Oral Antivirals

Medication Information Leaflet

What is this medication for?

  • This medication is used to treat viral infections. Examples include herpes simplex on the skin and mucous membranes (cold sores, genitals and anus); herpes zoster (shingles); and chickenpox. 
  • The viruses that cause these infections are similar. They are inactive in the body until an outbreak is triggered (e.g., during weakened immunity or stress). 
  • This medication is not a cure for the infection. However, treatment of a viral outbreak can speed up the healing of sores, decrease pain, and lower the risk of complications (e.g., infection spreading to the whole body, brain infection, persistent pain after sore healing).

How should I take/use this medication?

  • Acyclovir: Take this medication with or without food. However, taking this medication with food may decrease stomach upset.
  • Valaciclovir: Take this medication with or after food.
  • Drink plenty of fluids while taking this medication. 
  • When using this medication to suppress or control outbreaks, take it exactly as directed by your doctor. The dosage is based on your medical condition and response to therapy.
  • This medication works best when the amount of drug in your body is kept at a constant level. Therefore, take this medication at evenly spaced intervals, at the same time each day.
  • When treating an outbreak, complete the course of medication even if the symptoms have disappeared. 
  • Do not skip doses or stop treatment without your doctor’s approval. Stopping therapy earlier than recommended by your doctor may result in ineffective treatment.

What should I do if I forget to take/use this medication?

If you forget to take a dose, take it as soon as you remember. If it is almost time for the next dose, skip the missed dose. Then take your next dose at the usual time. 

Do not take two doses to make up for the missed dose. 

What precautions should I take?

  • Inform your healthcare professional if you:
    • Have kidney disease.
    • Are allergic to this medication or any of the other ingredients of this medication.
    • Are pregnant, planning to become pregnant, or breastfeeding.
  • If you are taking this medication for treatment of genital herpes, it is important to follow safe sex practices to lower the chance of spreading herpes to your partner, even if no symptoms are present. Always use a condom made of latex or polyurethane. When you have symptoms or are experiencing an outbreak, do not have sexual contact with your partner.

What are some common side-effects of this medication?

Common side effects include: 

  • Headache
    • You may take Paracetamol (if not allergic to Paracetamol) to treat the headache.
  • Nausea or vomiting, stomach upset, diarrhoea
    • Take the medication with or after food.
    • If diarrhoea occurs, drink sufficient water to prevent dehydration.
  • Tiredness or weakness

If any of the side effects persist or are severe, consult your doctor.

What are some rare but serious side-effects that I need to seek medical advice immediately?

If you experience any of these symptoms, you should stop your medication and see your healthcare professional immediately.

  • Allergic reaction
    • Swollen face/eyes/lips/tongue
    • Difficulty in breathing
    • Itchy skin rashes over your whole body
  • Mental or mood changes
    • Agitation
    • Confusion
    • Hallucinations
  • Severe tiredness and weakness, unusual bruising or bleeding, yellowing of eyes or skin, change in vision, difficulty speaking or thinking, change in balance, or fever
  • Unable to urinate, change in volume or darkening of urine or blood in urine
  • Seizures
  • Tremors, difficulty moving around, stiffness

What food or medication should I avoid when I take this medication?

Inform your doctor if you are taking or planning to take other medications, supplements, traditional medications and herbal remedies. 

How should I store this medication?

Store in a cool and dry place, away from direct sunlight. Keep this medication away from children. 

How should I throw away this medication safely?

Pack the medication into a bag and bring it back to the pharmacy where you received the medication from.



If you take more than the recommended dose, please seek medical advice immediately. The information provided on this page does not replace information from your healthcare professional. Please consult your healthcare professional for more information.

This article is jointly developed by members of the National Medication Information workgroup. The workgroup consists of cluster partners (National Healthcare Group, National University Health System and SingHealth), community pharmacies (Guardian, Unity and Watsons) and Pharmaceutical Society of Singapore. The content does not reflect drug availability and supply information in pharmacies and healthcare institutions. You are advised to check with the respective institutions for such information.

Last updated on Oct 2022

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