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What is acyclovir?

Acyclovir is an antiviral medication classified as a nucleoside analog. It acts against herpes simplex virus (HSV) and varicella-zoster virus (VZV).¹

The drug works by attaching to the viral DNA and inhibiting an enzyme producing new viruses within infected cells. Acyclovir slows viral multiplication, which stops the formation of new lesions. It can also suppress future outbreaks and reduce virus transmission to others.

However, acyclovir does not end HSV or VZV infections or completely prevent spreading them to others. After the initial infection, these viruses remain in the body for life. They mostly stay dormant but can become reactivated again.

Acyclovir is available only with a doctor’s prescription.

In the US, acyclovir is sold under the brand name Zovirax.

This guide pertains specifically to the oral forms of acyclovir (capsules, tablets, and liquid).

However, many other forms of acyclovir exist, such as topical ointment and cream and injectable formulations for intravenous administration in a hospital setting.

What is oral acyclovir used to treat?

Oral acyclovir is approved by the US Food and Drug Administration (FDA) for the treatment of the following conditions:²

  • Genital HSV infection

    • Initial episode - acute treatment

    • Recurrent episode - management

  • Herpes zoster (shingles) - acute treatment

  • Varicella (chickenpox) - acute treatment

  • Recurrent HSV labialis (cold sores) - Sitavig only

Although acyclovir won't cure these infections, it speeds up lesion healing and reduces the severity of the symptoms, such as pain and itching. It has also been shown to reduce the transmission of HSV between individuals.

Common off-label uses

Oral acyclovir is often prescribed as a short-term treatment for the following indications, which are not yet FDA approved:³

  • Oral HSV infections and recurrent episodes

  • Herpes zoster ophthalmicus (zoster affecting the eyes or near the eyes)

  • HSV keratitis (infection of the cornea of the eye)

Oral acyclovir dosage forms and strengths

Acyclovir tablets (generic, Zovirax) are formulated in the following dosing strengths:⁴

  • 400mg

  • 800mg

Acyclovir capsules (generic, Zovirax) are available in a 200mg strength.⁵

Acyclovir buccal tablets (Sitavig) are available in a 50mg strength.⁶

Acyclovir oral suspension (generic Zovirax) is formulated at 200mg/5mL concentration.⁷

Your acyclovir dosing will depend on several factors:

  • Your age

  • The health of your kidneys

  • Whether or not you are pregnant

  • The type of infection and condition being treated (HSV or VZV)

  • Whether you are being treated for your first-ever occurrence or outbreak,  a recurrence, or suppressing a future episode 

  • Any other medical conditions you might have (such as immunosuppression)

How do you take oral acyclovir?

Genital HSV

Acyclovir is most effective for recurrent HSV outbreaks when you start treatment within 24 hours of noticing the earliest signs. Delaying treatment reduces its effectiveness. As soon as you notice any symptoms, such as tingling, burning, pain, or blisters, take acyclovir as quickly as possible.⁸

Similarly, other conditions that acyclovir treats will respond best with early treatment:

Chickenpox (Varicella)

Start acyclovir immediately (within 24 hours of rash onset).⁹


Take acyclovir immediately (within 48–72 hours of rash onset).¹⁰

Oral HSV

Sitavig should be taken within 1 hour after symptom onset.¹¹

Acyclovir can be taken with or without food.  Swallow the medication whole with water (do not split or crush it). 

Be sure to read the information leaflet provided with acyclovir before you take it, particularly for the liquid and buccal tablet formulations, to ensure you receive the optimum dose for treating your condition.

If you take liquid acyclovir (the oral solution), shake the bottle thoroughly. Always use a medication measuring spoon or medicine cup to measure your dose. Household spoons are unsuitable for measuring medication.

Drink plenty of water while taking acyclovir to help maintain hydration and support the functioning of your kidneys.

Continue taking the medication as instructed by your doctor until you’ve completed the full course.

Do not change your dosage, skip a dose, or stop taking acyclovir early unless your doctor tells you to, or your symptoms may return. It’s important to complete the full course of medication prescribed, even if your outbreak has improved.

If your symptoms persist or worsen, notify your doctor.

Seeing results

Acyclovir therapy generally leads to a noticeable improvement in symptoms. However, the time to achieve results will vary depending on how quickly you started the medication and your overall health, as well as the type of HSV or VSV infection:

Herpes simplex

Acyclovir for HSV is most effective if taken within 24 hours of symptom onset. Most peoples’ symptoms start to improve after taking acyclovir for several days.¹² Those with recurrent episodes who take a daily suppression dose will typically notice fewer herpes outbreaks while they remain on the medication.

Herpes zoster (shingles)

Acyclovir therapy for shingles is most effective for skin healing and reducing viral shedding when started within 72 hours of developing lesions.¹³ It can also shorten the duration of time in which new blisters continue forming and lessen associated nerve pain (postherpetic neuralgia). Evidence indicates that in otherwise healthy individuals, herpes zoster may resolve without medication in about a month. Your doctor may prescribe acyclovir therapy to reduce zoster-associated pain and achieve a shorter healing time.

Varicella (chickenpox)

A 2005 independent review examining acyclovir’s effectiveness for treating varicella in otherwise healthy children and teens found that it reduced the number of days study participants had chickenpox-induced fever (shortening by about one day) and reduced the number of lesions. ​​However, its results for healing sores and reducing associated itching were unclear.¹⁴

Who should not take oral acyclovir?

Individuals with a history of an allergic reaction to acyclovir, valacyclovir, or a similar drug should not take this medication. Patients who have just received live varicella vaccine should wait 14 days before taking acyclovir.¹⁵ ¹⁶

Acyclovir should be taken with caution by those with:

Older adults, especially those with reduced kidney function, have a higher risk of medication side effects and may require dose adjustments. Acyclovir is removed from the body mainly by the kidneys.¹⁷

Potential side effects of oral acyclovir

When taken as indicated, acyclovir is usually well-tolerated. If your doctor prescribes acyclovir, they consider the benefits of taking the drug outweigh the possible risks.

Oral acyclovir treatment is associated with both common and less frequently seen side effects. There have also been reports of adverse effects that are rarely seen (and occasionally fatal), such as kidney toxicity.¹⁸

Common side effects observed with oral acyclovir include any of the following: 

  • Headache

  • Nausea, vomiting

  • Diarrhea

  • Malaise (discomfort, not feeling well)

Less common side effects include any of the following (and some may be severe):

General ailments

  • Fever 

  • Fatigue

Nervous system

  • Aggression, agitation

  • Dizziness

  • Tingling and numbness in the limbs 

Digestive system

  • Abdominal pain

  • Decreased appetite

Blood and lymphatic systems

  • Leg swelling (peripheral edema) 

  • Low iron levels 

  • Blood vessel inflammation

  • Low white blood cell or platelet count

  • Swollen lymph nodes

Liver, pancreas, kidney

Muscles and bones

  • Aches and pains 


  • Itchiness

  • Hair loss 

  • Patchy rash that may have a bull’s-eye appearance

  • Sensitivity to sunlight 

  • Itchy welts 


  • Visual abnormalities

Rare but severe side effects

If you experience any of the following rare but serious side effects while taking acyclovir, seek immediate medical attention. When you are able, inform your prescribing doctor.¹⁹

  • Extreme fatigue

  • Reduced, bloody, or dark urine

  • Severe back, stomach, abdominal, or side pain

  • Kidney failure 

  • Confusion, distress, visual hallucinations, delirium

  • Erratic movements

  • Difficulty speaking due to weakened muscles required for speech

  • Slow, fast, or irregular heartbeat

  • Hepatitis, yellowing of the eyes/skin (jaundice)

  • Altered brain function (encephalopathy)

  • Loss of muscle coordination

  • Anaphylaxis (severe allergic reaction)

  • Severe rash with blistering, skin peeling, and mucous membrane sores (Stevens-Johnson syndrome)

  • Sudden vision changes

  • Seizures

  • Coma

Acyclovir overdose

Acyclovir overdoses have been reported. The signs and symptoms of toxicity with acyclovir include any of the following:²⁰

  • Confusion

  • Agitation

  • Lethargy

  • Hallucinations

  • Slurred speech

  • Nausea, vomiting

  • Kidney injury

  • Seizures

  • Altered level of consciousness, coma

If you think you or someone else has taken too much acyclovir (or taken it by accident), call 911 or the Poison Control Center helpline at 1-800-222-1222 and get immediate medical attention.

Allergy information

Severe allergic reactions to acyclovir are rare but possible. Symptoms may include any of the following:

  • Itching and swelling of the face, mouth, or throat

  • Rash or hives

  • Severe dizziness

  • Difficulty breathing, wheezing

  • Rapid heart rate

  • Vomiting

  • Loss of consciousness

If you have any symptoms of an allergic reaction, seek immediate medical attention. 

Long-term use of oral acyclovir

Researchers have followed the use of acyclovir for treating HSV recurrences for up to 10 years in some individuals. The drug is effective and well-tolerated by most people. In early studies, many people remained symptom-free for up to five years without significant side effects.²¹ ²²

If you are prescribed acyclovir for long-term use, follow your healthcare professional's instructions for how and when to take the medication. Make sure to attend regular checkups to monitor your health and assess the dosing of and continued need for the medication.

Pregnancy category

Acyclovir was designated by the FDA as a pregnancy category B drug. That means there is a lack of adequate human research into its safety during pregnancy. However, animal studies do not demonstrate a risk to the fetus. Medical guidelines indicate that acyclovir should only be used during pregnancy if the prescribing doctor determines that the benefits exceed the potential risks.²³

Acyclovir and pregnancy

Studies of acyclovir taken in the first trimester of pregnancy have shown no increase in congenital anomalies.²⁴

Primary (first-time) HSV infections in early pregnancy may require acyclovir to prevent complications such as pneumonia. Recurrent HSV episodes during early pregnancy are not as risky for the fetus. However, outbreaks in the third trimester and at delivery risk transmitting the virus to the newborn, known as vertical transmission, can lead to life-threatening complications. Therefore, the American College of Obstetricians and Gynecologists recommends beginning antiviral medications at 36 weeks gestation in those with recurrent outbreaks to greatly reduce the chances of neonatal HSV infection.²⁵ ²⁶

Mothers with genital lesions during labor are recommended to have a cesarean delivery.

Your doctor might decide acyclovir is medically necessary at any time during your pregnancy after weighing the risks and benefits for you and your baby.


Acyclovir is present in breast milk in very small amounts, but there is no known risk of harm to the infant. The drug is sometimes given to newborns to treat or prevent neonatal herpes infections.²⁷

You should discuss with your doctor the risks and benefits of taking acyclovir while breastfeeding.

Missed doses

If you miss a dose of acyclovir, take it as soon as possible and then return to your regular medication schedule.

Do not take it if you remember the missed dose close to the time of the next scheduled dose. In that case, skip the missed tablet and resume your regular dosing schedule.

Drug interactions

The effectiveness of your medication can change, or the risk of side effects may increase due to certain drug interactions.

Taking any of the following medications while on acyclovir may cause negative effects:²⁸

  • Antivirals - valacyclovir (Valtrex), cidofovir, valganciclovir (Valcyte), ganciclovir 

  • Antiretrovirals - tenofovir (Viread), zidovudine (Retrovir, AZT)

  • Antifungals - amphotericin B

  • Antibiotics - bacitracin, gentamicin, neomycin, amikacin, trimethoprim-sulfamethoxazole (Bactrim), vancomycin, tobramycin

  • Blood pressure medications like captopril, chlorthalidone, enalapril (Vasotec), irbesartan (Avapro)

  • Blood thinners like warfarin (Coumadin)

  • Diabetes drugs like sitagliptin (Januvia)

  • Diuretics - furosemide (Lasix), bumetanide, spironolactone, hydrochlorothiazide, triamterene

  • Immunosuppressants - mycophenolate (CellCept), cyclosporine (Gengraf, Neoral), tacrolimus (Prograf)

  • Lithium

  • Live varicella vaccine

  • Non-steroidal anti-inflammatories - ibuprofen (Motrin, Advil), aspirin, naproxen (Aleve), celecoxib (Celebrex), meloxicam (Mobic)

  • Weight-management drugs, such as liraglutide (Saxenda)

This is not an exhaustive list, so please have a detailed conversation with your doctor about all products you use regularly or occasionally, including prescription and over-the-counter drugs, herbal products, supplements, and illicit drugs. 

Can I drink alcohol while taking oral acyclovir?

Drinking alcohol is not known to reduce acyclovir’s effectiveness, but using it may pose a risk for similar side effects associated with taking acyclovir, including drowsiness, dizziness, nausea/vomiting, and headache.²⁹

In addition, alcohol consumption is dehydrating, which can be especially hazardous to your kidney function while taking acyclovir. 

Speak to your doctor about whether or not you can safely drink alcohol while taking this medication.

What to discuss with your doctor before starting oral acyclovir

Before you begin taking acyclovir, inform your doctor of any medical issues you have, including any of the following:

  • History of an allergic reaction to acyclovir or valacyclovir

  • Immunodeficiency or suppression

  • Kidney disease 

  • History of a transplant

  • Liver disease

Also tell your doctor about:

  • Any prescription medications, over-the-counter medications, drugs, vitamins, or herbal supplements you use or plan to use

  • Any medication allergies

  • Any recent vaccinations or plan to be vaccinated with a live vaccine 

  • Factors that increase your risk of severe dehydration, such as intense activity like endurance or competitive sports or working outdoors

  • Whether you usually drive or operate heavy machinery (due to the potential for drowsiness)

  • Any plans for pregnancy or breastfeeding

When you speak with your doctor, be sure to review how best to stop the spread of infection both when you have symptoms and when you don’t. 

Stopping acyclovir

Speak with your doctor before you stop taking your prescribed course of acyclovir.

Stopping this medication won’t cause withdrawal symptoms but may have undesirable effects, such as the return or worsening of a herpes outbreak or an episode of shingles.

Drug approval history

In 1985, the US FDA approved Zovirax brand oral acyclovir capsules, making them the first oral treatment for HSV available to self-administer with a doctor’s prescription.³⁰

Sitavig was approved in 2013.³¹

Tips for taking oral acyclovir

  • Acyclovir works best for treating acute recurrent HSV outbreaks or shingles episodes when it is started within the first day of symptom onset. If you have been prescribed this medication to suppress future occurrences, make sure you have it readily available at the first sign of symptoms.

  • Avoid overexposure to the sun while taking acyclovir because it can cause photosensitivity (sun allergy). When you must be outside, wear sun protection with an SPF of 30 or higher and a hat and clothing covering your skin.

  • If you are an older adult or have a weakened immune system (due to HIV, a transplant, or a condition requiring immunosuppressant medication), stay in close communication with your prescribing doctor, as you may be at a greater risk of experiencing side effects while taking acyclovir.³³

  • Stay well hydrated while taking this medication. Familiarize yourself with over-the-counter and prescription medications that interact negatively with acyclovir, such as non-steroidal anti-inflammatories.

  • If you are being treated for recurrent HSV outbreaks, you will still be somewhat contagious while taking acyclovir, even if you are asymptomatic. If you are sexually active, ask your doctor about safer sex practices and ways to reduce the risk of HSV transmission.

Frequently asked questions

What is the most common side effect of acyclovir?

The most frequently reported side effects are nausea, vomiting, diarrhea, headache, and malaise (feeling mildly unwell).³³

Is acyclovir a type of antibiotic?

No. Acyclovir is an antiviral medication that works to slow the spread of the herpes simplex and varicella-zoster viruses. It can also be called an antimicrobial. Antibiotics treat infections with bacteria (and some protozoa) but not viral infections.


  2. (As above)

  3. Acyclovir - StatPearls


  5. (As above)

  6. Prescribing information Sitavig


  8. Genital herpes: Overview - - NCBI Bookshelf

  9. Antiviral therapy of varicella-zoster virus infections - Human Herpesviruses - NCBI Bookshelf

  10. Herpes Zoster Treatment & Management

  11. NDC 68712-049-02 Sitavig (ACYCLOVIR) Buccal Tablets 50 mg

  12. Antiviral therapy - Human Herpesviruses - NCBI Bookshelf

  13. Antiviral therapy of varicella-zoster virus infections - Human Herpesviruses - NCBI Bookshelf

  14. Acyclovir for treating varicella in otherwise healthy children and adolescents - PMC 

  15. Zovirax (acyclovir) dosing, indications, interactions, adverse effects, and more

  16. Ask the Experts: Varicella (Chickenpox) Vaccines

  17. Acyclovir for treating varicella in otherwise healthy children and adolescents - PMC


  19. Zovirax (acyclovir) dosing, indications, interactions, adverse effects, and more


  21. Valacyclovir for Herpes Simplex Virus Infection: Long-Term Safety and Sustained Efficacy after 20 Years’ Experience with Acyclovir | The Journal of Infectious Diseases

  22. Long-term suppression of recurrent genital herpes with acyclovir. A 5-year benchmark. Acyclovir Study Group


  24. Acyclovir Safe for Treating Herpes Infection in Early Pregnancy | AAFP

  25. Acyclovir (Zovirax®) / Valacyclovir (Valtrex®) - Mother To Baby | Fact Sheets - NCBI Bookshelf

  26. Mother-to-Child Transmission of Herpes Simplex Virus - PMC

  27. Acyclovir - Drugs and Lactation Database (LactMed®) - NCBI Bookshelf

  28. Acyclovir: MedlinePlus Drug Information

  29. Harmful Interactions | National Institute on Alcohol Abuse and Alcoholism (NIAAA)

  30. Federal Register :: Determination That ZOVIRAX (Acyclovir) Oral Capsules, 200 Milligrams, Was Not Withdrawn From Sale for Reasons of Safety or Effectiveness

  31. Drug Approval Package - FDA


  33. Zovirax (acyclovir) dosing, indications, interactions, adverse effects, and more

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.