Acyclovir is an antiviral medicine used to treat conditions caused by herpes viruses. Common side effects include nausea and vomiting, diarrhea, and headache. Serious side effects may include kidney damage. Acyclovir is available in multiple dosage forms, including a capsule, liquid, or tablet.

Uses

What is acyclovir used for?

Acyclovir is an antiviral medicine commonly used for the following conditions caused by herpes viruses.

Acyclovir may also be used for other conditions as determined by your healthcare provider.

How does acyclovir work (mechanism of action)?

Acyclovir is an antiviral medicine that stops the growth of herpes viruses and improves symptoms. It is important to note that acyclovir is not a cure. The herpes viruses remain in the body even if you do not have symptoms. 

How is acyclovir supplied (dosage forms)?

Brands and other names

  • Zovirax

Dosage forms and strengths

  • 200 mg oral capsules
  • 200 mg/5 mL oral suspension
  • 400 mg oral tablets
  • 800 mg oral tablets

Acyclovir is also available in injectable forms.

Acyclovir is also available for use on the skin. See Acyclovir topical for more information.

Acyclovir is also available as a buccal tablet that dissolves in the mouth. See Sitavig for more information.

How should I store acyclovir?

Acyclovir should be stored at room temperature, between 68 F to 77 F (20 C to 25 C). Some products may allow you to store it at slightly lower temperatures, such as down to 59 F (15 C). Protect from light and moisture.

Side Effects

What are the most common side effects of acyclovir?

The most common side effects of acyclovir are listed below. Tell your healthcare provider if you have any of these side effects that bother you.

  • Nausea
  • Vomiting
  • Diarrhea
  • Headache
  • General feeling of being unwell, tired, or not having your usual energy
  • Pain, itching, swelling, or redness near the injection site (injection)
  • Hives (injection)

There may be other side effects of acyclovir that are not listed here. Contact your healthcare provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088. In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of acyclovir?

While less common, the most serious side effects of acyclovir are described below, along with what to do if they happen.

Severe Allergic Reactions. Acyclovir may cause allergic reactions, which can be serious. Stop taking acyclovir and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Kidney Damage. Kidney damage can happen when taking acyclovir. Call your healthcare provider right away if you have any of the following symptoms of kidney damage.

  • Reduced need to pee
  • Swelling in your feet, ankles, or legs
  • Weakness or unusual tiredness
  • Difficulty catching your breath or chest pain/pressure
  • Confusion
  • Nausea
  • Seizures

Warnings & Precautions

Who should not use acyclovir?

Allergies to Ingredients. People who are allergic to any of the following should not take acyclovir.

  • Acyclovir
  • Valacyclovir
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific acyclovir products they stock.

What should I know about acyclovir before using it?

Do not take acyclovir unless it has been prescribed to you by a healthcare provider. Take it as prescribed.

Do not share acyclovir with other people, even if they have the same condition as you. It may harm them.

Keep acyclovir out of the reach of children.

Be sure to drink plenty of fluids while you are taking acyclovir.

If you are prescribed acyclovir to treat an outbreak, take it as soon as possible when you first have symptoms, such as burning, itching, or tingling, or when the sore appears.

Genital herpes can be passed to others during sexual activity even if you have no signs or symptoms. Ask your healthcare provider about safe sex practices, such as condoms. If you have symptoms or open sores from genital herpes, do not have sexual contact to avoid spreading it to others.

People who are 65 and older can be at greater risk for some side effects from acyclovir. Talk to your healthcare provider about your risks if you are in this age group.

If you are prescribed acyclovir oral liquid, use an accurate measuring device to measure your dose. A household spoon is not an accurate measuring device and may cause you to take the wrong dose. Ask your pharmacist to recommend an appropriate measuring device.

What should I tell my healthcare provider before using acyclovir?

Tell your healthcare provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if acyclovir is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your healthcare provider if you have any of the following.

Weak Immune System. Some people with a weak immune system who take acyclovir may develop a condition called thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). These conditions can cause small blood clots throughout the body, reducing blood flow to organs like the brain, heart, and kidneys. TTP and HUS are serious and can lead to death. Tell your healthcare provider if you have a weak immune system before taking acyclovir. 

Other Medicines and Supplements. Acyclovir may interact with other medicines and supplements. Before taking acyclovir, tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Vaccines. You may not be able to receive certain vaccines, such as the chickenpox (varicella) vaccine, right before or while taking acyclovir. Tell your healthcare provider about any vaccines you have recently received or have scheduled.

Pregnancy. It is not known if or how acyclovir could affect pregnancy or harm an unborn baby. Tell your healthcare provider if you are or plan to become pregnant. Your healthcare provider will advise you if you should take acyclovir while you are pregnant or trying to get pregnant.      

Breastfeeding. Acyclovir passes into breast milk. Tell your healthcare provider if you are breastfeeding or plan to breastfeed. Your healthcare provider will advise you if you should take acyclovir while breastfeeding. 

Interactions

Does acyclovir interact with foods or drinks?

There are no known interactions between acyclovir and foods or drinks.

It is unknown if drinking alcohol will affect acyclovir, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does acyclovir interact with other medicines (drug interactions)?

Always tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are taking probenecid, which is a medicine commonly used for gout, before taking acyclovir. Taking medicines that can harm the kidneys may also increase the risk of kidney damage with acyclovir.

This may not be a complete list of medicines that can interact with acyclovir. Always check with your healthcare provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much acyclovir?

If you or someone else has used too much acyclovir, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of acyclovir?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.