tablets/Capsules: For systemic infections such as genital herpes or shingles (typically 200-800 mg, taken 2-5 times daily depending on the indication)
Cream (topical): For local treatment of cold sores (applied 5 times daily)
Eye ointment: For herpes keratitis (corneal inflammation caused by herpes viruses)
Intravenous infusion: For severe infections such as herpes encephalitis or in immunocompromised patients

The exact dosage depends on the type and severity of the infection and the condition of the patient, especially kidney function. Dose adjustments are required in patients with impaired renal function.

Side Effects

Aciclovir is generally well tolerated. Possible side effects include:

Common (topical): Skin irritation, burning, or tingling at the application site
Common (oral): Nausea, vomiting, headache, diarrhea
Uncommon (intravenous): Kidney impairment (especially with insufficient fluid intake), neurological symptoms such as confusion or seizures
Rare: Allergic reactions, changes in blood count

Drug Interactions

Aciclovir may interact with other medications:

Probenecid: May slow the elimination of Aciclovir and increase its blood levels.
Nephrotoxic drugs (e.g., aminoglycosides, ciclosporin): Increased risk of kidney damage when used concurrently.
Mycophenolate mofetil: Possible mutual increase in blood levels.

Use During Pregnancy and Breastfeeding

Aciclovir should only be used during pregnancy after careful consideration of the benefits and risks and under medical supervision. Studies to date have not shown an increased rate of birth defects, but the available data remain limited. Aciclovir passes into breast milk during breastfeeding, so medical advice is essential before use.

References

World Health Organization (WHO) – WHO Model List of Essential Medicines, 23rd Edition (2023). Available at: https://www.who.int/publications/i/item/WHO-MHP-HPS-EML-2023.02
Elion GB. – The biochemistry and mechanism of action of acyclovir. Journal of Antimicrobial Chemotherapy, 1983;12(Suppl B):9-17. PubMed PMID: 6355051
De Clercq E, Li G. – Approved Antiviral Drugs over the Past 50 Years. Clinical Microbiology Reviews, 2016;29(3):695-747. PubMed PMID: 27281742

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