Aciclovir (or acyclovir) is an antiviral medicine.

It treats infections caused by the herpes virus (herpes simplex), including:

• cold sores
• genital herpes

Your doctor may prescribe aciclovir to prevent you getting these infections if you have had them before or you have a weak immune system.

It's also used to treat chickenpox and shingles.

Aciclovir is available on prescription. It comes as tablets, a liquid that you drink and a cream.

It's sometimes given by injection, but this is usually only done in hospital.

You can buy aciclovir cold sore cream at most pharmacies and supermarkets without a prescription.

• Start taking aciclovir as soon as you get the first signs of infection.
• For most infections, you should start to feel better after taking aciclovir for a few days.
• Common side effects include headaches, dizziness, and feeling or being sick.
• Wash your hands before and after using the cream.
• Brand names include Zovirax, Cymex Ultra and Virasorb.

Aciclovir can be taken by most adults and children.

Aciclovir is not suitable for some people.

To make sure aciclovir is safe for you, tell your doctor if you:

• have had an allergic reaction to aciclovir or any other medicine in the past
• have kidney problems
• are over 65 years old
• are pregnant, trying to get pregnant or breastfeeding

If your immune system is weakened (for example, if you have HIV or AIDS, or you have had a bone marrow transplant), talk to your doctor about the best type of aciclovir for you.

They may recommend tablets rather than the cream.

It's important to start taking (or using) this medicine as soon as you get the first signs of infection.

A cold sore usually starts with a tingling, itching or burning feeling.

Aciclovir tablets and liquid

Doses will vary, depending on why you're taking aciclovir. Your doctor will tell you how much to take and how often.

A single dose is generally between 200mg and 800mg, and may be lower for children.

You'll usually take aciclovir 2 to 5 times a day. Try to space the doses evenly throughout the day.

If you take aciclovir:

• 4 times a day – you could take it first thing in the morning, at midday, in the late afternoon and at bedtime
• 5 times a day – for example, you could take it at 7am, 11am, 3pm, 7pm and 11pm

You can take aciclovir with or without food. Drink plenty of water while taking this medicine to help keep your kidneys working well.

Keep taking the medicine until it's all finished or until your doctor or pharmacist tells you to stop taking it.

For treating a viral infection, you'll usually take aciclovir for 5 to 10 days. For prevention, you may need to take it for a long time.

Tablets: swallow the tablets whole with some water. If you find tablets difficult to swallow, you can dissolve them in water. Add a tablet to a small glass of water and stir. Drink all the liquid to make sure you get the full dose.

Liquid: use the measuring spoon or plastic syringe that came with your medicine. If you do not have a measuring spoon or syringe, ask your pharmacist for one. Do not use a kitchen teaspoon as you will not get the right amount of medicine.

Cream for cold sores

Wash your hands before and after using the cream.

Put a thin layer of cream on the cold sore 5 times a day. Do this every 4 hours – for example, at 7am, 11am, 3pm, 7pm and 11pm.

Do not put aciclovir cream in your mouth, eyes or vagina.

Use the cream for at least 4 days. If the cold sore has not healed by then, you can carry on using the cream for another 6 days.

If the sore still has not healed after a total of 10 days, stop using the cream and tell your doctor.

Cream for genital herpes

Wash your hands before and after using the cream.

Put a thin layer of cream on the affected area 5 times a day. Do this every 4 hours – for example, at 7am, 11am, 3pm, 7pm and 11pm.

Use the cream for at least 5 days. If the genital herpes sore has not healed by then, you can carry on using the cream for another 5 days.

If the affected area has still not healed after a total of 10 days, stop using the cream and tell your doctor.

What if I forget a dose of aciclovir?

If you forget a dose of aciclovir, take it (or use the cream) as soon as you remember, unless it's nearly time for your next dose. In this case, just skip the missed dose and continue with your next one as normal.

Never have 2 doses at the same time. Never have an extra dose to make up for a forgotten one.

If you forget doses often, it may help to set an alarm to remind you.

You could also ask your pharmacist for advice on other ways to remember your medicines.

What if I take or use too much?

Taking too much aciclovir by accident is unlikely to harm you, unless you take too much over several days.

Talk to your doctor or pharmacist if you're worried that you have had too much aciclovir.

Like all medicines, aciclovir can cause side effects in some people, although not everyone gets them.

Many people have no side effects at all, or only minor ones.

Common side effects (tablets and liquid)

These common side effects happen in more than 1 in 10 people who take aciclovir tablets or liquid.

Keep taking the medicine, but talk to your doctor or pharmacist, if these side effects bother you or do not go away:

• headaches
• feeling dizzy
• feeling or being sick (nausea or vomiting)
• diarrhoea
• skin being sensitive to sunlight

Common side effects (cream)

The cold sore cream is generally very safe and less than 1 in 100 people experience any side effects.

Keep using the medicine, but talk to your doctor or pharmacist, if these uncommon side effects bother you or do not go away:

• burning or stinging for a short time after applying the cream
• itchy, dry or flaky skin

Serious allergic reaction

In rare cases, aciclovir can cause a serious allergic reaction (anaphylaxis).

These are not all the side effects of aciclovir.

For a full list, see the leaflet inside your medicines packet.

What to do about:

• headaches – make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Paracetamol is safe to take with aciclovir if you need a painkiller. Speak to your doctor if this does not help or your headaches are severe.
• feeling dizzy – if aciclovir makes you feel dizzy, stop what you're doing and sit or lie down until you feel better. Do not drive, ride a bike, or use tools or machinery if you feel dizzy.
• feeling or being sick (nausea or vomiting) – stick to simple meals and do not eat rich or spicy food. It might help to take your medicine after you have eaten. If you're being sick, try small, frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
• diarrhoea – drink lots of fluids, such as water or squash, to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
• skin being sensitive to sunlight – stay out of bright sun and use a high factor sun cream (SPF 15 or above) even on cloudy days. Do not use a sun lamp or sun beds.
• burning or stinging feeling after applying the cream – this usually goes away after a short time. Ask a pharmacist or your doctor for advice if it continues to be a problem.
• itchy, dry or flaking skin – try using an unscented moisturiser. Do not apply the moisturiser at the same time as your aciclovir cream.

It's usually safe to use the cream or take aciclovir during pregnancy.

Speak to your doctor, who'll be able to explain the benefits and the risks of taking aciclovir when pregnant.

They can help you decide on the best treatment for you and your baby.

For more information about how aciclovir can affect you and your baby during pregnancy, read this leaflet on the best use of medicines in pregnancy (bumps) website.

Aciclovir and breastfeeding

It's usually safe to breastfeed while taking aciclovir.

If you're taking the tablets or liquid, some of the medicine passes into your breast milk.

This is in small amounts and is unlikely to harm your baby.

Talk to your doctor if you want to breastfeed while taking aciclovir. They can advise you on what's best for you and your baby.

Some medicines can affect the way aciclovir tablets or liquid work. They can also make you more likely to get side effects.

If you're using aciclovir cream on your skin, this is less likely to react with other medicines.

It's important to tell your doctor if you take any of the following medicines before taking aciclovir:

• cimetidine, a medicine for stomach ulcers
• mycophenolate mofetil, a medicine given after organ transplants
• probenecid, a medicine for gout
• aminophylline or theophylline, medicines for asthma

Mixing aciclovir with herbal remedies and supplements

There's very little information about taking herbal remedies and supplements while taking or using aciclovir.