Updated 03 June 2015

Cold sores/fever blisters

These are small red blisters with a white centre which crop up on or around the lips.


  • Fever blisters are caused by the Herpes simplex virus type 1 (HSV 1).
  • The infection is picked up by direct contact with someone with the virus. It can also be caught from someone who carries the virus but has no symptoms.
  • Like other viruses belonging to the herpes virus family, the virus remains in the body forever once acquired.
  • Fever blisters can be precipitated by stress, sun exposure and infection.
  • If caught early, acyclovir is an effective treatment.

Alternative names

Cold sores


Fever blisters or cold sores are small red blisters with a white centre which crop up on or around the lips. They can also be found inside the mouth, but this is less common.


Fever blisters are caused by the Herpes simplex virus type 1 (HSV 1). HSV 2 causes a small percentage of cold sores. The virus is spread between individual children by means of saliva contamination of toys, hands and other objects during play activities or children may pick up the virus by kissing someone who has the virus. The spread of infection can still occur from someone who does not have a visible sore, but has the virus in his or her saliva.

Once the virus is in the body it is there forever and most adults acquired it in early childhood. The virus acquired during childhood can cause a mild infection that goes unnoticed or a severe febrile illness with sores in the mouth known as primary oropharyngeal herpes. This disease is very noticeable since there are extensive blisters in the mouth and on the lips of the child, causing pain, discomfort and inability to eat or drink. Once recovery has occurred, oral herpes or fever blisters may recur in the form of fever blisters in some people in later years, while in others it lies dormant forever with no symptoms.


The first time a child gets oral herpes infection he or she will be very miserable, have a slight fever, complain of a sore mouth and gums and be unable to eat or drink because of pain when fluid or food comes into contact with the sores.

Parents will notice small blisters in and around the mouth, on the palate, tongue and throat, which then turn into shallow, painful sores. These lesions may be accompanied by fever and swollen glands. The sores will slowly get better and those on the skin will crust over and heal. The whole episode lasts seven to 10 days. In severe cases children may become severely dehydrated during the primary infection, necessitating admission to hospital and administration of fluids and nutrients with an intravenous drip.

Further flare ups of oral herpes at a later stage will manifest simply as a tingling sensation on the lip for a day or two which is followed by the development of a typical fever blister on the lip. Fever blisters are small groups of blisters on the lips that are caused by reactivation of herpes virus in an individual who had primary infection as a child. Sometimes in communities where there is little physical contact between individuals, some people may reach adulthood without experiencing primary infection with oral herpes.


Ideally, infection with HSV 1 or 2 can be prevented by stopping a child from kissing or touching someone with a noticeable fever blister. However, in practice this is almost impossible as someone without symptoms can transmit the virus anyway.

Avoid sharing eating utensils, particularly cups if possible.

Sun exposure definitely triggers cold sore recurrences in many people, so use a high factor sun-screen when outside.


If the sore is caught very early, during the stage when the only symptom is the tingling sensation at the lip margin, then an antiviral ointment called acyclovir may be effective in preventing a full cold sore from developing.

However, individuals often fail to recognise this stage and so nothing is done to prevent infection. Once the fever blister has developed, there is no cure, but the pain can be relieved by using ice over the area, simple painkillers such as paracetamol if necessary and any local anaesthetic cream which your pharmacist recommends.

When to see your doctor

  • If you or your child develop a painful sore on the eyelid, around the eye or complain about pain in the eye itself when he or she has a cold sore, you should see the doctor immediately, since this may be ocular herpes.
  • If the cold sores are extensive and are preventing you or your child from eating and drinking normally, you should see your doctor.

Reviewed by Dr Eftyhia Vardas BSc(Hons), MBBCh, DTM&H, DPH, FC Path (Virol), MMed (Virol), Clinical Virologist, Director HIV/AIDS Vaccine Division, Perinatal HIV Research Unit, Chris Hani Baragwanath Hospital, University of the Witwatersrand and senior lecturer, Department of Infectious Diseases, Faculty of Health Sciences, University of the Witwatersrand


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