Drug Notes - Acyclovir (By mouth)

Acyclovir (By mouth)

There may be other brand names for this medicine.

This medicine is not right for everyone. Do not use it if you had an allergic reaction to acyclovir or valacyclovir.

Your doctor will tell you how much medicine to use. Do not use more than directed.
Take this medicine at the first sign of a herpes breakout, or as soon as possible after you are diagnosed with herpes zoster. The medicine may not work if you wait longer than 3 days to start using it.
Oral liquid: Measure the oral liquid medicine with a marked measuring spoon, oral syringe, or medicine cup.
Drink extra fluids so you will urinate more often and help prevent kidney problems.
Missed dose: Take a dose as soon as you remember. If it is almost time for your next dose, wait until then and take a regular dose. Do not take extra medicine to make up for a missed dose.
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light.

Some medicines can affect how acyclovir works. Tell your doctor if you are using probenecid or medicines that weaken the immune system (including steroids, chemotherapy, or radiation).

Tell your doctor if you are pregnant or breastfeeding, or if you have kidney disease, dehydration, immune system problems, nervous system problems, or history of severe skin reactions to acyclovir or valacyclovir.
This medicine may cause the following problems:
- Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome
- Serious skin reactions, including acute generalized exanthematous pustulosis (AGEP), drug reaction with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis, and erythema multiforme
- Kidney problems
Do not have sex while you have herpes sores. Acyclovir will not stop the spread of herpes during sex.
Even if you have no signs of a herpes infection, it is still possible to spread the virus to others during sex. Talk with your doctor about ways to keep from spreading the virus.
Your doctor will do lab tests at regular visits to check on the effects of this medicine. Keep all appointments.
Keep all medicine out of the reach of children. Never share your medicine with anyone.

Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
Blistering, peeling, red skin rash
Change in how much or how often you urinate
Dark urine or pale stools, nausea, loss of appetite, stomach pain, yellow skin or eyes
Fast heartbeat, fainting, extreme weakness
Fever, chills, cough, sore throat, body aches
Pinpoint red spots on your skin, unusual bleeding or bruising, blood in your urine or stools
Problems with walking, speaking, or coordination, seeing or hearing things that are not there

Last Updated: 1/4/2026