What are the common side effects of valacyclovir?

Common Side Effects of Valacyclovir

Valacyclovir is generally well tolerated with headache, nausea, and abdominal pain being the most common side effects reported in more than 10% of patients. 1

Primary Side Effects

Valacyclovir's side effect profile is well-documented in clinical trials and includes:

• Most common (>10% of patients):

  • Headache
  • Nausea
  • Abdominal pain 1

• Less common but notable side effects:

  • Dizziness (reported in about 2% of patients) 2
  • Vomiting (3-6% of patients) 1
  • Diarrhea 2
  • Dyspepsia (indigestion) 2
  • Dry mouth 2
  • Flatulence 2

Special Considerations

Laboratory Abnormalities

Valacyclovir may occasionally cause laboratory abnormalities, including:

• Elevated liver enzymes (ALT elevation occurs in approximately 1.8% of patients) 1
• Changes in hemoglobin, white blood cell counts, and alkaline phosphatase (generally at similar frequencies to placebo) 1

Serious but Rare Side Effects

1. Neurotoxicity:

   • Can manifest as confusion, hallucinations, agitation, dysarthria, and altered consciousness
   • More common in elderly patients and those with impaired renal function
   • Typically occurs within 3 days of starting treatment 3

2. Thrombotic Thrombocytopenic Purpura/Hemolytic Uremic Syndrome:

   • Primarily reported with high-dose valacyclovir (8 grams/day)
   • Rarely reported with standard doses used for herpes infections 1, 4
   • Higher risk in immunocompromised patients, particularly those with advanced HIV disease 5

3. Acute Renal Failure:

   • Rare but serious potential complication
   • Risk increases with dehydration or pre-existing renal impairment 1

Risk Factors for Side Effects

Certain patient populations are at increased risk for developing side effects:

• Patients with renal impairment:

  • Higher systemic and CSF concentrations of acyclovir and its metabolites 6
  • Increased risk of neurotoxicity 3, 7
  • Require dose adjustment based on creatinine clearance

• Elderly patients:

  • More susceptible to neuropsychiatric side effects
  • Often have reduced renal function requiring dose adjustment

• Immunocompromised patients:

  • Higher risk of serious adverse events, particularly with high doses 5

Monitoring Recommendations

• No routine laboratory monitoring is needed for patients on standard doses unless they have substantial renal impairment 2
• For patients receiving high-dose IV acyclovir, monitoring of renal function is recommended at initiation and once or twice weekly during treatment 2
• Patients should be advised to maintain adequate hydration while taking valacyclovir

Clinical Pearls

1. The side effect profile of valacyclovir is similar to that of acyclovir but with better bioavailability allowing for less frequent dosing 5

2. Most side effects are mild and transient, resolving with continued use or upon discontinuation of the medication 2

3. Neuropsychiatric symptoms typically resolve within 1-2 weeks after discontinuation of the drug, with 74.4% of patients recovering within 7 days 3

4. The risk of serious side effects is significantly higher with doses used for CMV prophylaxis (8g/day) than with standard doses used for herpes labialis or genital herpes treatment 1, 5

By understanding the common side effects and risk factors, clinicians can appropriately counsel patients and monitor for potential complications while prescribing valacyclovir for herpes infections.