Cold sores

A cold sore is a small crop of blisters, usually on the border of the lip, caused by the herpes simplex virus — generally type 1. Most adults carry the virus. Most acquired it in early childhood from ordinary family affection, and a large proportion never get a single cold sore.

That is worth saying at the start because of the word involved. Carrying HSV-1 is close to being the human default, it is not a sexually transmitted infection in the ordinary sense, and nothing about having cold sores implies anything whatsoever about anybody's conduct.

How it behaves

The first infection usually passes unnoticed. Occasionally it arrives as a nastier illness in a small child — painful ulcers across the mouth and gums, fever, dribbling and a refusal to eat or drink. That version matters because of dehydration: a small child who will not drink because their mouth hurts needs watching, and needs seeing if they are taking almost nothing.

After that the virus settles in a nerve beside the face and stays. Cold sores are it reactivating, which happens in some people a few times a year, in others almost never.

The sequence is consistent, which is what makes it treatable:

• A warning — tingling, itching, burning or tightness at the spot, for a few hours up to a day before anything is visible.
• Blisters, small and clustered, which swell and then break.
• Weeping and crusting, which is the most infectious stage and the most uncomfortable.
• Healing, generally complete within seven to ten days, without scarring.

The thing the old advice got wrong

Writing on cold sores frequently concludes that there is no cure, so the only thing to do is wait them out. The first half is true and the second does not follow.

Antiviral cream works, but only if it is started at the warning stage. Aciclovir or penciclovir cream, from any pharmacy without a prescription, applied the moment the tingle starts and then five times a day, shortens the episode and can stop one developing. Applied once blisters have formed, it does very little — which is why so many people conclude it does not work. The trick is to have a tube already in the house, because by the time one is bought the window has closed.

Beyond that: antiviral tablets are more effective than cream and can be prescribed for severe episodes, and daily suppressive treatment is an option for anyone getting them frequently enough to be a real nuisance. Hydrocolloid cold sore patches cover the sore, reduce the mess and are reasonable. Ordinary painkillers help, and so does a lip balm to stop the crust cracking.

Triggers

Most people learn their own, and keeping track is worthwhile because several are avoidable.

Sunlight is the one worth acting on, and the most frequently missed — ultraviolet light is a reliable trigger, and a lip balm with a sun protection factor genuinely prevents episodes for people who get them on holiday or on bright winter days. Also: any feverish illness, which is where the name comes from; being run down or short of sleep; periods; and physical trauma to the lip, including dental work and cosmetic injections. Stress is widely blamed and probably does contribute.

Not passing it on, and the two situations that matter

While a cold sore is blistered or weeping, the virus is on the surface and transfers by direct contact. So during an episode: no kissing, no sharing of cups, cutlery, towels, flannels, razors or lip products, and hands washed after touching the sore or applying cream.

Two situations are not merely inconvenient:

Newborn babies must not be kissed by anyone with a cold sore. Neonatal herpes is rare and it is serious, and a baby in the first weeks of life has very little defence against a virus that is trivial in an adult. This extends to anyone handling the baby and to not letting a sore touch the baby's skin. It is a reasonable thing to say out loud to visiting relatives, awkward as that is.

The eye. Touching a cold sore and then rubbing an eye can transfer the virus to the cornea, which can scar it and threaten sight. A painful, red, watering, light-sensitive eye in someone with a cold sore needs same-day assessment, not a wait.

Two further points:

Anyone with eczema is at risk of the virus spreading across eczema-affected skin, which produces a condition needing same-day treatment — rapidly worsening, unusually painful eczema with clustered blisters and small punched-out sores. It is described under eczema, and it is a reason for someone with a cold sore to keep away from a child with active eczema.

Oral sex during an episode can give a partner genital herpes. This is not a marginal possibility: HSV-1 passed from mouth to genitals is now the commonest cause of genital herpes in the UK, particularly in younger people. Avoiding oral sex while a cold sore is present, and during the warning stage, is the straightforward answer.

What is not the test, and not the treatment

A cold sore is diagnosed by looking at it. Blood tests have no routine role — they can show that someone carries the virus, which most people do, without indicating anything useful about the sore in front of them. A swab is occasionally taken where the appearance is unusual.

On painkillers, one correction that circulates in older writing: Advil and ibuprofen are not two different options. Advil is a brand name for ibuprofen, and not one sold in the UK. Paracetamol or ibuprofen is the whole of it.

When to see someone

• A cold sore that has not healed within about ten days.
• Episodes frequent or severe enough to be a genuine problem — there is prescribed treatment for that.
• Any eye symptoms, urgently.
• A suppressed immune system, where episodes can be extensive and need prompt antiviral treatment.
• Widespread sores, or sores inside the mouth in an adult, which may be something else.
• A baby or small child with a painful blistering mouth who is drinking very little, or any cold sore in a newborn, which is urgent.
• Someone with eczema whose eczema is worsening rapidly and painfully with blisters — same day.