Aciclovir

An antiviral that only switches on inside infected cells

Aciclovir — acyclovir in American spelling — treats the two herpes simplex viruses behind cold sores and genital herpes, along with varicella-zoster, the virus that causes chickenpox and shingles. Its neat trick is where it becomes active: a viral enzyme that exists only in cells the virus has infected turns aciclovir into its working form, and that form blocks the copying of viral DNA. Untouched cells lack the enzyme and are left largely alone.

What aciclovir cannot do is clear the virus out. It hides in nerve cells between attacks, so treatment is about shortening an outbreak, easing it, and — taken every day — making recurrences less frequent.

Cream for a lip, tablets for the rest

Two forms, two jobs. Acyclovir Cream 5% comes in 5g and 10g tubes for a cold sore on the lip or around the mouth; it acts on the skin where it is spread, and very little of it reaches the rest of the body.

Tablets travel wherever blood goes, which is what genital herpes, shingles and chickenpox need. Aciclovir and Zovirax are the same medicine under two brand names, both in 200mg, 400mg and 800mg. For herpes simplex, 200mg and 400mg are the strengths used; shingles and chickenpox in adults call for the 800mg tablet.

Starting at the first tingle

With aciclovir, timing counts for more than the name on the box. Treated from the first tingle, itch or blister, a cold sore or genital outbreak is shorter and milder; shingles tablets work best when they begin within three days of the rash showing.

For a first herpes simplex outbreak the standard course is 200mg five times a day for five days, doses spaced about four hours apart with none overnight. Outbreaks that keep returning are suppressed with 400mg twice a day for several months. Shingles and adult chickenpox take 800mg five times a day for seven days.

Food makes no difference to how aciclovir works, and the kidneys clear it, so a glass of water with each dose is a habit worth keeping.

Side effects and interactions

Nausea, headache, diarrhoea, vomiting and tiredness are the usual complaints. Drowsiness, confusion, hallucinations and uncontrolled shaking are uncommon, and turn up mainly when the kidneys are clearing the drug more slowly than they should. A rash with swelling of the lips, face or throat, or difficulty breathing, needs urgent treatment.

Two medicines are worth knowing about: probenecid and cimetidine both slow aciclovir's removal from the body, which raises its level. Where the kidneys are not working well, a lower dose is used.