HSV-1 Antiviral Side Effects: What to Expect and When to Call Your Doctor

Acyclovir, valacyclovir, and famciclovir are among the safest long-term medications available — most people take them for years with no significant side effects. However, like all medications, HSV-1 antivirals can cause adverse effects in some patients. This guide reviews what you can expect, which side effects are rare but serious, and when you should call your doctor.

Are HSV-1 Antiviral Medications Generally Safe?

Yes. Acyclovir, valacyclovir, and famciclovir have been in clinical use for decades and have excellent safety profiles in immunocompetent patients. Long-term safety has been documented in studies of patients receiving daily suppressive therapy for years. None of these medications carry a boxed warning for use in immunocompetent adults. They are not controlled substances and are used across a wide age range, including during pregnancy (with appropriate discussion of risks and benefits with a doctor).

Common Side Effects of Acyclovir

These side effects occur in some patients taking oral acyclovir and are generally mild:

• Nausea (most common)
• Headache
• Diarrhea or abdominal discomfort
• Dizziness
• Fatigue

Taking acyclovir with food can reduce nausea. If nausea is persistent, speak with your doctor — valacyclovir or famciclovir may be better tolerated for some patients.

Common Side Effects of Valacyclovir (Valtrex)

Valacyclovir generally has a similar side effect profile to acyclovir since it converts to acyclovir in the body. Common side effects include:

• Headache (most commonly reported)
• Nausea
• Abdominal pain
• Dizziness in some patients

Common Side Effects of Famciclovir

Famciclovir is generally well-tolerated. Reported side effects in clinical trials were similar to placebo:

• Nausea
• Diarrhea
• Headache

Serious Side Effects: What to Watch For

Serious side effects from oral HSV antivirals are rare in immunocompetent patients but can occur, especially in patients with kidney disease or those on high doses:

• Neurotoxicity: Confusion, agitation, hallucinations, tremors, or altered consciousness. This is most common in patients with reduced kidney function or the elderly. If you experience any mental status changes while taking these medications, stop the medication and contact your doctor or go to the ER immediately. Mean onset is about 3 days after starting treatment.
• Acute kidney injury (AKI): Acyclovir can crystallize in the renal tubules, especially if taken without adequate hydration. Drink plenty of water while taking acyclovir, especially at higher doses. Signs include decreased urination, swelling, or back pain.
• Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS): Rare, primarily associated with high-dose valacyclovir (8g/day) in severely immunocompromised patients — not a risk at standard doses in healthy adults
• Allergic reactions: Rare; signs include rash, hives, swelling of face/tongue/throat, or difficulty breathing. Seek emergency care immediately.

When to Call Your Doctor

Contact your doctor if you experience any of the following while taking an HSV-1 antiviral:

• Confusion, agitation, unusual behavior, or hallucinations
• Signs of kidney problems: swelling, decreased urination, back or side pain
• Severe nausea, vomiting, or abdominal pain that doesn't resolve
• Skin rash or signs of allergic reaction
• Your HSV symptoms are not improving after 7–10 days of treatment (may indicate resistant virus)

Special Populations: Extra Caution Needed

Certain groups need extra monitoring when taking HSV-1 antivirals:

• Kidney disease: Dose adjustment is required for all three antivirals in patients with reduced kidney function (reduced creatinine clearance)
• Elderly patients: More prone to neurotoxicity; age-related decline in kidney function may necessitate lower doses
• Pregnancy: Acyclovir and valacyclovir are commonly used in pregnancy (particularly in the third trimester to prevent neonatal transmission). Discuss the risks and benefits with your OB/GYN.
• Immunocompromised patients: Higher risk of resistance, neurotoxicity at standard doses; closer monitoring required

The Bottom Line on Side Effects

For most healthy adults, HSV-1 antivirals are very well-tolerated medications with an excellent long-term safety record. Serious side effects are rare and primarily affect patients with kidney disease or the immunocompromised. Stay hydrated while taking acyclovir, don't take higher doses than prescribed, and contact your doctor at the first sign of unusual neurological symptoms. Once you've confirmed your prescription is safe for you, find it in stock using medfinder. For drug interaction information, see our guide on HSV-1 antiviral drug interactions.