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Acyclovir for Encephalitis


Generic NameBrand Name

How It Works

Acyclovir prevents herpes simplex virus—the virus that causes chickenpox, cold sores, and genital herpes—from reproducing. The medicine is given in a vein (intravenous, or IV) when used to treat encephalitis caused by these viruses.

Why It Is Used

Acyclovir is used to treat encephalitis caused by herpes simplex and varicella-zoster.

How Well It Works

To improve the chance of survival from herpes simplex encephalitis, acyclovir should be given as soon as the illness is diagnosed. The death rate from this form of encephalitis is less than 30% when prompt treatment is given, compared with 70% to 80% without treatment.1

The main things that affect the success of treatment include the age of the person and his or her level of consciousness. Confusion and disorientation (altered consciousness) are signs of encephalitis. People younger than age 30 and those who have a normal level of consciousness have better results than people older than 30 and those who have altered consciousness.2

People who get treatment with acyclovir immediately (within 4 days of symptoms) do better than people who wait longer for treatment.3 But even with treatment, serious mental and physical impairments can occur, such as paralysis, seizures, or hearing loss.

Side Effects

All medicines have side effects. But many people don't feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.

Here are some important things to think about:

  • Usually the benefits of the medicine are more important than any minor side effects.
  • Side effects may go away after you take the medicine for a while.
  • If side effects still bother you and you wonder if you should keep taking the medicine, call your doctor. He or she may be able to lower your dose or change your medicine. Do not suddenly quit taking your medicine unless your doctor tells you to.

Common side effects of this medicine include:

  • Pain, swelling, or redness where you got the shot.
  • Belly or stomach pain.
  • Nausea or vomiting.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

Other antiviral medicines that may be used to treat encephalitis include valacyclovir, penciclovir, and famciclovir.


Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.

Complete the new medication information form (PDF)Click here to view a form.(What is a PDF document?) to help you understand this medication.



  1. Jubelt B (2010). Viral infections and postviral syndromes. In LP Rowland, TA Pedley, eds., Merritt's Neurology, 12th ed., pp. 156–185. Philadelphia: Lippincott Williams and Wilkins.

  2. Roos KL, Tyler KL (2008). Meningitis, encephalitis, brain abscess, and empyema. In AS Fauci et al., eds, Harrison's Principles of Internal Medicine, 17th ed., vol. 2, pp. 2621–2641. New York: McGraw-Hill.

  3. Ropper AH, Samuels MA (2009). Acute encephalitis section of Viral infections of the nervous system, chronic meningitis, and prion diseases. In Adams and Victor's Principles of Neurology, 9th ed., pp. 716–723. New York: McGraw-Hill.


ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerW. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Last RevisedOctober 26, 2011

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