Cold sores

Symptoms

• Clusters of small, painful, fluid-filled blisters around the lips (during the first infection, blisters may form on the inside the lips, on the tongue, and on the gums)
• Difficulty eating or drinking because of pain
• In some cases, a fever may develop; the fever may be very high (40°C to 40.6°C), especially during the first infection

• Is under 3 months old
• Has repeated uncomfortable flare-ups, for instance, every 3 to 4 weeks
• Refuses to drink due to the pain from the blisters
• Has had a fever above 40°C for 48 to 72 hours, or their fever is accompanied by irritability or drowsiness
• Develops a blister near their eyes: The herpes virus can be spread to the eyes through contact with contaminated hands if the child scratches a cold sore elsewhere on their body. Your doctor may recommend that your child be seen by an ophthalmologist. The herpes virus can cause serious eye problems and could even blind your child.

What is a cold sore?

Cold sores are generally caused by a widespread virus known as herpes simplex virus 1. They can also be caused by herpes simplex virus 2, though this is less common. Herpes simplex virus 1 can also cause blisters to appear in the mouth (gums, palate, inside of the cheeks) and elsewhere on the body, on the skin or mucous membranes. The term cold sores is reserved for blisters that form near the lips.

It is estimated that 25% of children contract herpes simplex virus 1 by the age of 5. Although this infection is annoying, it is generally harmless in babies over 3 months of age and in children with a normally functioning immune system. However, herpes simplex can sometimes cause more serious infections, such as meningitis and encephalitis.

Once someone has been infected, the virus stays dormant in their body for life and can reactivate at any time. This is why cold sores reappear. Sometimes, people with the virus are asymptomatic and don’t realize they are a carrier. These people still have flare-ups where the virus is contagious, but don’t develop any cold sores. This is why the virus is so easy to spread.

Treatment

Some children experience more severe symptoms during their first herpes infection (more pain and sores), especially if blisters form inside their mouth and on their gums.

If this occurs, it’s a good idea to consult a doctor, especially if your child has a fever or can’t drink. The doctor may prescribe an antiviral medication to reduce the severity and duration of symptoms. Antivirals are most effective when started within the first 48 hours of a flare-up. For some children, a brief hospital stay may be necessary to start intravenous antiviral treatment.

Your child may develop cold sores again in the future after their first infection is cleared up, as the virus remains in the body for life. Flare-ups usually last less than a week and, unfortunately, antiviral medication is not very effective at this stage. Some ointments claim to reduce the duration of symptoms, but their effectiveness has not been proven.

In some cases, an antiviral medication can be prescribed if the child can recognize the first signs of cold sores (e.g., burning or tingling) and experiences significant discomfort during flare-ups. In rare cases, the doctor may prescribe a daily dose of antiviral medication to prevent flare-ups in children who get cold sores often, e.g., every month.

Care and practical advice

• To sooth their pain, give your child an ice cube or popsicle.
• Try to get them to drink often, even if they’re in pain, to avoid dehydration.
• If your child complains of pain or develops a fever, give them acetaminophen (e.g., Tylenol^®, Tempra^®) or ibuprofen (e.g., Advil^®, Motrin^®) as indicated and in the doses recommended for your child’s age and weight. Don’t give ibuprofen to babies under 6 months, and never give Aspirin^® (acetylsalicylic acid) to a child or teenager.

Prevention

• Don’t kiss your child on the mouth or near the eyes if you, they, or anyone else close to them has a cold sore, or if you get cold sores, especially if your child is under 3 months old. Babies of this age can become very ill if infected with the herpes simplex virus (e.g., complications from meningitis).
• Teach your child not to touch their cold sores.
• Clean your child’s toys regularly.
• Avoid sharing objects your child puts in their mouth (e.g., utensils, drinking glasses, toys).
• Wash your hands and your child’s hands regularly.

Scientific review: Dr. Isabelle Viel-Thériault, pediatrician and infectious disease specialist at CHU de Québec – Université Laval Research and copywriting:The Naître et grandir team Updated: April 2025

Photo: iStock.com/somethingway