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Acyclovir: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on May 26, 2022.

1. How it works

  • Acyclovir is an antiviral drug that has activity against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV).
  • Acyclovir acts as a specific inhibitor of herpesvirus DNA polymerase. This is a key enzyme in the lytic phase of infection by herpesviruses to ensure the replication of the virus. Activity against HSV is greater than activity against VZV.
  • Acyclovir belongs to the class of medicines called antivirals.

2. Upsides

  • Acyclovir may be used in the treatment of herpes zoster virus (shingles). Acyclovir does not completely rid the body of the herpes zoster virus.
  • Acyclovir may be used to treat initial and recurrent episodes of genital herpes. Acyclovir does not cure genital herpes or completely rid the body of the herpes virus.
  • Acyclovir may be used in the treatment of varicella (chickenpox).
  • Inhibits herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV).
  • Acyclovir is available in several different formulations including a tablet, capsule, suspension, buccal tablet, cream, and injection.
  • Generic acyclovir is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Nausea, vomiting, diarrhea, headache, and a general unwell feeling. Buccal tablets may cause mouth pain. Rarely, acyclovir may affect your kidneys or cause a bleeding disorder.
  • May not be suitable for some people, including those with kidney problems, who are immunosuppressed, taking certain medications, or in those who are dehydrated. In people with kidney disease, a dosage reduction is recommended.
  • Acyclovir is usually dosed five times daily (every 4 hours)
  • May interact with several other medications including probenecid, other antivirals, medications for bowel disease, injectable osteoporosis medications, and analgesics.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Acyclovir is an antiviral drug that treats infections caused by HSV-1, HSV-2, and varicella-zoster viruses such as chickenpox, genital herpes, and shingles. It is most effective when started within 48 hours of symptom onset.

5. Tips

  • Acyclovir treatment is best started within 72 hours of symptom onset (ie, rash, blisters, tingling, burning) and most effective if started within 48 hours of symptom onset.
  • Take acyclovir for the exact time prescribed by your doctor, even if your symptoms improve. Although acyclovir treats viruses, it is not effective against the flu or the common cold.
  • Shake acyclovir oral suspension well before using. Use a proper dosing syringe or medicine cup to correctly measure dosage, not a kitchen teaspoon.
  • If you have been prescribed buccal tablets to treat a cold sore, place the flat side of the buccal tablet against your upper gum, behind your lip, and in front of your canine tooth on the same side of your mouth as the cold sore. Close your mouth and gently press on the outside of your lip to hold the tablet in place for 30 seconds. Allow the tablet to slowly dissolve; eating and drinking may continue as normal while the buccal tablet is in place, but avoid brushing your teeth, chewing gum, or wearing an upper denture.
  • If you are taking acyclovir long-term and your weight changes, let your doctor know as acyclovir dosages are based on weight.
  • Try to maintain good hydration when taking acyclovir.
  • If you have a shingles rash or a cold sore, try and keep it as clean and dry as possible; although you must be careful not to transfer the virus to others, so always use disposable tissues or a separate towel that is washed by itself. Wearing loose clothing may help prevent irritation caused by a shingles rash.
  • Seek urgent medical advice if you have any signs of an allergic reaction (such as hives; difficulty breathing; facial or throat swelling), easy bruising or bleeding or difficulty urinating, swelling in your feet, or shortness of breath.
  • Herpes infections are highly contagious and you should cover your rash and avoid intimate contact. Also, avoid touching the rash and then your eyes. Wash hands frequently.
  • If you are taking acyclovir for genital herpes, it will not prevent you from passing the infection to your sexual partner. Sexual intercourse should be avoided while you have lesions or the first symptoms of an outbreak. Even if you have no symptoms, the transmission of genital herpes may still occur.
  • Talk to your doctor or pharmacist before buying any medication over the counter to check that it is compatible with acyclovir, including vitamins or herbal products.
  • Acyclovir may make your skin more sensitive to the sun and more likely to burn. Wear at least an SPF 30+ sunblock and sun-protective clothing when you need to go outside.

6. Response and effectiveness

  • May take up to two hours to reach peak plasma concentrations after oral acyclovir administration.
  • May take up to three days for symptom reduction; however, acyclovir should be taken until the course prescribed is completed. Acyclovir works best when started within 48 hours of symptom onset.
  • In herpes zoster (shingles) infections, acyclovir shortened the time it took lesions to scab over and decreased the time needed for the rash to heal and become pain-free. Adults older than 50 gained the most benefit from taking acyclovir.
  • In varicella infections (chickenpox), acyclovir shortened the time to 50% healing, reduced the number of lesions and vesicles, and decreased rates of fever and tiredness by day 2.
  • Acyclovir significantly reduced the duration of acute infection and duration of lesion healing in people with genital herpes. In some, the duration of pain and new lesion formation was decreased. When given daily for 4 months to 10 years, acyclovir reduced the frequency and severity of recurrent genital herpes in more than 95% of people.

7. Interactions

Medicines that interact with acyclovir may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with acyclovir. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with acyclovir include:

  • amphotericin
  • clozapine
  • hepatitis B medications, such as entecavir
  • HIV medications, such as tenofovir and zidovudine
  • gentamicin
  • NSAIDs, such as ibuprofen, diclofenac, or naproxen
  • other antivirals, such as valacyclovir
  • probenecid
  • theophylline
  • warfarin, an anticoagulant (blood thinner)
  • vaccinations, such as the varicella virus or zoster vaccine
  • other medications that are metabolized by CYP1A2 enzymes, such as theophylline and tizanidine.

Alcohol should be avoided while taking acyclovir because it weakens the immune system and may exacerbate the side effects of acyclovir.

Note that this list is not all-inclusive and includes only common medications that may interact with acyclovir. You should refer to the prescribing information for acyclovir for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use acyclovir only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: May 25, 2022.