Acyclovir: Everything You Need to Know

Acyclovir is an antiviral medicine that's very effective against herpesviruses and the varicella-zoster virus. It's used both to treat a first time infection and to prevent future outbreaks.
Acyclovir: Everything You Need to Know

Last update: 01 July, 2021

Acyclovir is a synthetic nucleoside analog drug that fights against the herpes simplex virus and the varicella-zoster virus. It’s a derivative of guanosine and it’s useful both to combat the infection for the first time and in subsequent outbreaks.

This medicine – also known as acycloguanosine – is recommended for treating infections such as shingles, herpetic stomatitis, genital herpes, cold sores, keratitis, herpes simplex type 1 and 2, and hepatic encephalitis.

It’s also useful as an antiviral treatment. This is mainly because it acts in a highly selective way (that is, it only attacks the viruses we’ve already mentioned) and has a low level of cytotoxicity (that is, it hardly affects uninfected cells).

Specifically, what the drug does is inhibit the replication of viral DNA, in order to prevent it from continuing to spread throughout the body, relieve pain, promote the healing of blisters, and shorten how long the condition lasts.

Let’s see more about it below.

What do you use it for?

Acyclovir is an effective treatment against herpes virus lesions
Herpes viruses are susceptible to acyclovir.

Acyclovir fights infections with the herpes virus and the varicella-zoster virus. It prevents them from spreading and also allows the recovery process to start.

Doctors use acyclovir to reduce pain and speed the healing of wounds or blisters in people who have been infected with the chickenpox or shingles viruses. It’s also effective in preventing outbreaks from reoccurring (in the case of genital herpes).

Although this drug is very useful in treating infections caused by the abovementioned viruses and in preventing outbreaks of genital herpes (in people who are already infected), it isn’t able to cure genital herpes or prevent its transmission to other people. For that reason, it can’t be used in a preventative way in healthy people.

In the case of infected people, in addition to following the doctor’s instructions for use, it’s essential to follow their recommendations in all matters relating to self-care, prevention, and other measures.

How do you administer it?

You should take the medication exactly as the doctor tells you, at the same time each day, and with plenty of fluids. And if you have any questions, you should consult with your doctor, or, failing that, the pharmacist. Don’t change the hours or dosages without their consent because you may worsen your situation and prolong the treatment.

When a person uses it to fight infection and they’ve already begun to manifest symptoms, acyclovir should be taken between 2 and 5 times a day, for anywhere between 5 and 10 days, depending on the case.

If it’s to prevent outbreaks of genital herpes, then the patient should take this medication 2 to 5 times a day for long periods of time that can be up to 1 year.


Acyclovir has several commercial presentations
It’s common for this drug to be taken orally.
  • Acyclovir can be found under the following brand names: Sitavig®, Zovirax® capsules, and Zovirax® tablets.
  • It has 3 formats which are all taken orally: tablets, capsules, and liquid. You can also find it in delayed-release tablets, creams, and preparations for intravenous administration.


According to the information sheet, the dosage of the drug depends on the degree of infection, the age of the patient, and other factors. So, for example, to treat a varicella-zoster infection of the skin and mucosa you need to take 200 mg every 4 hours (except at night) for 5 days.

To prevent a recurrent varicella-zoster infection in immunocompetent patients the dosage should be 200 mg every 6 hours, or 400 mg every 12 hours, interrupting the treatment at 6 or 12-monthly intervals.

Who shouldn’t take it or use it?

In principle, the following types of people shouldn’t take acyclovir without consulting their doctor:

  • Allergic people with hypersensitivity to acyclovir, valaciclovir or ganciclovir.
  • Dehydrated patients.
  • Pregnant or lactating women.
  • People who are intolerant to milk proteins.
  • Patients with neurological alterations in the presence of a cytotoxic agent.

To avoid negative interactions, it’s essential to inform your doctor about the medication you’re currently taking, as well as any nutritional supplements, herbal preparations, over-the-counter medications, and the like.

What are the possible side effects?

Acyclovir is generally a safe drug. However, it can cause mild and temporary side effects in some cases. Among the most common are the following:

  • (When taken orally)
    • Sickness
    • Dizziness
    • Vomiting
    • Diarrhea
    • Agitation
    • Tiredness
    • Hair loss
    • Vision problems
    • Stomach upset (abdominal pain)
    • Joint pain
  • (When given intravenously)
    • Sickness
    • Hypotension
    • Hematuria
    • Phlebitis

In other cases, acyclovir can cause serious side effects (difficulty breathing, swelling of the face and other parts of the body, confusion, speech difficulties, fast heartbeat, etc.). If this should occur then it’s essential to seek immediate medical help.

What happens if I miss a dose?

Doctors recommend that you take the missed dose as soon as you remember and take the remaining doses for that day at equal intervals. However, if it’s almost time for your next dose, then skip the missed dose and continue with your regular dosing schedule. Don’t double the dose to make up for the one that you missed.

If you have any questions, ask your pharmacist or doctor what you should do next. Avoid taking action on your own, as this could backfire.

What should I do in the case of an overdose?

As in other cases of poisoning, in the case of an overdose, you should seek immediate medical help.

If the person has lost consciousness, has seizures, or is experiencing breathing difficulties, then contact the emergency department immediately and follow the instructions they give you over the phone until the paramedic team arrives.

How to store and how to dispose of this medicine

Like other medicines, it should be stored in a dry place at room temperature, out of the reach of children and pets. Preferably, in its original container for greater safety.

Don’t store it in the bathroom, as the humidity could affect the medication.

Neither acyclovir nor any other medicine should be flushed down the toilet. They should be taken to a collection point (such as the pharmacy, or a recycling center). Try to find out about the drug return program in your community.

Final recommendations

The medicine should be taken whole with plenty of liquid (especially if you have a dry mouth). You shouldn’t crush or chew it. The delayed-release buccal tablets shouldn’t be swallowed either. Wait for it to dissolve completely before you brush your teeth.

Under no circumstances should acyclovir be given to your partner or to anyone else infected with the herpes virus or varicella-zoster virus without a prescription because it can be dangerous. Nor should it be administered as a preventive measure in healthy people.

Taking acyclovir doesn’t exempt you from maintaining the necessary precautionary and hygiene measures when having sexual intercourse. Consult your doctor with any questions you may have in this regard.

If you go for a consultation or to the emergency department for other reasons, tell them you’re taking (or have taken) acyclovir.

Este texto se ofrece únicamente con propósitos informativos y no reemplaza la consulta con un profesional. Ante dudas, consulta a tu especialista.