Aciclovir and antivirals

Antiviral medicines for herpes and shingles

Aciclovir revolutionised the treatment of herpes simplex (HSV) and varicella-zoster (VZV) viruses when it was introduced in the 1980s. Valaciclovir and famciclovir are prodrugs, meaning the body converts them into their active form after absorption, which allows less frequent dosing and better absorption than aciclovir itself. None of these medicines cure the latent virus, but they shorten outbreaks, reduce their severity, and lower the risk of passing on the virus when used suppressively.

How antivirals work

These medicines block the enzyme the virus uses to copy its DNA, which prevents the virus replicating during an active infection. They are not effective against the latent virus that hides in nerve tissue between outbreaks, which is why recurrences still happen once treatment stops.

Aciclovir cream for local treatment

Aciclovir cream is used mainly for cold sores and works best when applied as soon as the tingling sensation of an oncoming cold sore begins. It is applied several times a day for about five days, and gives only a modest benefit if started late. This cream is not suitable for genital herpes, which needs oral tablets instead.

Oral aciclovir tablets

Oral aciclovir tablets are used for a first episode or recurrence of genital herpes, for severe cold sores, for chickenpox in patients at higher risk, and as an emergency treatment for eczema herpeticum. For an outbreak, tablets are usually taken several times a day for about five days, while a lower dose taken less often is used for longer-term suppression of frequent recurrences. Because aciclovir needs to be taken often, sticking to the regimen can be a challenge, which is one reason valaciclovir is often preferred.

Valaciclovir (Valtrex)

Valaciclovir is converted into aciclovir once absorbed, and different regimens are used depending on the condition being treated: a short twice-daily course for a genital herpes outbreak, a longer twice-daily course for a first episode, a low once-daily dose for suppression, a three-times-daily course for shingles, and a very short high-dose course for an oral cold sore. Its better bioavailability means fewer doses are needed overall, which has made it the NHS first-line choice for many of these indications.

Famciclovir (Famvir)

Famciclovir is an alternative prodrug with a similar spectrum of use to valaciclovir, including three-times-daily courses for shingles and outbreak or suppression regimens for genital herpes. The choice between famciclovir and valaciclovir often comes down to cost and individual GP preference rather than any major difference in effectiveness.

Shingles treatment is time-critical

Antiviral treatment for shingles works best when started within about three days of the rash appearing, typically as a three-times-daily course of valaciclovir for about a week. Starting promptly reduces the duration of post-herpetic neuralgia, the nerve pain that can persist after shingles — see the shingles guide for more detail. People who are immunocompromised may need aciclovir given directly into a vein in hospital rather than as tablets.

Suppressive therapy for genital herpes

Daily low-dose valaciclovir can be used for people who have frequent recurrences of genital herpes, typically six or more a year. This approach reduces the number of outbreaks considerably and also reduces asymptomatic shedding of the virus, which helps protect a partner from infection. See the genital herpes guide for more information.

Special situations for antiviral treatment

A primary genital herpes outbreak in pregnancy, or an outbreak near labour, is treated with aciclovir or valaciclovir under a specialist protocol to help prevent neonatal herpes. People with kidney disease usually need a reduced dose, since a build-up of the drug risks affecting the nervous system, and the GP adjusts this according to kidney function. Drinking plenty of water is also advised, since crystals forming in the kidneys is a rare but recognised historical concern with these medicines.

Side effects of antivirals

The most common side effects are headache and nausea. Rarer side effects include rash, a rise in liver enzymes, and confusion, particularly in elderly or dehydrated patients. You should stop the medicine and seek help if you develop signs of an allergic reaction or notice a reduced urine output.

Interactions with other medicines

Interactions are generally uncommon, though probenecid can increase aciclovir levels in the body, and caution is needed when combining these antivirals with other drugs that can affect the kidneys.

What antivirals cannot do

Antivirals cannot eliminate the latent virus from the body, and they do not fully replace condoms for preventing sexually transmitted infections. They also cannot treat bacterial STIs such as chlamydia, which need antibiotics instead. Starting tablets early in an outbreak works better than starting them late, which is why some people who have recurrent genital herpes or cold sores keep a stock prescription at home to start treatment themselves as soon as symptoms begin, if their GP agrees to this approach.

Common questions about aciclovir and antivirals

What is the difference between aciclovir and valaciclovir?

Aciclovir (Zovirax) is taken multiple times a day, while valaciclovir (Valtrex) is a prodrug that the body converts into aciclovir, giving better absorption from a lower number of daily doses. Famciclovir (Famvir) is used for similar conditions but is taken slightly less often than aciclovir for outbreaks.

How quickly should I start antivirals for a cold sore?

Cream should be applied as soon as you feel the first tingling, several times a day for about five days. Tablets are used if the cold sore is severe or frequent, and should be started within a couple of days of it appearing, since starting earlier shortens the outbreak more effectively.

What dose of valaciclovir is used for genital herpes?

For an outbreak, valaciclovir is usually taken twice a day for about five days, or for longer at a higher dose for a first episode. For suppression of frequent recurrences, a lower dose is taken once a day, and the dose may need adjusting if you have kidney problems, which your GP will calculate.

What are the side effects of aciclovir?

Aciclovir is generally well tolerated, with headache, nausea and diarrhoea being the most common side effects. Kidney problems are rare but more likely if you become dehydrated or take a high dose with existing kidney failure, so it helps to stay well hydrated. Allergic reactions are rare, and there is no significant interaction with alcohol in moderation.

Can I take aciclovir in pregnancy?

Yes, aciclovir is used in pregnancy for a primary genital herpes outbreak and near delivery to help prevent neonatal herpes, guided by the obstetric team. Safety data for this use is reassuring, and genital herpes should not be left untreated in pregnancy.

Sources

• NHS — Aciclovir
• NHS — Valaciclovir