What Is Aciclovir? Uses and How It Works

DoktorABC editorial team

Accurate, up-to-date health information written by our editorial team and reviewed by UK-registered prescribers.

What are aciclovir tablets used for?

Aciclovir 400mg tablets and other tablet strengths are prescription-only medicines (POM). Per the current SmPC, they are indicated for:

• Treatment of HSV infections of the skin and mucous membranes, including initial and recurrent genital herpes
• Suppression (prevention of recurrences) of recurrent HSV infections in people with a normally functioning immune system
• Prophylaxis of HSV infections in immunocompromised patients
• Treatment of varicella (chickenpox) and herpes zoster (shingles)

Cold sores are also caused by herpes simplex virus, and a prescriber may consider aciclovir tablets appropriate when cold sore outbreaks are frequent or particularly severe, alongside the licensed indications above. Always check the exact wording of the current SmPC and PIL, since licensed indications can be updated.

How does aciclovir work?

Aciclovir works by targeting cells that are infected with a herpes virus. Once inside an infected cell, it is converted into an active form that blocks the enzyme the virus needs to copy its own DNA. This slows the virus down considerably, giving the immune system a better chance to bring the outbreak under control.

Aciclovir does not remove the herpes virus from the body. Herpes viruses become dormant in nerve cells after the first infection and can reactivate later, which is why outbreaks can recur over time. Aciclovir manages and shortens active outbreaks rather than curing the underlying infection.

How long does aciclovir take to work?

Aciclovir needs to reach a therapeutic level in the body before it can start slowing the virus down. When treatment is started early in an outbreak, ideally at the first signs such as tingling, itching, or the earliest appearance of a lesion, many people notice symptoms beginning to improve within a few days.

There is no single, universal timeframe that applies to everyone, since this depends on the condition being treated, how promptly treatment is started, and individual factors. Starting aciclovir as early as possible in an outbreak generally gives the best chance of a faster response. A prescriber can give more specific guidance based on your individual situation and the condition being treated.

What forms does aciclovir come in?

Aciclovir is available in several forms in the UK:

• Tablets: taken orally, working systemically throughout the body
• Cream: applied topically to the skin
• Other formulations, including intravenous preparations, which are used in hospital settings for more severe infections

This article covers the general picture across these forms. For dosing and purchase-specific detail, see the Aciclovir (Genital Herpes) page, the Aciclovir (Cold Sores) page, and the Aciclovir 5% Cream page.

Aciclovir cream vs tablets: what's the difference?

Aciclovir cream and aciclovir tablets work differently and are not interchangeable.

Topical aciclovir cream is applied directly to the skin to treat a visible outbreak locally. There are two distinct types of aciclovir cream available in the UK, and it is important not to confuse them:

• A prescription-only aciclovir 5% cream is licensed for the treatment of HSV skin infections including genital herpes and herpes labialis
• Certain aciclovir 5% cold sore cream products are licensed specifically for herpes labialis on the lips and face only, and some of these are available without a prescription

Neither type of cream is intended for use inside the mouth, in the eyes, or, in the case of the OTC cold sore cream, on the genital area.

Aciclovir tablets work systemically, throughout the body, rather than only at the site of application. Tablets are used for genital herpes and for cold sores when outbreaks are frequent or severe enough that a prescriber judges systemic treatment to be more appropriate than a topical cream alone.

Who needs a prescriber's input for aciclovir?

The right dose and course length of aciclovir depend on several factors, including which condition is being treated, how often outbreaks occur, and individual factors such as kidney function. This is why a prescriber assesses each case individually rather than working from a single standard course. A prescriber will also confirm whether tablets, cream, or another formulation is most appropriate, and whether any other medicines you take could interact with aciclovir.

Final thoughts

Aciclovir is a well-established antiviral medicine for herpes virus infections, most commonly genital herpes and cold sores, and in some cases chickenpox or shingles. It works by slowing the virus down at a cellular level rather than eliminating it from the body, and it tends to work best when started early in an outbreak. Because dosing and formulation depend on the specific condition and individual factors, a clinical assessment by a registered prescriber is the appropriate next step for anyone considering treatment.

DoktorABC connects UK patients with licensed prescribers who can assess suitability for aciclovir and issue private prescriptions where clinically appropriate.

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The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board