Acyclovir is an antiviral medication used to treat infections caused by herpes viruses, including genital herpes, shingles, and chickenpox. It doesn’t cure these infections, but it shortens outbreaks, reduces symptom severity, and can be taken long-term to suppress recurring flare-ups.
How Acyclovir Works
Acyclovir is unusually selective for an antiviral drug. It essentially stays inactive until it encounters a cell that’s already been infected by a herpes virus. Infected cells contain a virus-produced enzyme that “switches on” acyclovir by converting it into its active form. Normal, healthy cells barely activate the drug at all, which is why side effects tend to be mild compared to many other antivirals.
Once activated inside an infected cell, acyclovir interferes with the machinery the virus uses to copy its DNA. It gets incorporated into the growing viral DNA strand, then stops the chain dead because it’s missing a critical chemical piece that would let the next building block attach. The result: the virus can’t replicate, and the infection stalls. This is why timing matters so much. Acyclovir works best when viral replication is at its peak, which is during the earliest phase of an outbreak.
Genital Herpes
Genital herpes, caused by herpes simplex virus type 1 or type 2, is the most common reason acyclovir is prescribed. It’s used in three distinct ways depending on the situation.
For a first outbreak, the standard course is 400 mg taken three times daily for 7 to 10 days. First episodes tend to be the most painful and prolonged, and starting acyclovir early can significantly reduce healing time and symptom severity. For people who experience recurring outbreaks, there are two approaches. Episodic therapy means taking a short course (800 mg twice daily for 5 days, or 800 mg three times daily for just 2 days) at the first sign of a flare-up, such as tingling or burning. Suppressive therapy means taking 400 mg twice daily on an ongoing basis to reduce the frequency of outbreaks. Many people on suppressive therapy see outbreaks drop to near zero.
Suppressive therapy also plays a role during pregnancy. Starting at 36 weeks of gestation, a daily regimen of 400 mg three times a day can lower the risk of an active outbreak at the time of delivery, which is important because newborns are highly vulnerable to herpes infection.
Shingles
Shingles (herpes zoster) is a reactivation of the chickenpox virus that remained dormant in nerve cells, sometimes for decades. It typically causes a painful, blistering rash on one side of the body, and the pain can linger for weeks or months after the rash heals.
Acyclovir is one of the standard treatments. The critical factor is speed: treatment started within 48 hours of rash onset is significantly more effective at promoting rash healing and reducing pain duration than treatment started later. If you suspect shingles, getting evaluated quickly gives acyclovir the best chance of working. Treatment typically runs for 7 days at higher doses than those used for genital herpes, since the varicella-zoster virus requires more drug exposure to suppress.
Chickenpox
Chickenpox (varicella) is usually mild in healthy children, but acyclovir can be helpful in certain situations. It’s most commonly considered for teenagers and adults, who tend to have more severe cases, and for children with weakened immune systems who are at risk for complications like pneumonia or widespread skin involvement.
For children who do need treatment, efficacy depends heavily on early intervention. Starting within 24 hours of rash onset maximizes the benefit. In mild cases, oral acyclovir is given four times a day for 7 to 10 days, continuing until no new blisters have appeared for at least 48 hours. Children with severe disease, such as those with high fever, respiratory symptoms, deep or hemorrhagic skin lesions, or internal organ involvement, typically receive acyclovir intravenously in a hospital setting.
Cold Sores and Other Herpes Infections
Though not always listed as a primary labeled use for the oral capsule form, acyclovir is active against herpes simplex virus type 1, the usual cause of cold sores. Topical acyclovir cream is available specifically for cold sores, and oral acyclovir is sometimes prescribed for people who get frequent or severe outbreaks around the mouth. The same principle applies: earlier treatment produces better results.
In hospital settings, intravenous acyclovir is also used for serious herpes-related conditions, including herpes encephalitis (a rare but dangerous brain infection) and neonatal herpes. These situations are medical emergencies where the drug is given at higher doses under close monitoring.
Side Effects
Oral acyclovir is generally well tolerated. The most common complaints are nausea, vomiting, and stomach discomfort. Some people experience headache or fatigue. These tend to be mild and often improve as your body adjusts to the medication.
Rare but more serious side effects include confusion, hallucinations, trembling, and seizures. These neurological symptoms are more likely in older adults and in people with kidney problems, since acyclovir is cleared from the body primarily through the kidneys. If the kidneys can’t eliminate the drug efficiently, it builds up to higher levels in the blood.
Kidney-related side effects are the most important safety concern, particularly with the intravenous form. Acyclovir can crystallize in the kidneys if you’re not well hydrated, potentially causing damage. This is why staying well hydrated during treatment is consistently recommended. Drinking plenty of fluids helps your kidneys flush the drug through before it can cause problems. People with existing kidney disease may need adjusted doses.
What Acyclovir Does Not Do
Acyclovir does not eliminate herpes viruses from your body. These viruses establish lifelong latent infections in nerve cells, where they hide from both the immune system and antiviral drugs. Acyclovir only works on actively replicating virus, so once you stop taking it, outbreaks can return. It also does not prevent transmission of herpes to sexual partners on its own, though suppressive therapy does reduce the risk. It has no meaningful activity against non-herpes viruses like those that cause the common cold or flu.