Acyclovir is the generic name for Zovirax, a prescription medication used to treat certain virus infections.

The Food and Drug Administration (FDA) approved acyclovir to treat viral infections from the varicella virus that causes chicken pox and shingles, as well as infections from the virus that causes genital herpes.

Sometimes doctors prescribe acyclovir to treat herpes infections in people with HIV.

The drug works by preventing viruses from dividing and multiplying. The FDA approved acyclovir in the 1980s.

Acyclovir is available as a generic, made by several companies, or under the brand name Zovirax, made by GlaxoSmithKline and available in tablet, capsule, and liquid form.

Acyclovir is one of the oldest drugs used to treat herpes simplex viruses and remains the first line of treatment for these infections.

However, research shows that acyclovir is not as effective as it used to be.

A 2013 study, published in the journal Current Opinion in Infectious Diseases, found that acyclovir-resistant herpes strains could develop over time.

Resistance happens in people with a healthy immune system as well as in those with a weakened immune system.

Acyclovir Warnings

It’s important to know that acyclovir does not cure viral infections. However, it can make infections shorter and less serious for some people.

If you’re taking acyclovir for genital herpes, it can reduce the severity or prevent recurrences of a herpes outbreak.

If you’re taking acyclovir to treat chicken pox or shingles, the drug can reduce the severity of your infection.

It’s important to know that treatment with acyclovir works best when you start taking it as soon as possible after a rash appears.

This means within three days of a shingles rash and within 24 hours of a chicken pox rash.

It’s usually not necessary to treat young, healthy children with chicken pox, but older children or adults who get chicken pox may need treatment.

Drink plenty of fluids when taking this medication. Children younger than 2 should not take acyclovir.

Use acyclovir with caution if you have kidney disease or any condition that weakness your immune system. If you have these conditions, you could be at risk for serious reactions to acyclovir.

Ask your doctor for advice on practicing safe sex if you have a genital herpes infection. Genital herpes spreads through sexual activity, and taking acyclovir alone may not be enough to prevent it.

Acyclovir and Pregnancy

If you’re a woman, let your doctor know if you are or may be pregnant or if you’re breastfeeding.

Researchers have not studied acyclovir use by pregnant women, so there’s not enough evidence to say that it is safe to take during pregnancy.

Acyclovir also may pass into breast milk.

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Acyclovir Side Effects

The most common side effects of acyclovir treatment for genital herpes include nausea, vomiting, and diarrhea.

Shingles requires treatment with higher doses of acyclovir, and the most common side effects at higher doses are tiredness and malaise.

Tell your doctor if you have any side effects. Side effects that may be seen in all people using acyclovir include:

  • Nausea
  • Diarrhea
  • Vomiting
  • Headache
  • Dizziness
  • Tiredness
  • Muscle or joint aches
  • Visual changes
  • Fluid retention
  • Hair loss
  • Confusion
  • Changes in behavior

Serious side effects also can occur. If you have any of these side effects, call your doctor right away:

  • Severe rash, hives, or a rash that causes blisters and peeling
  • Yellowing of skin or eyes
  • Unusual bleeding or bruising
  • Seizure
  • Loss of consciousness
  • Swelling of face, lips, or tongue
  • Difficulty breathing
  • Decreased urine output or blood in the urine
  • Extreme sleepiness or confusion
  • Hallucinations
  • Tingling, numbness, or shakiness

Age matters, too. People older than 65 may have more side effects from acyclovir because their kidneys do not get rid of the drug as quickly as younger people’s do.

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Acyclovir Interactions

Some drugs may affect the way acyclovir works, and acyclovir may affect other drugs you are taking.

It’s very important to let your doctor know about all drugs you are taking, including any over-the-counter herbs or supplements.

Drugs that may interact with acyclovir include: