Causes of cold sores 

Cold sores are usually caused by the herpes simplex virus.

In most cases the virus is passed on in early childhood – for example, when a child is kissed by a family member or friend with a cold sore.

The virus passes through the skin and travels up the nerves, where it lies inactive (dormant) until it's triggered at a later date.

Cold sore triggers

Factors thought to trigger outbreaks of cold sores include:

  • having another infection, such as a respiratory tract infection
  • having a high temperature (fever)
  • emotional upset or psychological stress
  • tiredness and fatigue
  • an injury to the affected area
  • menstruation (periods)
  • strong sunlight

However, in many cases there's no obvious trigger for an outbreak.

Genital herpes

Cold sores can happen after having oral sex with a man or woman who has genital herpes.

Page last reviewed: 25/04/2016

Next review due: 01/04/2019