VALACYCLOVIR HYDROCHLORIDE
Clinical safety rating: safe
Human studies have proved safety
Valacyclovir hydrochloride is a prodrug of acyclovir. After oral administration, it is rapidly converted to acyclovir, which inhibits viral DNA polymerase, leading to chain termination and inhibition of viral DNA replication.
| Metabolism | Valacyclovir is rapidly and extensively converted to acyclovir and L-valine by first-pass intestinal and hepatic metabolism via hydrolysis (likely by esterases). Acyclovir is partially metabolized to inactive metabolites by aldehyde oxidase and alcohol dehydrogenase. |
| Excretion | Renal excretion: >90% as unchanged drug and inactive metabolite (9-carboxymethoxymethylguanine). Biliary/fecal: <2%. |
| Half-life | Terminal elimination half-life: 2.5–3.3 hours in patients with normal renal function; prolonged to 14 hours in renal impairment (CrCl 15–30 mL/min). |
| Protein binding | 13–18% bound to plasma proteins (primarily albumin). |
| Volume of Distribution | 0.6–0.9 L/kg, indicating distribution into total body water; penetrates well into tissues including skin and genital lesions. |
| Bioavailability | Oral bioavailability: 54.5% (10% coefficient of variation) after conversion to acyclovir; food does not significantly alter AUC. |
| Onset of Action | Oral: Clinical effect (viral suppression) begins within 2–4 hours after initiation; time to maximum plasma concentration (Tmax): 1–2 hours. |
| Duration of Action | Duration: 8–12 hours for antiviral effect based on dosing interval (e.g., twice-daily for herpes zoster). In renally impaired, duration extended due to prolonged half-life. |
| Molecular Weight | 360.8 |
500 mg orally twice daily for recurrent genital herpes; 1 g orally twice daily for herpes zoster; 1 g orally three times daily for herpes simplex encephalitis or immunocompromised patients.
| Dosage form | TABLET |
| Renal impairment | CrCl ≥50 mL/min: no adjustment; CrCl 30-49 mL/min: 1 g every 12h for herpes zoster or 500 mg every 12h for genital herpes; CrCl 10-29 mL/min: 1 g every 24h for herpes zoster or 500 mg every 24h for genital herpes; CrCl <10 mL/min: 500 mg every 24h for herpes zoster or 500 mg every 24h for genital herpes. |
| Liver impairment | No dosage adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B). Not studied in severe hepatic impairment (Child-Pugh C); use with caution. |
| Pediatric use | For herpes labialis (≥12 years): 2 g orally every 12h for 1 day; for chickenpox (2-<18 years): 20 mg/kg orally three times daily for 5 days (max 1 g per dose); for genital herpes (12-18 years): 500 mg orally twice daily for 3-5 days. |
| Geriatric use | Dose selection based on renal function; monitor creatinine clearance and adjust dose accordingly. Elderly patients may have reduced renal function; use the lowest effective dose and consider increased risk of neurological adverse effects (e.g., hallucinations, confusion, agitation). |
| 1st trimester | Use only if potential benefit justifies potential risk to fetus; limited human data show no increased risk of major malformations. |
| 2nd trimester | Use only if clearly needed; no evidence of fetal harm in animal studies; crosses placenta. |
| 3rd trimester | Use only if clearly needed; may accumulate in fetal tissues; no known adverse fetal effects. |
Clinical note
Probenecid may decrease acyclovir clearance increasing its levels Maintain adequate hydration to prevent nephrotoxicity especially in elderly or dehydrated patients.
| Placental transfer | Valacyclovir crosses the placenta; cord blood concentrations are approximately 1-2 times maternal plasma concentrations after repeated dosing. |
| Breastfeeding | Valacyclovir is excreted into breast milk in small amounts; peak milk levels occur 2-4 hours after dose. The relative infant dose is approximately 1-2% of maternal weight-adjusted dose, which is unlikely to cause adverse effects. However, caution is advised in neonates or infants with renal impairment. |
■ FDA Black Box Warning
WARNING: THROMBOTIC THROMBOCYTOPENIC PURPURA/HEMOLYTIC UREMIC SYNDROME (TTP/HUS) has been reported in immunocompromised patients at high doses (8 g/day) for prolonged periods.
| Common Effects | Varicella zoster virus (VZV) infections |
| Serious Effects |
Hypersensitivity to valacyclovir, acyclovir, or any component of the formulation
| Precautions | Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) risk in immunocompromised patients, Central nervous system adverse effects (e.g., agitation, hallucinations, confusion, encephalopathy) especially in elderly or renally impaired patients, Renal impairment: reduce dose; acute renal failure may occur, particularly in patients with preexisting renal disease or taking nephrotoxic drugs, Co-administration with nephrotoxic drugs increases risk of renal impairment, Use caution in patients with electrolyte abnormalities or significant dehydration |
Loading safety data…
| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | FDA Pregnancy Category B. No evidence of fetal harm in animal studies. Limited human data; available studies do not show increased risk of major birth defects or miscarriage. First trimester exposure: risk of neural tube defects not increased. Second and third trimesters: no specific fetal risks identified. Use only if clearly needed. |
| Fetal Monitoring | Monitor maternal renal function (serum creatinine) and hepatic function (ALT, AST) periodically. In pregnancy, monitor for signs of genital herpes lesions at delivery to assess need for C-section. Fetal monitoring as standard prenatal care. |
| Fertility Effects | No known adverse effects on human fertility. Animal studies showed no impairment of fertility at clinically relevant doses. |
| Food/Dietary |
| No significant food interactions. Take with or without food. Maintain adequate hydration (avoid excessive caffeine or alcohol) to prevent renal toxicity. |
| Clinical Pearls | Valacyclovir is a prodrug of acyclovir with higher oral bioavailability. Dose adjustment required for CrCl <30 mL/min; avoid in end-stage renal disease. Use cautiously in elderly with renal impairment due to risk of neurotoxicity (confusion, hallucinations). Not effective for HSV encephalitis; IV acyclovir preferred. For herpes zoster, start within 72 hours of rash onset for maximal benefit. Safety in pregnancy not fully established; use only if benefit outweighs risk. |
| Patient Advice | Take valacyclovir exactly as prescribed; do not skip doses and complete the full course even if symptoms improve. · Drink plenty of fluids (e.g., water) during treatment to prevent kidney damage. · Inform your doctor if you have kidney problems, HIV, or are post-transplant. · This medication does not cure herpes infections and does not prevent transmission; use barrier methods during sexual activity. · Common side effects include headache, nausea, and diarrhea. Report confusion, hallucinations, or unusual bleeding/bruising to your doctor. · Avoid alcohol because it may worsen side effects or interact with the medication. · Store at room temperature away from moisture and heat. |