Valtrex Does Not Cause Constipation
Constipation is not a recognized adverse effect of valacyclovir (Valtrex) based on FDA labeling and clinical trial data. The most common adverse reactions reported in clinical trials are headache, nausea, and abdominal pain—not constipation 1.
Evidence from FDA Drug Labeling
The FDA-approved prescribing information for Valtrex clearly documents the adverse reaction profile across multiple large clinical trials 1:
Most common adverse reactions (>10% incidence):
- Headache (14-38% depending on indication)
- Nausea (5-15%)
- Abdominal pain (3-11%)
Gastrointestinal effects documented in clinical trials:
- Nausea
- Vomiting (3-6%)
- Abdominal pain
- Diarrhea (5% in pediatric trials)
Notably, constipation is completely absent from the comprehensive adverse event listings across all clinical trial populations, including:
- Cold sores treatment (n=609)
- Genital herpes initial and recurrent episodes (n=1,856)
- Suppressive therapy trials (n=669)
- Herpes zoster treatment (n=967)
- Pediatric populations (n=177)
Clinical Guidelines Confirm Safety Profile
Multiple CDC and infectious disease guidelines consistently describe valacyclovir's adverse reactions as "infrequent" and specifically mention only nausea and headache as occasional side effects 2, 3. The 2009 CDC/NIH guidelines state: "Acyclovir, valacyclovir, and famciclovir are occasionally associated with nausea or headache" 3—again, no mention of constipation.
Rare Gastrointestinal Complications
While constipation is not associated with valacyclovir, one case report documented Clostridium difficile colitis (which causes diarrhea, not constipation) in a patient taking valacyclovir 4. This is an extremely rare complication and presents with the opposite symptom profile.
Clinical Bottom Line
If a patient taking Valtrex reports constipation, look for alternative causes rather than attributing it to the medication. Common culprits include:
- Other medications the patient is taking
- Dehydration (especially relevant since adequate hydration is recommended with valacyclovir to prevent renal complications)
- Dietary factors
- Underlying medical conditions
The extensive clinical trial database and FDA labeling provide no evidence linking valacyclovir to constipation.