Acyclovir belongs to the family of medications known as antivirals. Acyclovir is used to treat and prevent genital herpes. It is also used for the acute treatment of herpes zoster (shingles) and varicella (chickenpox). All these infections are caused by viruses. Acyclovir helps healing by stopping the virus from multiplying and spreading to nearby healthy cells.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

Genital herpes: The usual recommended adult dose for treating genital herpes is 200 mg every 4 hours, 5 times daily for 10 days. The usual recommended adult dose for preventing genital herpes infection from coming back (called suppressive therapy) varies but usually starts at 200 mg 3 times daily. You should start acyclovir as early as possible following the onset of signs and symptoms.

Shingles: The recommended dose for treating shingles is 800 mg every 4 hours, 5 times daily for 7 to 10 days. Treatment should be started within 48 hours of onset of the rash or lesions for maximum benefit, however it may be started as late as 72 hours after lesions begin to form.

Chickenpox: The recommended dose for chickenpox is based on body weight. The usual dose is 20 mg per kilogram of body weight (not to exceed 800 mg) 4 times daily for 5 days. Therapy should be started within 24 hours of onset of the rash.

Tablets should be swallowed whole, with water.  If you are using the suspension, the bottle should be shaken well before use. Use an oral syringe to measure each dose of the suspension, as it gives a more accurate measurement than household teaspoons.

Many things can affect the dose of a medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not take acyclovir if you are allergic to acyclovir, valacyclovir, or any of the ingredients of the medication.

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• diarrhea
• dizziness
• general feeling of discomfort or illness
• hair loss
• headache
• increased sensitivity to sun (photosensitivity)
• lack of energy
• nausea
• skin rash
• tingling, pricking, or numbness
• upset stomach
• vomiting
• weakness

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• agitation
• confusion
• fever
• hallucinations (seeing or hearing things that aren't really there)
• lack of coordination
• shortness of breath
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of kidney failure (e.g., side pain between ribs and hip, decreased urine production, swelling, fatigue, abdominal pain)
• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• symptoms of a serious allergic reaction (such as swelling of the face, lips, tongue, or throat; skin rash; hives; or difficulty breathing)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Drowsiness/reduced alertness: Acyclovir may affect the mental or physical abilities needed to drive or operate machinery. Avoid driving, operating machinery, or performing other hazardous tasks until you have determined how this medication affects you.

Genital herpes: To reduce the risk of spreading the virus, wash your hands immediately after touching your skin sores. You should avoid intimate contact when live lesions are visible on your skin. The herpes virus can still be spread even when you do not have blisters or sores.

Kidney function: Kidney disease or reduced kidney function may cause this medication to build up in the body, causing side effects. If you have reduced kidney function or kidney disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

People with reduced kidney function generally require a lower dose of this medication.

Long-term use: The effect of long-term use of this medication has not been established. Your doctor may periodically stop your medication to reassess your need for continuous treatment.

Pregnancy: This medication is not recommended for use during pregnancy unless your doctor has determined that the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: This medication passes into breast milk. If you are a breast-feeding mother and are taking acyclovir, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using this medication have not been established for children less than 2 years of age.

Seniors: Seniors are more likely to have reduced kidney function and therefore are at greater risk of experiencing the side effects of acyclovir. Lower doses may be needed.

There may be an interaction between acyclovir and any of the following:

• cladribine
• clozapine
• foscarnet
• mycophenolate
• tenofovir
• theophylline
• tizanidine
• varicella virus (shingles) vaccine
• zidovudine
• zoster (chicken pox) vaccine

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.