Cold sores: why they appear, how long they last and when to see a doctor

The virus behind cold sores is herpes simplex virus type 1 (HSV-1). MSD Manuals writes that worldwide it is found in more than a third of children and around two thirds of adults. Most people catch it in childhood through close skin contact, such as a kiss from someone with a cold sore. The first infection often passes unnoticed; in some children, though, it causes many painful sores inside the mouth and on the gums, with a fever and feeling unwell (herpetic gingivostomatitis). MSD writes that the sores in this first episode can last 10 to 19 days and can make eating and drinking so difficult that the child becomes dehydrated.

After the first infection the virus does not leave the body. It retreats to a cluster of nerve cells (a ganglion) near the base of the brain, belonging to the nerve that carries sensation from the face, and lies dormant there. From time to time it wakes up, travels back along the nerve fibre and causes blisters again on the same patch of skin. This is why a cold sore comes up at the same spot on the lip each time: the virus is waiting in the nerve that supplies that area. MSD adds that the reactivation often cannot be put down to any single cause.

MSD lists the known triggers as too much sun, an illness with a fever, emotional stress, physical trauma such as dental treatment, medicines that suppress the immune system and hormonal changes such as menstruation. The NHS also names illness, sunlight and periods. This is why people say a cold sore comes after a chill or a feverish illness: an illness with a fever gives the virus a good opportunity to wake up.

A typical cold sore follows a timetable. According to the NHS, it starts with a tingling, itching or burning feeling; within the next 48 hours one or more painful, small, fluid-filled blisters appear. The blisters burst and crust over. MSD writes that the tingling lasts from a few minutes to a few hours and that the scab falls off after about 5 to 10 days. Some episodes fade away with only tingling and redness and no blisters at all. The NHS expects a cold sore to start healing within 10 days.

Telling a cold sore from a mouth ulcer is where people most often get confused, and it can be done at home. The first clue is the location: according to MSD, recurrent cold sores appear on the border between the red of the lip and the skin (the vermilion border), and only very rarely on the hard palate. The NHS describes mouth ulcers as appearing inside the mouth, on the inside of the cheeks and lips, or on the tongue. The second clue is how it starts: a cold sore is announced by tingling and appears first as blisters, whereas an ulcer appears as a sore straight away. The third is contagion: the NHS stresses that mouth ulcers are not contagious, whereas cold sores are.

Some conditions look like a cold sore but are something else. According to the NHS list for telling them apart, a red spot filled with pus on the face may be a spot or a boil, a blister on the inside of the lip may be a mouth sore (for a fluid-filled blister caused by biting the lip, see our mucocele page), and red sores on the face that crust over into golden-brown patches may be impetigo, a bacterial skin infection. A crack that opens at the corner of the mouth, rather than on the edge of the lip, is usually not a cold sore but a separate condition.

How well creams and tablets work tends to be overstated. In two large studies, around 1,400 adults who often got cold sores applied either 5% aciclovir cream or a cream with no medicine in it at the start of an episode. The average duration of the cold sore fell from 4.8 to 4.3 days in the first study and from 5.2 to 4.6 days in the second; in other words, by about half a day. The cream did not stop blisters and sores from forming. In two studies testing a high-dose, one-day course of prescription antiviral tablets, episodes were about 1 day shorter. MSD also writes that antiviral medicines can clear the symptoms 1 to 2 days sooner and work best when started within a few hours of the symptoms beginning, before the blisters appear.

Cold sores have three serious extensions beyond the lip, and these are not for watching at home. The first is the eye: MSD writes that the same virus can infect the clear front layer of the eye (the cornea), and that if it is left untreated the cornea can become cloudy and cause marked loss of vision. The second is the newborn baby: the NHS says that someone with a cold sore kissing a baby can cause a very dangerous infection in the baby (neonatal herpes), and that babies are most at risk in their first 6 weeks. The third is skin with eczema: MSD writes that in babies and adults with atopic eczema the virus can spread over the eczema and cause a severe illness, and that these people should keep away from anyone with an active cold sore. There is one more, very rare condition: MSD writes that the same virus can also infect the brain (herpes encephalitis), that this starts with confusion, fever and seizures and can be fatal; in that situation, call 112.

The situations in the first list are the normal course of a cold sore; they clear up with care at home and, if needed, a cream from the pharmacy. If even a single point from the second list applies, do not wait.

The sequence below follows both what you can observe at home and the points a doctor will look at. The first step deliberately rules out the dangerous situations.

1. 1

   First, the eye, baby and eczema question

   Pain, redness, watering, sensitivity to light or a change in vision in the eye; a baby under 6 weeks old at home; sores spreading over skin with eczema. If any of these applies, do not try to tell things apart at home; act on the emergency criteria above.

2. 2

   Find the location: the edge of the lip or inside the mouth

   Look in a mirror. If the sore is on the border where the red of the lip meets the skin of the face, or just outside it, a cold sore is likely. If it is on the inside of the cheek or lip, on the tongue, or on the soft, moist tissue inside the mouth, a mouth ulcer is likely. MSD writes that recurrent cold sores appear on the lip border and rarely on the hard palate.

3. 3

   Think back to how it started: was there tingling

   A cold sore usually announces itself with tingling, burning or itching lasting from a few minutes to a few hours and appears first as blisters. A mouth ulcer appears as a sore straight away, without a blister stage like this.

4. 4

   Look at its appearance: a cluster of blisters or a single sore

   A cold sore forms a cluster of small, fluid-filled blisters that then crust over. Golden-brown crusts suggest impetigo, a single spot with pus suggests a pimple, and a clear blister on the inside of the lip after biting it suggests a mucocele.

5. 5

   Watch how it goes: the 10-day and 3-week thresholds

   The NHS expects a cold sore to start healing within 10 days and a mouth ulcer to clear within 1 to 2 weeks. If a cold sore has not improved within 10 days, see a doctor; if a sore inside the mouth lasts longer than 3 weeks, see a dentist or GP, as the NHS advises.

6. 6

   If it is still unclear: a sample from the sore

   MSD writes that the diagnosis is usually made from the appearance, and that when in doubt a swab from the sore can be tested in a laboratory for the virus (PCR). This is particularly useful for sores that come back often or appear in an unusual place.

No treatment gets rid of cold sores completely; the virus stays in the nerve. The options are about shortening the episode, reducing the pain and making frequent recurrences less frequent.

The following are common habits that make a cold sore last longer, spread it, or let a serious condition slip by unnoticed.

The sequence below is for a recurrent cold sore in an adult with a normal immune system. In babies, pregnant women and people with a weakened immune system, the doctor's plan comes first.

1. The first hours: tingling

   Burning, itching or tingling on the lip. MSD writes that this stage lasts from a few minutes to a few hours. This is when antiviral treatment works best, and it is also when you become contagious.

2. The first 2 days: blisters

   According to the NHS, one or more painful, fluid-filled blisters appear within the next 48 hours. The pain is usually at its worst at this stage.

3. The following days: bursting and crusting

   The blisters burst, leave a shallow sore and crust over. Do not pick the scab off; the skin underneath is healing.

4. Days 5 to 10: the scab falls off

   MSD writes that the scab falls off after about 5 to 10 days. The NHS writes that a cold sore usually clears within 10 days and advises seeing a doctor if it has not started to heal within that time. It remains contagious until it has fully healed.

A normal cold sore usually clears up with care at home alone or with a cream from the pharmacy; what widens the scope is how often the episodes come and any conditions that go with them. These are the items that determine it:

How often the episodes come

Cold sores that come up a few times a year and cold sores that recur very often call for different approaches; with frequent recurrences, a doctor may discuss continuous preventive treatment.

The product used

Care at home, antiviral cream and prescription tablets are separate items; tablets need a doctor's examination.

Whether a diagnostic test is needed

For sores in an unusual place or with an unusual appearance, a swab test is a separate item.

Involvement of the eye or skin

An infection that spreads to the eye or to skin with eczema falls within the remit of eye and skin specialists and changes the scope considerably.