acyclovir vs famciclovir
Side-by-side comparison of acyclovir and famciclovir Data from FDA drug databases (Orange Book, NDC Directory, recalls, shortages) covering 20,000+ approved drugs, plus CMS pricing; see our methodology.
Zovirax
Famvir
Acyclovir is an antiviral medicine. It is used to treat infections caused by certain viruses.
Famciclovir is an antiviral medicine. It helps to treat infections caused by certain viruses.
Acyclovir is used to treat shingles, which is caused by herpes zoster. It also treats genital herpes, both the first time you have it and when it comes back. Acyclovir can also treat chickenpox.
This medicine treats herpes infections. It can treat cold sores, genital herpes, and shingles. It can also prevent genital herpes outbreaks. This medicine may not work for everyone, including first-time genital herpes, eye shingles, or certain immunocompromised patients.
Acyclovir stops the virus from growing and spreading. It does this by interfering with the virus's ability to make copies of itself. This helps your body fight off the infection.
Famciclovir stops the virus from multiplying. It does this by blocking a key enzyme the virus needs to make copies of itself. This helps your body fight off the infection.
- • Malaise (feeling unwell)
- • Nausea
- • Diarrhea
- • Headache
- • Nausea
- Tiredness 7,612
- Diarrhea 7,064
- Using the medicine for something it's not approved for 5,797
- Lung infection 5,474
- Feeling sick to your stomach 5,330
- Tiredness 185
- Diarrhea 179
- Feeling sick to your stomach 145
- Headache 135
- The medicine is not working 133
Acyclovir can cause nervous system problems, especially in older adults or people with kidney problems. Tell your doctor if you have kidney problems before taking this medicine.
If you have kidney problems, make sure your doctor knows. Taking too much famciclovir when you have kidney problems can damage your kidneys.
Tell your doctor if you are pregnant or plan to become pregnant. It is not known if acyclovir will harm an unborn baby. Talk to your doctor about the risks and benefits of taking acyclovir while breastfeeding.
Tell your doctor if you are pregnant or plan to become pregnant. Untreated herpes during pregnancy can harm the baby. It is not known if famciclovir passes into breast milk.
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How to Read This acyclovir vs famciclovir Comparison
acyclovir is classified in the Antiviral (Nucleoside Analog) drug class, while famciclovir sits within the Antiviral (Nucleoside Analog) class. Because both drugs share the same classification, they are often considered interchangeable in theory — but clinical outcomes rarely track that cleanly. Both drugs are prescription-only, so a licensed provider must authorize use.
Adverse event totals above are pulled from the FDA's Adverse Event Reporting System (FAERS). For these top-ranked reactions alone, acyclovir has 31,277 submissions while famciclovir has 777. Those figures reflect cumulative reporting volume — not per-patient risk — so older, widely dispensed drugs typically look worse on count alone. No direct interaction between these two drugs is listed in our FDA-derived dataset, though co-prescription still warrants pharmacist review. Serious warnings, pregnancy guidance, and contraindications can differ even when indications overlap.
A table cannot substitute for clinical judgment. Effectiveness, tolerability, drug-drug interactions with your other medications, kidney and liver function, pregnancy status, insurance formulary, and price all feed into a decision that only a licensed prescriber can make responsibly. Data here is sourced from FDA Structured Product Labels (SPL) and FAERS, both of which update as manufacturers and clinicians submit new information. This page is for educational purposes only, is not medical advice, and should not be used to self-switch between acyclovir and famciclovir — always consult your physician or pharmacist first.
Important: This comparison is for informational purposes only. Drug effects vary between individuals. Always consult your doctor or pharmacist for personalized medical advice.