Does Valtrex Cause Diarrhea? What You Should Know About This Side Effect

October 11, 2025 •

3 min read

Clinical trials and patient reports have documented diarrhea as a possible side effect of the antiviral medication Valtrex (valacyclovir). While generally mild and temporary, understanding its likelihood and management is crucial for those taking the medication to treat herpes infections.

Quick Summary

Valtrex can cause mild, infrequent diarrhea, though its occurrence varies by individual and dosage. Strategies exist to manage this gastrointestinal side effect, and serious complications are rare.

Key Points

Diarrhea is a Known Side Effect: Clinical trials and patient reports confirm that Valtrex (valacyclovir) can cause diarrhea, though it is not the most common adverse event.
Occurrence is Often Mild and Infrequent: For most people, if diarrhea occurs, it is generally mild and temporary, resolving as the body adjusts to the medication.
Management Strategies are Available: Mild cases can be managed with dietary adjustments (like the BRAT diet), increased hydration, and taking the medication with food.
Certain Factors Increase Risk: Higher dosages, long-term use, kidney problems, and compromised immune systems can increase the likelihood and severity of side effects.
Rare, Serious Complications Can Occur: In rare instances, Valtrex use has been associated with severe conditions like C. difficile colitis, which requires immediate medical attention.
Acyclovir Has Similar Side Effects: Valtrex and its parent compound, acyclovir, have comparable side effect profiles, including diarrhea, but differ in dosing frequency.
Medical Consultation is Key: For severe, bloody, or persistent diarrhea, or signs of dehydration, it is important to contact a healthcare provider immediately.

Understanding the Link Between Valtrex and Diarrhea

Valtrex, also known by its generic name valacyclovir, is an antiviral medication prescribed for various herpes simplex virus infections, including shingles, cold sores, and genital herpes. The drug functions by converting to acyclovir in the body, which then inhibits viral replication. Gastrointestinal issues are recognized as potential side effects of Valtrex, with diarrhea being one among them, alongside nausea, vomiting, and abdominal pain.

How Common is Diarrhea with Valtrex?

Diarrhea is a reported side effect of Valtrex, but it's not usually considered one of the most frequent. Some studies indicate that the rate of diarrhea in patients taking valacyclovir was similar to those taking a placebo. However, both FDA labeling and post-market reports confirm that diarrhea can occur. For example, in clinical trials involving young children (1 month to under 12 years), 5% of participants experienced diarrhea. Long-term use or a compromised immune system, such as in patients with advanced HIV, may slightly increase the risk of side effects, including those affecting the digestive system.

Why Does Valtrex Cause Diarrhea?

While the exact reasons are not fully understood, it's thought that Valtrex or its active component, acyclovir, might directly affect the intestinal lining or alter the balance of gut bacteria. In rare instances, Valtrex has been linked to Clostridium difficile colitis, a severe infection leading to significant diarrhea and colon inflammation. This risk is higher in older or immunocompromised individuals.

Factors That Influence Diarrhea Risk

The likelihood and severity of diarrhea from Valtrex can be influenced by several factors:

Dosage: Taking higher doses of the medication may increase the chance of experiencing gastrointestinal side effects. For example, a 1-gram daily dose has been associated with more frequent abdominal pain compared to a 500-milligram dose.
Kidney Function: The kidneys are important for clearing Valtrex from the body. If kidney function is impaired, the drug can build up, potentially increasing the risk of side effects, including severe gastrointestinal issues. Dose adjustments may be necessary for those with kidney problems.
Overall Health: Individuals with weakened immune systems may have a higher risk of more severe side effects.

A Comparison of Antivirals: Valtrex vs. Acyclovir

Comparing Valtrex to acyclovir, its parent drug, can be helpful. Both medications have similar side effect profiles. However, Valtrex is often preferred due to its less frequent dosing.


Side Effect	Valtrex	Acyclovir
Headache	Common	Common
Nausea	Common	Common
Diarrhea	Reported	Reported
Dosing Frequency	Less frequent, often twice a day	More frequent, up to 5 times a day
Bioavailability	Higher	Lower

Managing Diarrhea While Taking Valtrex

If you experience mild to moderate diarrhea while on Valtrex, several approaches can help manage symptoms while you continue your prescribed treatment.

Dietary Adjustments:

Consider the BRAT diet, which includes bland, low-fiber foods like bananas, rice, applesauce, and toast.
Avoid foods that can aggravate symptoms, such as spicy, greasy, or dairy products, as well as caffeine and alcohol.
Eating smaller, more frequent meals can aid digestion.

Hydration:

It's vital to stay well-hydrated by drinking clear fluids like water, broth, or sports drinks. Dehydration is a risk with diarrhea and can also affect kidney function, potentially increasing drug side effects.
Be aware of dehydration signs, such as dizziness, dry mouth, or reduced urination, and seek medical advice if they occur.

Medication Considerations:

Taking Valtrex with food can help minimize stomach upset.
For mild to moderate diarrhea, over-the-counter options like loperamide may be used, but always consult with your doctor first.

When to Seek Medical Attention

While mild diarrhea from Valtrex is not uncommon, certain symptoms require prompt medical evaluation. Contact your healthcare provider if you experience:

Diarrhea that is severe, persistent, or worsening
Fever
Significant abdominal pain
Bloody or black, tarry stools
Signs of significant dehydration, such as feeling dizzy when standing or notably decreased urination
Any unusual changes in mental state, such as confusion or hallucinations, especially in older adults

Conclusion

Diarrhea is a possible, though typically mild and infrequent, side effect of Valtrex. Strategies such as taking the medication with food and maintaining adequate hydration can help manage mild symptoms. It's important to be aware of potential risks and management strategies to ensure you complete your treatment effectively and safely. Always discuss any side effects you experience with your healthcare provider, particularly if they are severe or do not improve.

Frequently Asked Questions

While diarrhea is a documented side effect, it is not considered one of the most common ones. Some studies found the incidence was not significantly different from a placebo group, though it has been reported in clinical and post-marketing settings.

For mild diarrhea, focus on increasing your fluid intake to prevent dehydration and modifying your diet with bland foods, such as those in the BRAT diet (bananas, rice, applesauce, toast). Taking Valtrex with food may also help reduce stomach upset.

You should contact your doctor if the diarrhea is severe, persistent, or bloody. Also, seek medical attention if you experience a fever, severe abdominal pain, or signs of dehydration like dizziness or reduced urination.

Over-the-counter anti-diarrheal medications like loperamide can be used for mild-to-moderate diarrhea, but only after consulting with your healthcare provider. Always follow their guidance and discontinue use if instructed.

Yes, higher doses of Valtrex may increase the risk of certain gastrointestinal side effects. Studies have noted that side effects like abdominal pain and dizziness occurred more frequently in patients on higher daily doses.

Yes, older adults may be more sensitive to the side effects of valacyclovir, including potential central nervous system changes and kidney problems. This heightened sensitivity is often due to age-related changes in kidney function, which can affect drug clearance.

Bloody diarrhea is not a common side effect but can indicate a more serious underlying issue, such as Clostridium difficile colitis, which has been associated with Valtrex use in rare cases. This symptom requires immediate medical evaluation.