Aciclovir (Zovirax): cold sores, shingles and herpes treatment

Aciclovir is an antiviral, a different kind of medicine from an antibiotic. The NHS lists cold sores, shingles, chickenpox, genital herpes and herpes simplex infections of the eye among its uses, and it can also be given to prevent infection in people with a weakened immune system.

The NHS page does not explain how it works. In practice what matters to a patient is timing, which is why the NHS instructions about starting treatment promptly are so specific.

For skin infections such as shingles or genital herpes, the NHS says to start the tablets as soon as possible after they are prescribed, and to follow your doctor's instructions about how many and how often. Quantity and length depend on the infection, so there is no single number to quote here.

Drink plenty of water while you are taking aciclovir tablets. The NHS stresses this particularly if you have kidney problems.

• Missed dose: take it when you remember unless the next is close; never take two together.

• One extra tablet is unlikely to cause trouble, but ask a doctor or NHS 111 if you have taken more than prescribed and have symptoms such as nausea or vomiting.

For cold sores and genital herpes, the cream goes on as a thin layer 5 times a day with 4 hours between uses, ideally as soon as you feel itching or tingling. The NHS suggests seeing a doctor if the sore has not healed after 9 days for a cold sore or 10 days for genital herpes.

Eye ointment is used as a strip of about 1 cm under the eyeball, 5 times a day, roughly every 4 hours, and continued for at least 3 days after the eye has healed. Excess cream or ointment is unlikely to be harmful.

The NHS lists a previous allergic reaction to aciclovir or valaciclovir as a reason not to take it, and asks people with certain medical conditions to talk to a doctor first.

Mention kidney problems in particular, because dehydration and kidney function affect how the tablets are handled.

Tablets commonly cause headache, dizziness or fainting, nausea or vomiting, diarrhoea, stomach ache, rash, itching, tiredness, fever and sensitivity to light. Cream and ointment may sting or burn mildly at first, which usually passes quickly.

• Call NHS 111 for tiredness with itching and yellowing of the skin (a liver problem), passing less urine (a kidney problem), or tiredness with easy bruising or frequent illness (a blood change).

• Call 999 for anaphylaxis: swollen throat or tongue, a rash, or difficulty breathing.

• If you feel dizzy or faint, avoid driving, cycling or using machinery.

Aciclovir is generally permissible in pregnancy, but speak to a pharmacist or doctor first. Cream and ointment are generally safe while breastfeeding provided the baby does not touch the treated skin; tell your doctor before taking tablets.

The NHS names probenecid, cimetidine and mycophenolate mofetil as medicines that can interact. You can eat normally and drink alcohol. Suspected side effects can be reported through the Yellow Card scheme.