Acyclovir (Zovirax, Sitavig) can treat certain viral infections. Common side effects include diarrhea, headaches, and malaise. Hair loss, anemia, and other effects can occur but are less common.

A person may use acyclovir as a cream, in a suspension, intravenously, or as tablets. The drug treats viral infections, including genital herpes, chickenpox, and shingles.

Below, learn about the safety of acyclovir, alternatives, and when side effects require medical care.

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Acyclovir is an antiviral drug that people typically take in pill form, and doctors sometimes prescribe it in combination with other medications.

It treats certain viral infections, including genital herpes, chickenpox, shingles, and cold sores.

Acyclovir is not a cure for herpes, however — the virus remains in the body for a long time and can cause recurring symptoms. The drug instead reduces the virus’s ability to multiply, and this eases symptoms, including pain from sores or blisters. Acyclovir can also help speed healing.

It is important to note that acyclovir does not prevent the virus from spreading.

Healthcare providers may also use acyclovir to treat common infections in people with HIV.

Learn more about acyclovir here.

Acyclovir has a few common, minor side effects, as well as less common and rare side effects. These can be more serious.

Common

Some common side effects of acyclovir include:

Less common

Some less common side effects of acyclovir include:

  • inflammation at the injection site, if the person receives the drug intravenously
  • a rash
  • transaminitis, which involves having unusually high levels of liver enzymes

Rare

The following side effects of acyclovir are rare:

Minor side effects of acyclovir can be easy to manage with home care techniques.

A doctor or pharmacist can help provide guidance about managing nausea, vomiting, diarrhea, and headaches.

It is important to note, however, that some over-the-counter medications that can reduce these symptoms may interact with acyclovir.

Rarely, this drug causes an allergic reaction. If this happens and seems likely to be severe, seek emergency medical care. Symptoms of a severe allergic reaction include:

  • a rash, such as hives
  • difficulty breathing
  • swelling of the tongue or throat

People taking other medications should speak with a doctor before using acyclovir, as there may be a risk of interactions. For example, intravenous acyclovir can interact with probenecid (Probalan), a drug used in treating gout.

Also, the drug may pose a risk to people who are pregnant, nursing, or have certain health issues. Learn more below.

Kidney conditions

People with kidney problems should speak with a doctor before using acyclovir. The drug can decrease kidney function and worsen any existing kidney disease.

The doctor may recommend a decreased dosage of the medication.

Pregnancy and breastfeeding

Consult a doctor before using acyclovir during pregnancy.

The Food and Drug Administration (FDA) classify acyclovir as a category B drug during pregnancy. This means that animal studies have not shown that it poses a risk to a developing fetus, but there has been too little research in humans to conclude that it is safe.

One study found no association between taking acyclovir during the first trimester of pregnancy and an increase in birth abnormalities.

There is also a risk that taking acyclovir while breastfeeding may pass the drug on to the infant.

Long-term effects

Few studies have investigated the long-term effects of acyclovir.

Research from 1988 suggests that using acyclovir for the long-term treatment of genital herpes successfully prevented recurrences without causing notable side effects.

However, this research is quite old and involved a very small group of participants — 131 people. More research into the long-term effects of acyclovir in humans is necessary.

According to the Centers for Disease Control and Prevention (CDC), antiviral chemotherapy can help patients with severe herpes symptoms.

Meanwhile, other antivirals, such as valacyclovir and famciclovir, can also help manage herpes symptoms.

For people looking into the treatment for cold sores around the mouth, it is worth noting that an over-the-counter or prescription treatment may reduce the healing time, but by about 1 day.

It is important to receive urgent medical attention if symptoms of a serious allergic reaction develop.

Seek medical care if acyclovir causes any of the following:

  • mood or behavior changes
  • unexplained fatigue
  • changes in vision
  • symptoms of kidney problems, such as blood in the urine or pain in the back or side

Acyclovir is an antiviral drug often used as a treatment for the symptoms of herpes.

It is generally safe, but people who have kidney disease or are pregnant or breastfeeding should consult a healthcare provider before taking acyclovir.

This is also important for anyone who is taking other medications or supplements.