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Cold Sores: Causes, Treatment and Protecting Newborn Babies

Cold sores are small blisters around the mouth caused by the herpes simplex virus. Most heal on their own, but they are contagious, and they can be dangerous for newborn babies.

Updated September 2026·By the OHA editorial team·6 min read·How we write
Key takeaways
  • Cold sores are small blisters on or around the lips, caused by the herpes simplex virus, which stays in your body for life.
  • They start with tingling or itching, then blister, crust over and heal, usually within 1 to 2 weeks.
  • Cold sores are contagious until they have completely healed. Do not kiss anyone, and never kiss a newborn baby, while you have one.
  • Antiviral creams work best started at the first tingle. Antiviral tablets are an option for severe or frequent cold sores.
  • See a doctor straight away if a cold sore is near your eye, or if a young baby has sores, a fever or seems unwell.

A cold sore is a small blister, or group of blisters, on the lips or around the mouth. NHS inform (Scotland) says cold sores are caused by the herpes simplex virus (HSV) and usually clear up without treatment within 7 to 10 days. They are also called fever blisters (American Dental Association) or oral herpes (healthdirect Australia).

Cold sores are not the same as mouth ulcers. The NHS says a blister on the inside of the lip or mouth is more likely to be a mouth ulcer, and red sores that become crusty, golden-brown patches may be impetigo, a bacterial skin infection.

What causes cold sores?

Most cold sores are caused by herpes simplex virus type 1 (HSV-1) (NHS inform, healthdirect). NHS inform says type 2 (HSV-2) can sometimes cause them, for example after oral sex with someone who has genital herpes.

The NHS says most people catch the virus as children, through close contact such as a kiss from someone with a cold sore. NHS inform explains that the virus then lies inactive (dormant) in the nerves until something triggers it. It stays in your body for life.

The virus is very common, but many people who have it never get a cold sore. The World Health Organization (WHO) estimates that 64% of people under 50 worldwide had HSV-1 in 2020, and healthdirect says about 8 in 10 people have it by adulthood. The Oral Health Foundation (UK) says only about 1 in 3 people who catch the virus go on to develop cold sores.

The first infection

The first infection often causes no symptoms (NHS inform, Raising Children Network). When it does, NHS inform says it is most likely in children under 5 and can cause herpes simplex gingivostomatitis: swollen, sore gums with small, painful sores in and around the mouth, often with a sore throat, swollen glands and a high temperature. It can last 7 to 14 days, and young children are particularly at risk of dehydration if drinking hurts. NHS inform says this is rare in adults. The WHO says later outbreaks are usually shorter and milder than the first.

How does a cold sore develop?

  1. Tingling. A tingling, itching or burning feeling where the sore is about to appear (NHS).
  2. Blisters. Over the next 48 hours, small, painful, fluid-filled blisters appear (NHS), most often where the lip meets the skin (Oral Health Foundation).
  3. Crusting. The blisters burst and crust over into a scab (NHS), usually within 2 to 3 days (Raising Children Network).
  4. Healing. Sources give different times: about a week (American Dental Association, Oral Health Foundation), 7 to 10 days (NHS inform, Raising Children Network), within 10 days (NHS) or within 2 weeks (healthdirect). healthdirect says they do not leave scars.

What triggers a cold sore?

NHS inform, healthdirect and the Oral Health Foundation list another illness such as a cold, flu or fever; stress and tiredness; strong sunlight (healthdirect adds cold or wind); periods and other hormonal changes; and injury to the area. NHS inform says that often there is no obvious trigger.

How do cold sores spread?

The NHS says cold sores are contagious from the first tingle until they have completely healed, and can spread to other people and other parts of your body. The Oral Health Foundation says they are most infectious when blisters are present, and NHS inform says when they burst. healthdirect, the WHO and Raising Children Network say the virus can also spread when there are no visible sores.

To avoid spreading it, the NHS and healthdirect advise:

  • do not kiss anyone, or have oral sex, until the sore has completely healed (the NHS says you could give your partner genital herpes)
  • try not to touch, pick or lick the sore, and wash your hands whenever you do touch it
  • do not share cups, cutlery, towels, toothbrushes, razors or cold sore creams (healthdirect)

Protecting newborn babies

The NHS says kissing a baby when you have a cold sore can lead to neonatal herpes, which is very dangerous to newborn babies. It is rare (the WHO estimates about 10 in every 100,000 births), but a newborn's immune system is not fully developed, so they can become seriously ill very quickly. The NHS says the first 6 weeks are thought to be the riskiest time.

The NHS advises:

  • Anyone with a cold sore, or who has recently had one, should not kiss the baby.
  • Everyone should wash their hands before handling the baby.
  • Discourage anyone other than close family or carers from kissing the baby. The safest place for a kiss is the top of the head.
  • If you have a cold sore, cover it, and wash your hands before touching or breastfeeding your baby.
  • If you have a herpes blister on your breast, do not feed from that breast until it has healed.

If you are pregnant and have ever had genital herpes, the NHS says to tell your midwife or doctor. See also pregnancy and oral health.

How are cold sores treated?

Cold sores cannot be cured, but the WHO says antiviral medicines can make outbreaks shorter and less severe.

From a pharmacy

The NHS says a pharmacist can recommend creams to ease pain, antiviral creams to speed up healing, and cold sore patches to protect the skin. Use antiviral cream at the first tingle: the NHS says it does not always work after blisters appear. The NHS says not to use aciclovir cream, a common antiviral, inside the mouth or near the eyes, and to get medical advice before using a cold sore cream if the sore is severe or your immune system is weakened.

Antiviral tablets

This differs by country. In the UK, the NHS says a GP may prescribe antiviral tablets if cold sores are very large, painful or keep coming back. In Australia, healthdirect says a single dose of antiviral tablets is available from a pharmacist without a prescription. In the US, the American Dental Association says your dentist may prescribe antiviral medicine.

Easing the discomfort

  • Paracetamol or ibuprofen can ease pain (NHS). Do not give children aspirin: the NHS says not under 16, and Raising Children Network (Australia) says not under 12 unless a doctor prescribes it.
  • Drink plenty of fluids and eat cool, soft foods; avoid acidic or salty foods if they sting (NHS, NHS inform).
  • Use a lip balm with sun protection (SPF 15 or above) in the sun (NHS).

healthdirect says there is no evidence that the supplement lysine prevents cold sores.

When to see a pharmacist or doctor
  • The cold sore has not started to heal within 10 days (NHS), or has not healed after 2 weeks (Oral Health Foundation).
  • It is very large or painful, spreading, or looks infected, with redness, pus or a fever (NHS, healthdirect).
  • You or your child also have swollen, painful gums and sores inside the mouth (NHS).
  • You have a weakened immune system, for example from chemotherapy, diabetes or HIV (NHS, NHS inform).
  • You get cold sores often (healthdirect), or your child cannot drink enough even after pain relief (Raising Children Network).
  • You are pregnant and have a cold sore. The NHS says pregnant women with cold sores may be referred to hospital for advice or treatment.
Get help urgently
  • A cold sore near the eye. healthdirect says see a doctor immediately, as it can lead to severe complications.
  • A newborn or young baby. The NHS says call a GP or NHS 111 straight away if a baby will not settle, is not feeding as well as usual, has a temperature of 38°C or more, or has a rash or sores on the skin, eyes or inside the mouth. Call 999 (or your local emergency number) if a baby is floppy, hard to wake, having trouble breathing or has a seizure.
  • A seizure, confusion, drowsiness or unusual behaviour in a child with cold sores. Raising Children Network says see a GP or go to an emergency department, as these can be signs of a rare brain infection (encephalitis).

Frequently asked questions

Can I go to work or school with a cold sore?

Yes. NHS inform says there is no need to stay away, but avoid close contact such as kissing until the sore has healed.

Can I kiss my baby if I have a cold sore?

No. The NHS says people with a cold sore, or who have recently had one, should not kiss a baby.

Can I stop cold sores coming back?

Not completely. The Oral Health Foundation and healthdirect suggest avoiding your triggers, using a lip balm with sun protection, managing stress and looking after your general health.

Sources

  1. Cold sores, NHS, accessed September 2026
  2. Neonatal herpes (herpes in a baby), NHS, accessed September 2026
  3. Aciclovir, NHS, accessed September 2026
  4. Cold sore, NHS inform (Scotland), accessed September 2026
  5. Cold sores, Oral Health Foundation, accessed September 2026
  6. Cold sores, healthdirect Australia, accessed September 2026
  7. Cold sores: children and teens, Raising Children Network (Australia), accessed September 2026
  8. Cold sores, American Dental Association (MouthHealthy), accessed September 2026
  9. Herpes simplex virus, World Health Organization, accessed September 2026

This article is general information about oral health. It is not a diagnosis or a substitute for advice from a dentist or doctor who knows your situation. If you have pain, swelling, bleeding that does not settle, or a problem that is getting worse, see a dental professional. In an emergency, contact your local emergency service.

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