What is Aciclovir?

Aciclovir is an antiviral medicine used to treat infections caused by the herpes simplex virus (HSV), including genital herpes and cold sores. It is available as a generic prescription medicine and is just as effective as branded products such as Zovirax. Aciclovir does not cure herpes, but it helps to reduce symptoms, shorten the duration of outbreaks and lower the risk of future flare-ups.

What is Aciclovir used for?

Aciclovir tablets treat and help prevent infections caused by HSV. In men, it is commonly prescribed for genital herpes, which causes painful blisters or sores around the genitals or anus. It can also be used to treat cold sores around the mouth and to reduce the number of future outbreaks when taken regularly.

How does Aciclovir work?

After swallowing the tablet, aciclovir is converted inside infected cells into its active form, aciclovir triphosphate. This blocks an enzyme called viral DNA polymerase, which the herpes virus needs to make new genetic material. By stopping viral DNA replication, the medicine prevents the virus from multiplying so that the immune system can clear the infection more effectively. Starting treatment early can shorten an outbreak and help sores heal more quickly, usually within about five to ten days.

When is treatment needed?

Treatment is recommended when genital herpes symptoms appear or if outbreaks recur frequently. Early signs include tingling, itching or burning around the genitals before blisters appear. Antiviral tablets can reduce discomfort, speed healing and lower the chance of passing the virus to a partner. People with weakened immune systems may need longer or preventive courses, which can be obtained from their GP. 

Transmission advice

The risk of passing the virus on is highest while blisters or sores are present, but herpes can occasionally spread even when there are no visible symptoms. Using condoms and avoiding sexual contact during an outbreak helps reduce transmission.

How to take Aciclovir

Take the tablets exactly as prescribed with a full glass of water. Swallow whole and try to space doses evenly through the day. Complete the full course even if symptoms improve sooner.

Hydration guidance

Drink plenty of fluids while taking aciclovir. Good hydration supports kidney function and helps the body process the medicine safely, reducing the risk of kidney-related side effects. If you have kidney disease, your doctor may adjust the dose.

Availability

In the UK, aciclovir tablets are prescription-only. They can be supplied through an online pharmacy such as 123Meds after an appropriate consultation with a clinician or pharmacist.

Alternative medicines

Other antiviral options include valaciclovir (Valtrex) and famciclovir. These medicines work in a similar way and may be prescribed when convenient dosing or specific clinical factors make them suitable. All require a prescription.

Possible side effects

Common side effects include headache, nausea or diarrhoea. Very rarely, kidney problems or neurological effects can occur, especially if you become dehydrated or have pre-exsisting kidney disease. Please disclose all medical conditions to your 123Meds prescriber to ensure that appropriate treatment is provided and seek urgent medical advice from your GP or NHS111 if you notice unusual symptoms.

Start treatment quickly

Begin taking the tablets as soon as you notice the first signs of a cold sore, such as tingling, itching or a burning sensation around the lips. Early treatment gives the best chance of shortening the outbreak.

Typical adult dose

For an active cold sore, a common prescription is 400 mg three times a day for five days (roughly every eight hours). Swallow each tablet whole with a full glass of water.

Stay well hydrated

Drink plenty of fluids while on aciclovir to help protect your kidneys and ensure the medicine is processed safely.

Reduce the risk of spreading the virus

Cold sores remain contagious from the first tingle until they are completely healed. Avoid kissing, sharing utensils or towels, and close contact with babies, pregnant women, or anyone with a weakened immune system.

Follow your prescription

Take the tablets exactly as directed and complete the full course, even if the cold sore starts to improve sooner. Do not change the dose or frequency without medical advice.

Active ingredient

Each tablet contains aciclovir, the antiviral medicine that treats infections caused by the herpes virus.

Inactive ingredients

The tablets also contain other substances such as colloidal anhydrous silica, magnesium stearate, polyvidone and sodium starch glycollate. These inactive ingredients help bind the tablet and ensure it dissolves correctly.

Exact ingredients can vary slightly between manufacturers, so always check the packaging or patient information leaflet if you have allergies or intolerances.

Usage reminder

Take this medicine exactly as prescribed by your 123Meds prescriber to get the best results.

Side Effects of Aciclovir Tablets

Aciclovir is generally well tolerated, but rarely serious side effects can occur.

Seek urgent medical advice from your GP/NHS111/A&E immediately if you notice any of the following serious effects:

• Signs of kidney problems, such as very little or painful urination, swelling of the feet or ankles, extreme tiredness, or shortness of breath
• Unusual bleeding or bruising or tiny purple or red spots under the skin
• Confusion, hallucinations, or sudden changes in behaviour
• Symptoms of a serious allergic reaction such as hives, difficulty breathing, or swelling of the face, lips, tongue or throat.
  
  

Common side effects

Some people may experience milder effects, including:

• Nausea, vomiting or diarrhoea
• Mild Headache
  
  

These effects are usually temporary and often improve as your body adjusts to the medicine.