VALACYCLOVIR HYDROCHLORIDE

Clinical safety rating: safe

Human studies have proved safety

How it works

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.

What the body does with it

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.

Classification & Brands

Dosing & administration

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).

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Probenecid may decrease acyclovir clearance increasing its levels Maintain adequate hydration to prevent nephrotoxicity especially in elderly or dehydrated patients.

Breastfeeding	Valacyclovir is excreted into breast milk. M/P ratio approximately 1.0. A nursing infant would receive about 3-14% of the maternal dose. Considered compatible with breastfeeding by the American Academy of Pediatrics. Monitor infant for rash, diarrhea, or fussiness.
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.

Warnings & precautions

■ 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.

Side Effect Profile

Common Effects	Varicella zoster virus (VZV) infections
Serious Effects

Absolute Contraindications

["Hypersensitivity to valacyclovir, acyclovir, or any component of the formulation"]

Clinical Precautions

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"]

Clinical Tips & Counseling

Drug interactions

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