Valacyclovir vs Acyclovir : Same or Different?

valacyclovir vs acyclovir

Valacyclovir and acyclovir are antiviral drugs often used to treat herpes virus infections such as herpes simplex (which causes cold sores and vaginal herpes), herpes zoster (which causes shingles), and varicella zoster (which causes chickenpox). Both medications are efficacious, however there are significant differences in their pharmacology, administration, and overall efficacy.


  • Mechanism of Action: Valacyclovir is a prodrug of acyclovir. After oral administration, it is rapidly converted into acyclovir in the body through first-pass intestinal and hepatic metabolism.
  • Bioavailability: Valacyclovir has much higher oral bioavailability, approximately 55%. This allows more of the drug to be absorbed into the bloodstream compared to acyclovir.
  • Metabolism: Valacyclovir is converted to acyclovir and L-valine. The acyclovir is then processed similarly to the direct administration of acyclovir.


  • Mechanism of Action: Acyclovir works by inhibiting viral DNA replication. It is a guanine analogue that gets incorporated into viral DNA, leading to chain termination.
  • Bioavailability: Acyclovir has relatively low oral bioavailability, around 10-20%. This means that when taken orally, only a small percentage of the drug reaches the bloodstream.
  • Metabolism: It is not metabolized by the liver and is excreted unchanged in the urine.

Administration and Dosage

Because of changes in bioavailability, the dosing regimens for valacyclovir and acyclovir varies significantly:

Acyclovir: Depending on the condition being treated, it is often necessary to take many doses throughout the day, often 2-5 times.
Valacyclovir: Because of improved absorption, valacyclovir can often be used less frequently, typically 1-2 times per day.
Valacyclovir’s less frequent dosing can enhance patient compliance, making it easier for people to stick to their treatment regimen.

Clinical Efficacy and Use

Both drugs are successful in treating herpes virus infections; however, their distinct pharmacokinetic characteristics can influence therapeutic decisions.

Acute treatment: Acyclovir and valacyclovir are both effective at treating acute outbreaks. Valacyclovir’s enhanced bioavailability enables more convenient dosage, which is especially useful during an acute episode where compliance is critical.
Suppressive Therapy: Valacyclovir’s reduced dose frequency makes it the ideal choice for long-term suppressive therapy to prevent recurring outbreaks.

Side Effects and Safety

The side effect profiles of acyclovir and valacyclovir are similar because valacyclovir is converted to acyclovir in the body. Common adverse effects include nausea, headaches, and abdominal pain. More serious but uncommon adverse effects include renal toxicity and neurotoxicity, which occur at high doses or in patients with renal impairment.

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