Aciclovir, also spelled acyclovir, is an antiviral medicine. Most people who take aciclovir tablets do not experience serious side effects, although side effects can occur . When side effects do occur, they can include nausea, headache, dizziness, vomiting, diarrhoea and stomach pain.
Aciclovir helps treat herpes virus infections and can improve symptoms, but it does not eliminate the virus from the body . If you are an adult in the UK taking or considering aciclovir, it can help to know which side effects are more common and usually manageable, and which symptoms may need prompt medical attention.
This guide explains the common and less common side effects of aciclovir tablets, rare but serious reactions, factors that may increase the risk of problems, ways to manage mild side effects, when to seek medical advice, and how to report a suspected side effect.
Key things to know
- Many people tolerate aciclovir well when it is used as prescribed , although side effects can occur
- Commonly reported side effects include nausea, headache, dizziness, vomiting, diarrhoea and stomach pain
- Rare but serious effects can include allergic reactions, kidney problems and changes affecting blood cells or the liver
- Taking aciclovir as prescribed and maintaining adequate hydration may help reduce the risk of dehydration-related problems during treatment
Common Side Effects
- Nausea
- Headache
- Dizziness
- Vomiting
- Diarrhoea
- Stomach pain
These are among the more commonly reported side effects of aciclovir tablets. If you experience one of these side effects, it does not necessarily mean that anything serious is wrong . Mild symptoms may settle while you are taking the medicine.
If a side effect is severe, persistent or worrying you, speak with your doctor or pharmacist. They can advise whether you should continue treatment or whether anything needs to be changed.
For more information about headaches associated with aciclovir, see our dedicated guide linked below . Other possible side effects are covered further down the page.
Does the Dose Affect the Likelihood of Side Effects?
Aciclovir doses vary depending on the infection being treated. It may be prescribed for viral infections such as cold sores, genital herpes, shingles and chickenpox , but the recommended dose and treatment length can differ between conditions.
Some side effects may be more likely with higher doses. The risk of certain problems can also be higher in people with kidney problems, particularly if the dose is not adjusted appropriately. This is why your prescriber chooses a dose based on your condition, medical history and other factors.
Taking more aciclovir than prescribed is unlikely to improve effectiveness and may increase the risk of side effects and may increase the risk of side effects. Follow the dose and treatment length provided by your prescriber rather than changing the amount yourself.
It is also important to follow the instructions you have been given about how long to take aciclovir. If you are feeling better, do not take extra doses to try to prevent symptoms from returning, and do not change or stop your treatment without checking with your doctor or pharmacist first.
Less Common Side Effects
- Fatigue or feeling generally unwell
- Skin rash
- Sensitivity to light
- Joint pain
These side effects are reported less often than the common effects listed above, but they can still occur while taking aciclovir. A mild rash does not necessarily indicate an allergic reaction , although it is worth mentioning to your doctor or pharmacist.
If any of these symptoms are troublesome, do not improve, or become worse, speak with your pharmacist or doctor. If you notice increased sensitivity to light, simple measures such as wearing sunglasses and avoiding prolonged exposure to bright sunlight may make you more comfortable.
Serious Side Effects: What to Watch For
Serious side effects with aciclovir are uncommon, but it is useful to know the warning signs. This can help you recognise when a symptom needs medical attention rather than assuming it is a routine side effect.
Signs of an Allergic Reaction
A rash, itching, or swelling can sometimes be a sign of an allergic reaction. Swelling of the face, lips, tongue or throat, or difficulty breathing, needs urgent medical attention.
If you develop these symptoms or think you may be having a severe allergic reaction, call 999 or seek emergency medical attention immediately .
Signs of Kidney Problems
Aciclovir is mainly cleared from the body through the kidneys, so kidney problems are an important consideration, particularly in people who already have reduced kidney function.
Signs that may need medical attention include:
- Passing significantly less urine than usual or being unable to pass urine
- Swelling, particularly around the legs, ankles or eyes
- Feeling unusually unwell or weak
Staying well hydrated is generally recommended during treatment with aciclovir , unless a healthcare professional has advised you to limit your fluid intake. If you notice a significant change in how much you are urinating or develop swelling, contact your doctor promptly.
Signs of Blood or Liver Effects
Rarely, aciclovir can be associated with changes affecting blood cells or the liver.
Unusual bruising or bleeding may need medical assessment, while yellowing of the skin or whites of the eyes may be a sign of jaundice and should be checked promptly.
Very rare blood disorders, including thrombotic thrombocytopenic purpura (TTP), have been reported , particularly in people with weakened immune systems. If you develop unusual bleeding, severe bruising or other concerning symptoms, seek medical advice.
Signs of Neurological Effects
Confusion, agitation, hallucinations, tremors or unusual drowsiness are rare but recognised effects of aciclovir. They are more likely to occur with higher doses, particularly in people with reduced kidney function, because the medicine may build up in the body.
If you or someone you are caring for develops confusion, unusual behaviour, hallucinations or severe drowsiness while taking aciclovir, seek medical advice promptly.
Who May Be at Higher Risk of Side Effects
- People with existing kidney impairment or kidney disease
- Older adults
- People who are dehydrated at the start of treatment
- People taking other medicines that can affect kidney function
These factors do not automatically mean that you cannot take aciclovir. However, your prescriber may need to consider them when deciding whether aciclovir is suitable and what dose is appropriate. In some cases, kidney function may also need to be monitored.
People with a weakened immune system may also be prescribed aciclovir to treat or prevent certain herpes virus infections. If this applies to you, make sure your prescriber knows about your medical history and all the medicines you take.
Before starting aciclovir, tell your doctor or pharmacist about any kidney problems, other medical conditions and medicines you are taking. This gives them the information they need to assess whether the treatment is suitable and how it should be used.
Reporting a Side Effect
If you experience a side effect while taking aciclovir, particularly one that is unusual or not listed in the Patient Information Leaflet , you can report it to the MHRA through the Yellow Card scheme. You should also speak with your prescriber or pharmacist about any symptoms that concern you.
You do not need to be certain that aciclovir caused the side effect before making a report. Reporting suspected side effects helps the MHRA monitor the safety of medicines and identify possible patterns over time.
If you are unsure whether a symptom should be reported, your pharmacist or doctor can help you decide what to do.
Managing Side Effects Effectively
- Take your tablets as directed. If they cause stomach discomfort, taking them with food may help some people, although aciclovir tablets can generally be taken with or without food
- Drink enough fluids during treatment to stay hydrated, unless your doctor has advised you to restrict your fluid intake , unless your doctor has advised you to restrict your fluid intake
- Get some rest if you develop a headache, dizziness or fatigue
- Speak with your pharmacist before taking a painkiller or another medicine for your symptoms, particularly if you take other medicines or have an existing health condition
Mild side effects may improve as treatment continues. However, there is no need to simply put up with symptoms that are persistent, getting worse or affecting your daily life. If this happens, contact your pharmacist or prescriber for advice.
No clinically significant interaction between alcohol and aciclovir is known . However, staying well hydrated is still important while you are taking the medicine. If you have kidney problems or have been advised to limit your fluid intake, follow the advice given by your healthcare professional.
For more detailed information about aciclovir and headaches, see our aciclovir headache guide .
Available Cold Sore Treatments
| Treatment | Price from |
|---|---|
| Aciclovir tablets | £12.99 |
| Aciclovir 5% cream | £8.99 |
| Valaciclovir | £18.99 |
| Valtrex | £34.99 |
Prices update live and may change.
Medical Disclaimer
This page provides general information about aciclovir tablet side effects. It is not a substitute for personalised medical advice, diagnosis, or treatment.
If you are pregnant, planning a pregnancy, or breastfeeding, speak with your doctor before taking aciclovir . Aciclovir may be prescribed during pregnancy or breastfeeding where considered clinically appropriate , but your healthcare professional can advise whether it is suitable for you based on your individual circumstances.
The side effects described on this page are grouped into common, less common, and serious effects for general guidance. They are not a complete list of every possible side effect.
If you develop signs of a serious allergic reaction, such as swelling of the face or throat or difficulty breathing, seek emergency medical attention immediately. If you notice reduced urination, unusual bruising or bleeding, yellowing of the skin or eyes, or confusion, seek prompt medical advice.
If you have questions about your specific treatment, side effects, or whether aciclovir is suitable for you, speak with your prescriber, pharmacist, or GP.
