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 of aciclovir tablets and liquid happen in less than 1 in 10 people. There are things you can do to help cope with them:
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.
If you take the combined contraceptive pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
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.
If you take the combined contraceptive pill or progestogen-only pill and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Skin becomes sensitive to sunlight
Stay out of bright sun and use a high factor sun cream (SPF 30 or above) even on cloudy days. Do not use a sun lamp or sun beds.
Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Side effects (genital herpes cream and cold sore cream)
These uncommon side effects of the genital herpes cream and cold sore cream happen in less than 1 in 100 people:
Burning or stinging for a short time 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 flaky skin
Try using an unscented moisturiser. Do not apply the moisturiser at the same time as your aciclovir cream.
Keep using the medicine, but speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Common side effects (eye ointment)
Side effects of the eye ointment are usually mild. These common side effects happen in more than 1 in 10 people:
Burning or irritation in your eye, or feeling as if there's something in your eye
This usually goes away after a short time. Ask a pharmacist or your doctor for advice if it continues to be a problem.
Eyes being more sensititive to light
This usually goes away after a short time. Ask a pharmacist or your doctor for advice if it continues to be a problem.
Serious allergic reaction
In rare cases, aciclovir can cause a serious allergic reaction (anaphylaxis).
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Other side effects
These are not all the side effects of aciclovir. For a full list, see the leaflet inside your medicines packet.
You can report any suspected side effect using the Yellow Card safety scheme.