VALTREX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for VALTREX (VALTREX).
Valacyclovir, a prodrug of acyclovir, is phosphorylated intracellularly to acyclovir triphosphate, which competitively inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination.
| Metabolism | Valacyclovir is rapidly converted to acyclovir by first-pass intestinal and hepatic metabolism via valacyclovir hydrolase. Acyclovir is partially metabolized by aldehyde oxidase and alcohol dehydrogenase, with minor oxidation products. |
| Excretion | Renal elimination: >90% as valacyclovir metabolites (primarily acyclovir, 9-carboxymethoxymethylguanine, and 8-hydroxy-9-carboxymethoxymethylguanine) via active tubular secretion and glomerular filtration. Fecal excretion: <5%. |
| Half-life | Terminal elimination half-life of acyclovir (active metabolite) is 2.5–3.3 hours in adults with normal renal function; prolonged to 14 hours in end-stage renal disease. |
| Protein binding | Acyclovir: 9–33% (concentration-dependent, saturable); valacyclovir: minimal (~15%). Primarily binds to albumin. |
| Volume of Distribution | Acyclovir: 0.7–1.2 L/kg (apparent Vd), indicating distribution into total body water and intracellular compartments. |
| Bioavailability | Oral valacyclovir: ~54% (converted to acyclovir); absorption enhanced by food (bioavailability not significantly altered). Acyclovir oral bioavailability is ~10–20%; valacyclovir is a prodrug to improve absorption. |
| Onset of Action | Oral: clinical effect (symptom relief) for herpes labialis occurs within 24–48 hours; for herpes zoster, symptom improvement begins within 2–3 days. Peak plasma concentrations reached 1–2 hours after oral administration. |
| Duration of Action | Dosing interval of 8–12 hours for most indications (e.g., herpes zoster, genital herpes) due to rapid elimination; duration of clinical effect supports thrice-daily dosing. In immunocompromised patients, extended intervals may be needed with renal impairment. |
| Molecular Weight | 382.46 |
For herpes zoster: 1 g orally three times daily for 7 days. For genital herpes initial episode: 1 g orally twice daily for 10 days. For recurrent genital herpes: 500 mg orally twice daily for 3 days. For herpes labialis: 2 g orally twice daily for 1 day. For varicella: 20 mg/kg orally three times daily for 5 days (max 1 g three times daily).
| Dosage form | Tablet |
| Renal impairment | CrCl 30-49 mL/min: same dose as normal but interval every 12 hours. CrCl 10-29 mL/min: same dose every 24 hours. CrCl <10 mL/min: 50% of dose every 24 hours. For hemodialysis: administer dose post-dialysis. |
| Liver impairment | No dosage adjustment required for Child-Pugh Class A or B. For Child-Pugh Class C: reduce dose by 50% and consider extending dosing interval. |
| Pediatric use | For varicella (≥2 years): 20 mg/kg orally three times daily for 5 days (max 1 g three times daily). For herpes labialis (≥12 years): 2 g orally twice daily for 1 day. For genital herpes (≥12 years): same dosing as adults. For neonates (up to 3 months): 20 mg/kg orally three times daily. For children <2 years: not established for all indications. |
| Geriatric use | Due to age-related renal impairment, base dosing on creatinine clearance; consider monitoring renal function and adjusting dose accordingly. |
| 1st trimester | Valacyclovir is generally considered safe in the first trimester based on large observational studies; however, use only if clearly needed. The drug crosses the placenta. No increased risk of major birth defects has been consistently demonstrated. |
| 2nd trimester | Considered safe; data from the Antiretroviral Pregnancy Registry and other studies show no significant increase in congenital anomalies. Use if potential benefit justifies potential risk. |
| 3rd trimester | Considered safe; no increased risk of adverse fetal outcomes. Use during late pregnancy may be considered for suppression of herpes simplex virus recurrences near term to reduce the need for cesarean delivery. |
Clinical note
Comprehensive clinical and safety monograph for VALTREX (VALTREX).
| Placental transfer | Valacyclovir crosses the placenta. Cord blood concentrations are approximately 30-70% of maternal serum concentrations, indicating significant placental transfer. |
| Breastfeeding |
■ FDA Black Box Warning
Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS), including fatal cases, have occurred in immunocompromised patients receiving high doses.
| Serious Effects |
Hypersensitivity to valacyclovir or acyclovir
| Precautions | Renal impairment (dosage adjustment required), central nervous system effects (agitation, hallucinations, confusion, encephalopathy), thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS, especially in immunocompromised high-dose patients), fetal/neonatal risk (use only if benefit outweighs risk), geriatric use (higher risk of CNS effects and renal dysfunction). |
| Food/Dietary | No significant food interactions. Absorption is not affected by food. Avoid excessive alcohol as it may worsen dehydration and kidney stress. |
Loading safety data…
| Valacyclovir is excreted into breast milk in small amounts. The relative infant dose is approximately 1-2% of the maternal weight-adjusted dose, which is below the 10% threshold commonly considered safe. No adverse effects in breastfed infants have been reported. Use with caution. |
| Lactation Rating | L2 (Possibly Safe) |
| Teratogenic Risk | Valacyclovir is classified as FDA Pregnancy Category B. Animal studies have not demonstrated teratogenicity or fetotoxicity. There are no adequate and well-controlled studies in pregnant women. However, data from the Valacyclovir Pregnancy Registry and observational studies suggest no increased risk of major birth defects compared to the general population. Risk cannot be ruled out; use only if clearly needed. |
| Fetal Monitoring | No specific maternal or fetal monitoring required beyond standard prenatal care. For women treated during pregnancy for herpes simplex or varicella-zoster, monitor for maternal adverse effects (e.g., renal function, CNS effects) and fetal growth via routine ultrasound if indicated. |
| Fertility Effects | No known adverse effects on fertility in animal studies. There are no human data on fertility impact. The drug does not affect spermatogenesis or oogenesis in preclinical studies. |
| Clinical Pearls | Valacyclovir is a prodrug of acyclovir with higher oral bioavailability. Dose adjustment required for CrCl <30 mL/min. Start within 72 hours of symptom onset for herpes zoster, within 24-48 hours for herpes simplex outbreaks. Adequate hydration is critical to prevent crystalluria. Not FDA approved for use in immunocompromised patients with disseminated herpes. |
| Patient Advice | Take exactly as prescribed; do not skip doses or stop early even if symptoms improve. · Drink plenty of water during treatment to prevent kidney damage. · Start medication at the first sign of an outbreak (tingling, burning) for best results. · Do not take with other nephrotoxic drugs without consulting your doctor. · Avoid sexual contact during herpes outbreaks to prevent transmission. |