8 Famciclovir (Famvir) Side Effects You Should Know About

Famciclovir (Famvir) is an antiviral medication used to treat herpes virus infections, such as genital herpes, cold sores, and shingles in adults. You’ll usually take a short course of famciclovir to treat herpes infections. But you can also take it daily to prevent future outbreaks.

Whether you’re taking famciclovir short or long term, it’s helpful to know about its possible side effects. Keep in mind that brand-name Famvir is no longer available. So we’ll refer to it as famciclovir, the available generic form.

Famciclovir side effects at a glance

The lists below include common and rare famciclovir side effects reported by people taking it. Keep in mind that you may experience side effects not listed here, since everyone responds to medication differently.

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Mild famciclovir side effects:

• Headache

• Nausea

• Vomiting

• Diarrhea and gas

• Fatigue

• Itchy skin

• Painful periods

• Dizziness

• Drowsiness

Rare but serious famciclovir side effects:

• Liver problems

• Kidney problems

• Low blood cell counts

• Confusion, especially in older adults

• Heart palpitations

• Allergic reactions, such as hives or severe skin problems

Next, we’ll explain eight of these famciclovir side effects in more detail.

1. Headache

Headache is one of the most common famciclovir side effects. It was reported by up to 40% of people in clinical studies. And the longer you take famciclovir, the more likely it is you’ll experience this side effect.

Headaches are usually mild and go away on their own. You can often alleviate headache discomfort with medication-free strategies, such as drinking plenty of fluids, resting, or applying a warm or cold compress.

If the headache continues to be bothersome, consider trying an over-the-counter (OTC) pain reliever, such as acetaminophen (Tylenol) or ibuprofen (Advil, Motrin). Follow the dosage and precautions on the product label, or check with a pharmacist to make sure it’s a safe choice for you.

If your headache feels severe or isn’t going away, contact your healthcare team.

2. Nausea and vomiting

Nausea and vomiting are also possible side effects of famciclovir.

You can take famciclovir with or without food. But taking it with food may help you avoid or lessen nausea. Sometimes, side effects like nausea will go away on their own as your body gets used to the medication.

If you feel queasy after taking famciclovir, it may help to choose small meals consisting of bland foods until you’re feeling better. Some people find relief by sipping on ginger or peppermint tea.

If you’re throwing up, do your best to drink fluids to prevent becoming dehydrated. Also, if you vomit after taking famciclovir, check with your prescriber before you take another dose. You should also let them know if your nausea or vomiting isn’t going away.

3. Diarrhea

Some people experience diarrhea while taking famciclovir, but it’s not as common as nausea and headache. You may also feel more gassy than usual in some cases. In clinical trials, diarrhea occurred in up to 9% of people who took famciclovir.

Diarrhea is unpleasant, but in most cases, it isn’t serious and goes away on its own. You can usually manage diarrhea at home and help your body recover by staying hydrated. Opt for water or fluids with electrolytes. OTC anti-diarrhea medications ,such as loperamide (Imodium A-D) or bismuth subsalicylate (Pepto-Bismol), are also options to consider.

If diarrhea doesn’t improve, tell your healthcare team. They may recommend other ways to manage it. Also, be sure to tell them if your diarrhea is accompanied by fever, bloody stools, or severe stomach pain. These symptoms may suggest a more serious problem (unrelated to famciclovir) that needs medical care.

4. Fatigue

If you’re feeling unusually tired and lacking in energy while taking famciclovir, it could be a side effect of the medication. Fatigue isn’t as common as other famciclovir side effects. But it’s more likely if you’re taking daily famciclovir to prevent outbreaks.

If you’re taking famciclovir for shingles (herpes zoster), keep in mind that fatigue can also be a symptom of shingles itself. For shingles, you’ll usually take famciclovir for 7 days. Even if you feel tired, it’s best to finish the entire course of treatment. This will help ensure you effectively treat shingles, which should help you get your usual energy back.

5. Itchy skin

Itchy skin can be a side effect of famciclovir, but it wasn’t common in studies.

Keep in mind that itching can also be a symptom of the herpes virus infections that famciclovir treats. So the itchiness may ease as the medication starts working to treat the infection.

Still, there are things that can help manage itching in the meantime. You can try soaking in an oatmeal bath. A topical herpes treatment may also help. In more severe cases, your prescriber may suggest an oral steroid medication such as prednisone to help ease itching and swelling.

Do your best to avoid scratching a herpes rash, as this could introduce germs and lead to secondary infections. It can also spread the infection if you touch other body parts after scratching.

6. Liver problems

Liver problems are unlikely but possible famciclovir side effects. Some participants in clinical trials had blood test results that showed elevated liver enzymes (proteins). This can sometimes be a sign of early liver problems.

Also, some people have reported jaundice (yellowing of the skin and eyes) while taking famciclovir. Jaundice can be a symptom of liver problems, but not always. And it isn’t known for sure if the medication was the reason people had jaundice in these cases.

As a precaution, your prescriber may avoid famciclovir if you have a history of liver problems. They can prescribe a safer treatment option for you, if needed.

While rare, watch for symptoms of liver problems while taking famciclovir, including:

• Abdominal pain

• Dark urine

• Yellow-looking skin or eyes

• Nausea and vomiting

Seek medical care right away if you develop these symptoms.

7. Kidney problems

Famciclovir isn’t known to directly cause new kidney problems. But, there have been cases of sudden or short-term (acute) kidney failure in people who already had kidney issues before taking famciclovir. In most cases, this was associated with taking higher doses of famciclovir than recommended. This is because famciclovir gets cleared from the body by the kidneys. So if your kidneys aren’t working well, famciclovir levels can build up in the body and cause further damage.

Because of this risk, it’s important to tell your healthcare team about any kidney problems you’ve had in the past. They may prescribe a lower or less frequent famciclovir dosage for you, depending on your kidney function.

If you notice symptoms of acute kidney failure (also known as acute kidney injury) while taking famciclovir, it’s best to seek emergency care right away. Symptoms may include:

• Peeing less than usual

• Swelling in your lower legs or feet

• Shortness of breath

• Fatigue

• Chest pain

• Nausea

• Confusion

If you develop acute kidney failure, your prescriber will recommend stopping famciclovir. You’ll also likely need care in the hospital for a few days, but most people make a full recovery.

8. Low blood cell counts

Famciclovir side effects may lower the number of blood cells in your body, but this is rare. Symptoms may include extreme tiredness or more frequent infections. Some people who take famciclovir may experience low platelets (thrombocytopenia), which may cause unexpected bruising or bleeding.

In clinical studies, this side effect was only reported in people who took famciclovir long term. Short-term use of this medication shouldn’t cause low blood cell counts in most cases.

Be sure to follow your provider’s recommendations for getting routine blood work done. These blood tests often include a complete blood count (CBC). The results help inform you and your prescriber about your current health as well as how treatments or other factors may be affecting your body.

If low blood cell counts become a concern during famciclovir treatment, your prescriber may recommend stopping it.

When should you seek medical care for famciclovir side effects?

Most common famciclovir side effects (nausea and headache) are mild and relatively easy to manage at home. But if any side effects persist or worsen, be sure to let your healthcare team know. And if any symptoms feel severe or life-threatening, seek medical care right away.

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Written by:

Patricia Weiser, PharmD

Patricia has 16 years of pharmacist experience. She previously held positions as a community pharmacist, pharmacy manager, and hospital pharmacist.

Edited by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

Reviewed by:

Christina Aungst, PharmD

Christina Aungst, PharmD, is a pharmacy editor for GoodRx. She began writing for GoodRx Health in 2019, transitioning from freelance writer to editor in 2021.

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