Acyclovir Patient Tips

Medically reviewed by Carmen Pope, BPharm. Last updated on July 25, 2025.

1. How it works

• Acyclovir is an antiviral medication that is used to treat herpes simplex types 1 and 2 (HSV, cold sores and genital herpes), and varicella zoster virus (VZV, chickenpox or shingles).
• Acyclovir works by selectively targeting the enzyme thymidine kinase (TK) encoded by HSV and VZV. This viral enzyme converts acyclovir into acyclovir monophosphate which eventually gets converted into acyclovir triphosphate which stops replication of herpes viral DNA by competitive inhibition of DNA polymerase, incorporation into and termination of the growing viral DNA chain, and inactivation of viral DNA polymerase. Acyclovir is more effective against HSV compared to VZV due to its more effective phosphorylation by viral TK.
• Acyclovir belongs to the drug class called antivirals. It may also be called a purine nucleoside.

2. Upsides

• Acyclovir is used to treat varicella zoster virus (chickenpox and shingles). Acyclovir does not completely rid the body of the herpes zoster virus. The injection may be used in immunocompromised patients with VZV infections
• Acyclovir is 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.
• Inhibits herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV). Acyclovir injection may be used to treat initial and recurrent mucosal and cutaneous herpes simplex in immunocompromised patients, herpes simplex encephalitis, or neonatal herpes simplex virus infection.
• Acyclovir is available as a tablet (400 mg, 800 mg), capsule (200 mg), suspension (200 mg/5 mL), buccal tablet (50 mg), cream, ointment, and injection (50 mg/mL).
• 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 feeling unwell are the most common side effects reported. Buccal tablets may cause mouth pain. Rarely, acyclovir may affect your kidneys or cause a bleeding disorder.
• Nausea, vomiting, dizziness, somnolence, hallucinations, confusion, and coma were reported more frequently in elderly subjects.
• Do not use in those with previous hypersensitivity reactions to acyclovir or valacyclovir.
• Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS), which has resulted in death, has occurred in immunocompromised patients receiving acyclovir therapy.
• May not be suitable for some people, including those with kidney problems, who are immunosuppressed, taking certain medications, or those who are dehydrated.
• In people with kidney disease, a dosage reduction is recommended. An increased risk of renal dysfunction may be seen in patients receiving other medications that adversely effect the kidneys.
• Oral acyclovir is usually dosed five times daily (every 4 hours) for 7 to 10 days.
• Acyclovir injection is usually given every 8 hours. A maximum dose equivalent to 20 mg/kg every 8 hours should not be exceeded for any patient.
• May interact with several other medications including probenecid, other antivirals, medications for bowel disease, injectable osteoporosis medications, and analgesics.
• Acyclovir injection is for intravenous infusion only. Give over at least an hour to minimize kidney damage. Administration of acyclovir by intravenous infusion must be accompanied by adequate hydration.
• Approximately 1% of patients receiving intravenous acyclovir have manifested encephalopathic changes characterized by either lethargy, obtundation, tremors, confusion, hallucinations, agitation, seizures, or coma. Acyclovir should be used with caution in those patients who have underlying neurologic abnormalities and those with serious renal, hepatic, or electrolyte abnormalities, or significant hypoxia.
• Safety and use of oral formulations in children under 2 years has not been established.
• There are no adequate and well-controlled studies in women. Acyclovir passes into breast milk and should only be administered with caution to a breastfeeding mother.

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 medication that is used to treat herpes simplex types 1 and 2 (HSV, cold sores and genital herpes), and varicella zoster virus (VZV, chickenpox or shingles). It is most effective when started within 48 hours of symptom onset. Nausea, vomiting, diarrhea, headache, and feeling unwell are the most common side effects reported. Rarely, acyclovir may affect your kidneys or cause a bleeding disorder.

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 duration 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 the 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, 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.

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Frequently asked questions

• Can you drink alcohol while taking acyclovir?
• What would be the benefits of taking valacyclovir vs acyclovir?

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.

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