What are the side effects of aciclovir?

DoktorABC editorial team

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

Are aciclovir tablets generally well tolerated?

Yes. Aciclovir has been prescribed in the UK for decades, and its safety profile is well documented. According to the current Summary of Product Characteristics (SmPC) for aciclovir 400 mg tablets, the majority of reported side effects are mild and resolve on their own without treatment needing to be stopped.

The SmPC notes side effects can vary in how often they occur depending on the dose used and treatment duration, so a short course taken for a single outbreak will not necessarily carry quite the same profile as longer-term suppressive therapy. This article looks at the side effect profile of aciclovir tablets as a whole, rather than focusing on one particular condition, since the tablet and its effects are the same whatever it has been prescribed for.

What are the common side effects of aciclovir?

The following are classified as common side effects in the SmPC, meaning they affect between 1 and 10 in every 100 people who take aciclovir:

• Headache and dizziness
• Nausea, vomiting, diarrhoea, and abdominal pain
• Itching and rashes, including increased sensitivity to sunlight
• Fatigue and fever

These effects tend to be mild and often settle as the body adjusts to the medicine. Taking tablets with food can help with nausea, and staying well hydrated supports the body in processing aciclovir comfortably.

What are the less common but important side effects of aciclovir?

Some side effects occur less often but are worth being aware of, particularly for anyone with existing kidney problems or anyone who may not be drinking enough fluids during treatment.

Skin reactions: uncommon effects include hives (urticaria) and a more noticeable, diffuse pattern of hair loss. The SmPC is careful to note that the link between this hair loss and aciclovir itself is uncertain, since the same pattern has been associated with a wide range of other medicines and health conditions. Rarely, swelling beneath the skin (angioedema) has also been reported.

Kidney function: aciclovir leaves the body mainly through the kidneys, so anyone with existing renal impairment, and elderly patients whose kidney function may naturally be somewhat reduced, can be more sensitive to its effects. The SmPC lists rare increases in blood urea and creatinine (markers of kidney function), and very rarely, acute kidney failure and renal pain. Where these occur, they are generally reversible once treatment is reviewed, and they are more likely in people taking higher oral doses without adequate fluid intake.

Neurological effects: very rarely, effects such as confusion, agitation, tremor, or unusual drowsiness have been reported. Again, these are seen more often in people with reduced kidney function or other predisposing factors, and they are generally reversible once treatment stops.

When should you seek urgent medical help?

Contact a prescriber, pharmacist, or NHS 111 promptly, or seek emergency care, if you notice:

• Signs of a serious allergic reaction, such as swelling of the face, lips, mouth or throat, difficulty breathing, or a widespread rash
• Signs of a severe skin reaction
• A marked reduction in how much urine you are passing, which can be a sign of kidney problems
• Confusion, unusual drowsiness, agitation, or hallucinations, particularly alongside existing kidney problems
• Yellowing of the skin or eyes
• Any symptom that feels severe, is getting worse, or is not improving as expected

This list reflects the current PIL and SmPC but is not exhaustive, so always read the patient information leaflet that comes with your specific medicine and speak to a prescriber or pharmacist about anything that concerns you.

Practical tips for taking aciclovir

Stay well hydrated

The SmPC specifically recommends maintaining adequate hydration, particularly for anyone taking higher oral doses, since this supports the kidneys in clearing aciclovir from the body and may help reduce the risk of kidney-related side effects.

Complete the course as prescribed

It can be tempting to stop once symptoms start to improve, but finishing the course exactly as your prescriber has directed gives the medicine the best chance of working properly. This applies to short episodic courses; longer suppressive regimens are managed differently and your prescriber will advise on this separately.

Keep to a consistent routine

Taking aciclovir at similar times each day, and with food if nausea is a problem, can help minimise gastrointestinal side effects.

Tell your prescriber about other medicines

This includes anything bought over the counter or taken as a herbal remedy, as a small number of medicines can affect how aciclovir is processed by the body.

Does it matter whether aciclovir is used for a single outbreak or long-term suppression?

Aciclovir may be prescribed as a short course to treat a single outbreak, or as ongoing suppressive therapy to reduce how often outbreaks happen. The overall pattern of side effects is broadly similar between the two, though the SmPC notes that how often adverse events occur can vary depending on the reason for treatment and how long it continues. This article does not go into dosing detail for either approach, since that depends on individual circumstances. For guidance specific to your situation, see our Aciclovir 400 mg (Genital Herpes) and Aciclovir 400 mg (Cold Sores) pages.

Final thoughts

Aciclovir has a long history of safe use, and most people taking it experience only mild, manageable side effects, if they notice any at all. Knowing the difference between the common, expected effects and the rarer signs that need prompt attention can make treatment feel far more straightforward. Always read the patient information leaflet supplied with your specific product, as formulations can vary slightly between manufacturers, and speak to a prescriber or pharmacist if anything during treatment concerns you.

DoktorABC connects UK patients with licensed prescribers and GPhC-registered pharmacies who can advise on aciclovir treatment and answer any questions about side effects.

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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