Cold sores: why they come back and what helps
A cold sore starts with a tingle, becomes a cluster of small blisters on the lip and crusts over within days. The virus behind it never leaves the body, which is why the same spot flares up again in the same circumstances. This page covers what triggers a flare, what actually shortens it, how to avoid passing it on, and the situations in which a cold sore is a medical problem rather than a cosmetic one.
How do I recognise a cold sore?
Most people notice the warning first: tingling, itching or a burning tightness on one spot of the lip or the skin just beside it, several hours before anything is visible. Then small fluid-filled blisters appear in a cluster, usually on the border between lip and skin.
The blisters break, weep and form a yellow-brown crust. The area is tender and can feel taut. An uncomplicated cold sore in an adult typically heals over about a week to ten days and does not leave a scar. Recurrences tend to come back in almost exactly the same place, because the virus travels back down the same nerve.
A first infection can look quite different, particularly in children: multiple painful ulcers inside the mouth and on the gums, fever, swollen neck glands and difficulty eating or drinking. That picture is worth having assessed, mainly because young children can become dehydrated when swallowing hurts.
Why do cold sores keep coming back?
Cold sores are caused by herpes simplex virus, usually type 1. Most people acquire it in childhood through ordinary close contact, often without ever noticing an illness. After the first infection the virus stays permanently in a nerve cluster near the face and stays dormant there.
Reactivation happens when local or general defences dip. The best-recognised triggers are strong sunlight or ultraviolet exposure, another infection such as a cold or flu, fever, physical or emotional stress, tiredness, cold wind, dental treatment or an injury to the lip, and in some women the days around a period. Some people can name their trigger precisely; others never find a pattern.
How often flares happen varies enormously and generally reduces over the years. Frequent, severe or unusually widespread recurrences are worth mentioning to a doctor, because they can occasionally be the first sign that the immune system is under strain for another reason.
What actually shortens an episode?
Antiviral creams containing aciclovir or penciclovir are sold at pharmacies without a prescription. Their effect is modest and depends almost entirely on timing: applied at the first tingle, before blisters form, they can shorten the episode by roughly a day. Started once the crust has formed, they achieve very little.
Hydrocolloid cold sore patches cover the lesion, reduce picking and make it less obvious, and they keep the crust from cracking. A simple lip balm prevents splitting, and paracetamol or ibuprofen helps if the area is painful. Do not pick the crust, and do not share lip balm, towels or cutlery while the sore is active.
Oral antiviral tablets are prescription medicines and work best when started very early in a flare, or taken continuously as suppression when recurrences are frequent. Lysine supplements, tea tree oil and similar remedies are widely promoted; the evidence behind them is weak, and none of them removes the virus from the body. Nothing available today eradicates it.
How do I avoid passing it on?
The virus spreads through direct contact with the sore or with the fluid from the blisters, so it is most contagious from the first tingle until the crust has fully healed. Avoid kissing during that time, do not share towels, razors, cups, cutlery or lip products, and wash your hands after touching the sore or applying cream.
Two situations deserve special care. Newborn babies must not be kissed by anyone with an active cold sore: herpes infection in the first weeks of life can be serious, and any suspicion of it is a reason for urgent medical assessment. People with eczema can develop a widespread, painful infection of broken skin, so keep an active sore away from them.
Do not touch the sore and then rub your eyes. Herpes infection of the eye can damage sight and needs prompt specialist treatment. Oral sex while a cold sore is active can transfer the virus to the genital area, which is one of the ways genital herpes is acquired.
When do I need medical help without delay?
Seek urgent medical care, on the same day, if a cold sore is accompanied by a painful red eye, blurred vision, watering or unusual sensitivity to light. Herpes affecting the eye threatens sight and is treated by an eye service, not with a cream from the shelf.
Call 112 or 999 or go to an Emergency Department if there is a severe headache with neck stiffness, sensitivity to light, confusion or a seizure alongside a herpes infection, or if a baby under three months old is unwell, feeding poorly, unusually sleepy or has a fever. Widespread painful blisters over eczema, with fever and feeling very unwell, also need emergency assessment.
Arrange an assessment promptly, rather than treating again, if a sore has not healed after two to three weeks, if sores are spreading beyond the lip, if you have frequent or severe recurrences, or if you take medicines that suppress the immune system or are having chemotherapy. A single ulcer or lump on the lip that persists for weeks is not a cold sore and must be examined.
Cold sores in pregnancy, in babies and with reduced immunity
A cold sore on the lip in pregnancy is not dangerous for the pregnancy itself, and most treatment decisions are straightforward. What matters is contact precautions once the baby is born: anyone with an active sore should avoid kissing the baby, cover the sore where possible and wash their hands before handling the child.
In newborns, herpes infection can be severe and can involve the brain and internal organs. The warning signs are non-specific and easily missed: poor feeding, floppiness, unusual sleepiness, fever or low temperature, a rash of blisters. Any of these in a small baby is a reason for emergency assessment, and it is always better to be sent home reassured than to wait.
People whose immune system is suppressed by medicines, chemotherapy or advanced illness can develop larger, longer-lasting and more painful sores that spread beyond the usual area. In that group, flares are usually treated with oral antiviral medicines under medical supervision rather than with creams.
Preventing recurrences
Where sunlight is the trigger, a lip balm with a high sun protection factor used before going out prevents a good proportion of flares, and it is the single most useful preventive measure for people who reliably break out on holidays or on the water. Protecting the lips from cold wind works on the same principle.
Sleep, managing periods of high stress and treating other illnesses promptly all reduce how often the virus reactivates, though none of it is a guarantee. Keep your own lip balm, and replace it after a flare so you are not repeatedly reapplying the same tube to a healing sore.
If flares are frequent, severe, or predictably triggered by a known event such as dental surgery or a skiing trip, a doctor can consider preventive antiviral treatment: either a short course started around the trigger or a longer suppressive course. That decision needs a doctor who knows your history, your kidney function and your other medicines.
Are cold sores contagious once the crust has formed?
The sore remains contagious until the crust has fully healed, although the risk falls as it dries. The most infectious period runs from the first tingle through the weeping blister stage. Avoid kissing and sharing towels, cups and lip products until the skin is intact again.
Do antiviral creams work?
They shorten an episode modestly and only if applied at the very first tingle, before blisters appear. Once crusting has started, the benefit is minimal. They do not remove the virus from the body and do not prevent future flares.
Can I catch a cold sore from sharing a cup?
It is possible, because the virus is spread by contact with the fluid from a blister, though direct contact such as kissing is far more common. The virus does not survive long on dry surfaces. Sharing towels, razors and lip products while a sore is active is best avoided.
Are cold sores and genital herpes the same virus?
They are closely related. Cold sores are usually herpes simplex type 1 and genital herpes has traditionally been type 2, but type 1 now accounts for a large share of genital infections, mainly through oral sex during an active or developing cold sore.
Can cold sores be cured for good?
No treatment currently available removes herpes simplex from the body. The virus stays dormant in a nerve and can reactivate. What treatment can do is shorten flares and, with preventive antiviral medicine prescribed by a doctor, make them much less frequent.