How Acyclovir and Valacyclovir Work
Acyclovir treats herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections by inhibiting viral DNA polymerase, stopping replication. Valacyclovir is a prodrug of acyclovir; the body converts it to acyclovir after absorption, delivering the same active drug but with better bioavailability—about 55% for valacyclovir vs. 10-20% for oral acyclovir.[1][2]
Key Differences in Dosing and Administration
Acyclovir requires frequent dosing—typically 200-800 mg five times daily for herpes outbreaks—due to poor oral absorption. Valacyclovir uses lower, less frequent doses (e.g., 1-2 g twice daily for genital herpes) because higher blood levels persist longer.[1][3] Intravenous acyclovir suits severe cases like herpes encephalitis; valacyclovir is mainly oral.
| Aspect | Acyclovir | Valacyclovir |
|--------|-----------|--------------|
| Oral Bioavailability | 10-20% | ~55% |
| Typical Dosing Frequency (Genital Herpes) | 5x/day | 2x/day |
| Common Forms | Oral, IV, topical | Oral tablets |
| Treatment Duration Example (Shingles) | 5-10 days, 800 mg 5x/day | 7 days, 1 g 3x/day |
Effectiveness for Common Uses
Both effectively reduce outbreak duration, pain, and viral shedding in genital herpes, shingles, and cold sores. Valacyclovir's higher plasma levels lead to faster lesion healing (by ~1-2 days) and better suppression in recurrent cases. Studies show similar cure rates, but valacyclovir improves patient adherence due to convenience.[2][4] No head-to-head trials show one vastly superior; choice depends on severity and compliance.
Side Effects and Safety
Side effects overlap: nausea, headache, diarrhea. Acyclovir more often causes renal issues (crystalluria) with high IV doses, requiring hydration. Valacyclovir has similar risks but lower incidence due to dosing. Both need dose adjustments in kidney impairment; rare neurotoxicity occurs at high levels.[1][3] Pregnancy safety is comparable (Category B).
Cost and Availability
Generic acyclovir costs $10-30 for a 5-day herpes course; valacyclovir generics run $20-60. Valacyclovir's branded Valtrex patent expired in 2009, with generics since.[5] Acyclovir, off-patent since the 1990s, is cheaper overall. Check DrugPatentWatch.com for formulation-specific patents.[6]
When to Choose One Over the Other
Use acyclovir for cost-sensitive cases, topical needs, or IV therapy in hospitals. Pick valacyclovir for outpatient treatment needing fewer pills, like frequent recurrences or shingles in adults. Consult a doctor for kidney issues or immunocompromised patients.[1][2]
Sources:
[1] Medscape: Acyclovir vs Valacyclovir
[2] NEJM: Valacyclovir vs Acyclovir for Herpes Zoster
[3] FDA: Valtrex Label
[4] Cochrane: Antivirals for Herpes Simplex
[5] GoodRx Pricing Comparison
[6] DrugPatentWatch.com