What causes cold sores?

Cold sores are inflammation of the lips and gums caused by the herpes virus, which is spread through close personal contact like kissing. 


Some children don't show any symptoms from the infection, while others will develop small, fluid-filled blisters on the lips, chin, cheeks, or in the nostrils. Cold sores typically will last for 10-14 days.  

Other symptoms can include:

  • fever 
  • irritability / more moody than usual 
  • painful mouth ulcers and lip blisters 
  • poor appetite or drinking less.
Mouth with red, sore on the lip

(DermNet NZ)


Cold sores are caused by a virus, so there are no antibiotics that can treat the infection. 

Make sure your child keeps hydrated while they have cold sores. They may not want to drink because their mouth is sore so provide pain relief if needed, so your child will want to drink.  

Talk to your doctor or pharmacist about pain reliefAlways make sure to follow the recommended dose for your child’s age and weight, and check to make sure the medication hasn’t expired. 


You can prevent your child from being exposed to the herpes virus, which causes cold sores. Make sure they avoid: 

  • kisses or close contact with anyone who has a cold sore 
  • sharing drinking cups or bottles
  • sharing towels and face flannels.

It’s also important for you to wash your hands before and after caring for young children and babies. 

When to visit a doctor

Take your child to see a doctor if they show signs of getting dehydrated. Signs can include: 

  • dry mouth and tongue 
  • sunken eyes 
  • cold hands and feet 
  • unusual sleepiness or lack of energy 
  • dry nappies 
  • not passing as much wee as usual or passing a small amount of dark yellow wee.