Acyclovir (acyclovir inf/oral): Uses, Dosage & Side Effects

What it's for (Indications)

Acyclovir, available in both oral and intravenous formulations (e.
g.
, Acylex 200mg tablets), is a highly effective antiviral agent primarily indicated for the treatment and prophylaxis of infections caused by herpes simplex virus (HSV) types 1 and 2, and varicella-zoster virus (VZV).
Specific indications include the treatment of initial and recurrent genital herpes in immunocompetent and immunocompromised individuals, suppression of recurrent genital herpes, and treatment of mucocutaneous HSV infections in immunocompromised patients.
Furthermore, it is prescribed for the management of herpes zoster (shingles) and varicella (chickenpox).
Intravenous acyclovir is crucial for severe infections such as herpes simplex encephalitis, neonatal herpes simplex, and severe VZV infections in immunocompromised patients, offering a systemic approach to critical viral diseases where rapid and potent antiviral action is essential.
Its utility extends to prophylaxis in specific high-risk populations, such as organ transplant recipients, to prevent CMV infection, although ganciclovir is often preferred for CMV.

Dosage Information

Type	Guideline
Standard	Dosage of acyclovir is highly dependent on the specific indication, route of administration, patient age, immune status, and renal function, necessitating careful individualization. For oral administration, typical adult dosages include: for genital herpes treatment, 200 mg orally five times daily for 5-10 days, or 400 mg three times daily for 5 days; for suppression of recurrent genital herpes, 400 mg twice daily; for herpes zoster, 800 mg orally five times daily for 7-10 days; and for varicella, 20 mg/kg (up to 800 mg) four times daily for 5 days. Intravenous acyclovir, reserved for severe infections, is often administered at 5-10 mg/kg infused over 1 hour every 8 hours, adjusted for renal impairment. For herpes simplex encephalitis, dosages can reach 10 mg/kg every 8 hours for 10-21 days. Pediatric dosages are weight-based and vary significantly. Dose adjustments are mandatory for patients with impaired renal function to prevent drug accumulation and potential toxicity, often requiring reduced frequency or dosage based on creatinine clearance. Hydration is critical during intravenous therapy to minimize renal complications.

Type

Guideline

Standard

Dosage of acyclovir is highly dependent on the specific indication, route of administration, patient age, immune status, and renal function, necessitating careful individualization. For oral administration, typical adult dosages include: for genital herpes treatment, 200 mg orally five times daily for 5-10 days, or 400 mg three times daily for 5 days; for suppression of recurrent genital herpes, 400 mg twice daily; for herpes zoster, 800 mg orally five times daily for 7-10 days; and for varicella, 20 mg/kg (up to 800 mg) four times daily for 5 days. Intravenous acyclovir, reserved for severe infections, is often administered at 5-10 mg/kg infused over 1 hour every 8 hours, adjusted for renal impairment. For herpes simplex encephalitis, dosages can reach 10 mg/kg every 8 hours for 10-21 days. Pediatric dosages are weight-based and vary significantly. Dose adjustments are mandatory for patients with impaired renal function to prevent drug accumulation and potential toxicity, often requiring reduced frequency or dosage based on creatinine clearance. Hydration is critical during intravenous therapy to minimize renal complications.

Safety & Warnings

Common Side Effects

Acyclovir is generally well-tolerated, but various side effects can occur, ranging from mild to severe, depending on the route of administration, dosage, and individual patient factors.
Common side effects observed with oral acyclovir include nausea, vomiting, diarrhea, abdominal pain, headache, and skin rash.
Less commonly, dizziness, fatigue, and pruritus may occur.
With intravenous acyclovir, which is used for more severe infections, adverse effects can be more pronounced and may include local reactions such as phlebitis or inflammation at the injection site.
Systemic side effects with IV administration can encompass acute renal failure due to crystallization of the drug in renal tubules, especially with rapid infusion or inadequate hydration, neurological effects like lethargy, confusion, hallucinations, tremors, seizures, or coma (particularly in elderly patients or those with pre-existing renal impairment), and hematologic changes such as decreases in red blood cell count, white blood cell count, or platelets.
Close monitoring for these serious adverse events is imperative, especially in hospitalized or immunocompromised patients receiving high-dose intravenous therapy.
Allergic reactions, though rare, can also manifest as urticaria, angioedema, or anaphylaxis.

Serious Warnings

Black Box Warning: Acyclovir does not carry an FDA-mandated Black Box Warning. However, clinicians and patients should be aware of several 'Serious Warnings' related to its use, particularly with the intravenous formulation and in vulnerable populations. These warnings include the potential for nephrotoxicity, which can manifest as acute renal failure, especially in patients who are dehydrated, have pre-existing renal impairment, or receive rapid intravenous infusions or high doses. Adequate hydration is paramount during intravenous administration to mitigate this risk, and dosage adjustments based on creatinine clearance are essential for all routes of administration in renally impaired patients. Furthermore, neurological adverse effects such as lethargy, confusion, hallucinations, agitation, tremors, and seizures have been reported, predominantly in elderly patients, those with underlying neurological conditions, or individuals with impaired renal function, where drug accumulation is more likely. Close clinical monitoring for these central nervous system toxicities is crucial, and drug discontinuation often leads to resolution of symptoms. While rare, thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) has occurred in severely immunocompromised patients receiving high-dose acyclovir, underscoring the need for vigilant hematological monitoring in this specific group. Patient education regarding signs of these serious complications and the importance of hydration is key to safe acyclovir therapy.
Acyclovir therapy requires careful consideration of several warnings to ensure patient safety and optimize clinical outcomes.
Renal impairment is a significant concern; intravenous acyclovir, particularly in higher doses or in patients with pre-existing renal dysfunction, can lead to nephrotoxicity, including acute renal failure, due to crystallization of the drug in renal tubules.
Adequate hydration during intravenous infusion is crucial to prevent this complication, and dosage adjustments based on creatinine clearance are mandatory for all routes of administration.
Neurological effects, such as lethargy, confusion, hallucinations, agitation, tremors, and seizures, have been reported, especially in elderly patients, those with renal impairment, or patients receiving high doses.
These symptoms typically resolve upon discontinuation of the drug.
Patients should be advised to maintain adequate fluid intake, especially when taking oral acyclovir, to reduce the risk of renal complications.
Caution is advised when administering acyclovir to patients with underlying neurological abnormalities, hepatic impairment, or electrolyte imbalances.
While acyclovir can reduce the frequency and severity of herpes outbreaks, it is not a cure and does not prevent the transmission of herpes simplex virus to others.
Patients should be counseled on safe sex practices and avoiding contact with lesions.
Prolonged or repeated courses of acyclovir in severely immunocompromised individuals may lead to the development of drug-resistant viral strains, necessitating careful monitoring and consideration of alternative therapies if resistance is suspected.

How it Works (Mechanism of Action)

Acyclovir is a synthetic purine nucleoside analogue that exerts its potent antiviral activity through selective inhibition of viral DNA synthesis. Its mechanism of action is highly specific for herpesviruses, distinguishing it from general cytotoxic agents. The drug itself is a prodrug, requiring initial phosphorylation by the viral enzyme thymidine kinase (TK) to become active. In cells infected with herpes simplex virus (HSV) or varicella-zoster virus (VZV), viral TK efficiently converts acyclovir into acyclovir monophosphate. Host cellular enzymes then further phosphorylate acyclovir monophosphate into acyclovir triphosphate. This active form, acyclovir triphosphate, acts as a competitive inhibitor of viral DNA polymerase, the enzyme responsible for replicating viral genetic material. Moreover, acyclovir triphosphate is incorporated into the growing viral DNA chain, leading to premature chain termination because it lacks a 3'-hydroxyl group required for further nucleotide addition. This dual mechanism—competitive inhibition of viral DNA polymerase and DNA chain termination—effectively halts viral replication, significantly reducing viral load and clinical manifestations of the infection, while having minimal impact on uninfected host cells due to its selective activation by viral enzymes.