Skip to Content

Acyclovir: 6 things you should know

Medically reviewed by C. Fookes, BPharm Last updated on Nov 14, 2018.

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 works in a number of different ways to prevent these viruses from replicating. 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).
  • Acyclovir is available in a number of 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.
  • Acyclovir is usually dosed five times daily.
  • May interact with a number of other medications including probenecid, other antivirals, medications for bowel disease, injectable osteoporosis medications, and analgesics.

Notes: 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. For a complete list of all side effects, click here.

4. Bottom Line

Acyclovir is an antiviral drug that helps treat chickenpox, genital herpes, and shingles infections. 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 of 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, 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 capsules [Package Insert] Revised 12/2017. Carlsbad Technology, Inc.

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.

Copyright 1996-2019 Revision date: November 14, 2018.

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