Acyclovir vs. Valacyclovir

What's the Difference?

Acyclovir and Valacyclovir are both antiviral medications commonly used to treat infections caused by the herpes virus, including genital herpes, cold sores, and shingles. However, there are some differences between the two drugs. Acyclovir is the older and more widely available medication, available in various forms such as oral tablets, topical creams, and intravenous injections. Valacyclovir, on the other hand, is a prodrug of Acyclovir, meaning it is converted into Acyclovir in the body. This conversion allows for better absorption and higher levels of the active drug in the bloodstream, resulting in improved effectiveness and a longer duration of action. Valacyclovir is typically taken orally in tablet form. While both medications are generally well-tolerated, Valacyclovir may have a slightly lower risk of side effects due to its improved bioavailability. Ultimately, the choice between Acyclovir and Valacyclovir depends on the specific condition being treated and the individual patient's needs.


Drug ClassAntiviralAntiviral
Brand NamesZovirax, SitavigValtrex
UsesTreatment of herpes simplex virus (HSV) infections, shingles, and chickenpoxTreatment of HSV infections, shingles, cold sores, and genital herpes
AdministrationOral, topical, intravenousOral
Active FormAcyclovirValacyclovir
ConversionN/AConverted to acyclovir in the body
Half-life2.5-3.3 hours2.5-3.6 hours
Side EffectsNausea, vomiting, headache, dizzinessNausea, vomiting, headache, dizziness
Drug InteractionsProbenecid, cimetidineProbenecid, cimetidine

Further Detail


Acyclovir and Valacyclovir are antiviral medications commonly used to treat infections caused by the herpes simplex virus (HSV). While both drugs are effective in managing herpes outbreaks, they differ in terms of their chemical structure, mechanism of action, dosing regimen, and bioavailability. Understanding the unique attributes of each medication can help healthcare professionals and patients make informed decisions regarding their use.

Chemical Structure

Acyclovir, also known as acycloguanosine, is a synthetic nucleoside analogue of guanosine. It is structurally similar to the natural nucleoside, but with an additional acyclic side chain. Valacyclovir, on the other hand, is an L-valyl ester prodrug of acyclovir. This means that valacyclovir is converted into acyclovir in the body through a process called first-pass metabolism. The conversion allows for improved oral bioavailability and prolonged antiviral activity.

Mechanism of Action

Both acyclovir and valacyclovir exert their antiviral effects by selectively inhibiting viral DNA synthesis. Once inside infected cells, these medications are phosphorylated by viral thymidine kinase, an enzyme specific to HSV. The phosphorylated form of acyclovir and valacyclovir competes with natural nucleotides for incorporation into the growing viral DNA chain. However, acyclovir triphosphate, the active form of acyclovir, has a higher affinity for viral DNA polymerase than valacyclovir triphosphate. This difference in affinity contributes to the higher potency of acyclovir against HSV.

Dosing Regimen

Acyclovir is available in various formulations, including oral tablets, capsules, topical creams, and intravenous (IV) formulations. The dosing frequency depends on the indication being treated. For the treatment of genital herpes, the typical oral dose is 200 mg five times daily for five days. For the suppression of recurrent genital herpes, the recommended dose is 400 mg twice daily. Valacyclovir, on the other hand, is primarily available as an oral tablet. The dosing regimen for valacyclovir is generally more convenient, with a lower frequency of administration. For the treatment of genital herpes, the recommended dose is 1 gram twice daily for 10 days, while for the suppression of recurrent genital herpes, the dose is 500 mg once daily.


One of the key differences between acyclovir and valacyclovir is their bioavailability. Acyclovir has poor oral bioavailability, with only about 15-30% of the drug being absorbed from the gastrointestinal tract. This low bioavailability necessitates higher doses and more frequent administration. In contrast, valacyclovir has significantly higher oral bioavailability, ranging from 55-77%. The improved bioavailability of valacyclovir allows for lower doses and less frequent dosing, making it a more convenient option for patients.


Both acyclovir and valacyclovir are indicated for the treatment and suppression of genital herpes, as well as the treatment of herpes labialis (cold sores) and herpes zoster (shingles). Additionally, acyclovir is approved for the treatment of varicella-zoster virus (VZV) infections in immunocompromised patients. Valacyclovir, due to its improved bioavailability, is often preferred for the treatment of recurrent genital herpes and for long-term suppression therapy.

Side Effects

Common side effects of both acyclovir and valacyclovir include headache, nausea, vomiting, diarrhea, and abdominal pain. These side effects are generally mild and transient. However, acyclovir may cause more frequent adverse effects, such as malaise, dizziness, and rash. Rare but serious side effects of both medications include allergic reactions, kidney dysfunction, and neurological symptoms. It is important to consult a healthcare professional if any concerning side effects occur.


Acyclovir and valacyclovir are both effective antiviral medications used to manage herpes infections. While acyclovir has been available for a longer time and is available in various formulations, valacyclovir offers improved bioavailability and a more convenient dosing regimen. The choice between the two medications depends on factors such as the specific indication, patient preference, and cost considerations. Healthcare professionals can guide patients in selecting the most appropriate treatment option based on individual needs and circumstances.

Comparisons may contain inaccurate information about people, places, or facts. Please report any issues.