Cold Sores Guide: Causes, Spread and Care

Cold sores, also called fever blisters or oral herpes, are very common and usually clear up on their own. This guide covers what causes them, how to avoid passing them on, and when to get help, with links to all our doctors' answers about cold sores and oral herpes.

This is general information, not medical advice. It does not replace a visit with a doctor or clinic.

Contents

• What causes cold sores?
• What do they look like?
• How long are they contagious?
• Common triggers
• Self-care and treatment
• When to see a doctor
• Start here: our key pages
• All doctor answers on this topic (34)

What causes cold sores?

Cold sores are caused by herpes simplex, usually HSV-1. Most people pick up the virus as children through close contact such as kissing. Once you have it, it stays in the body for life and can cause a cold sore from time to time.

What do they look like?

A cold sore usually starts with tingling, itching or burning. Within about 48 hours, small fluid-filled blisters appear, then burst and crust over. They usually start to heal within 10 days. Not every mouth sore is a cold sore: a blister inside the mouth is often a mouth ulcer.

How long are they contagious?

From the first tingle until the sore has completely healed. Don’t kiss anyone or have oral sex during that time, because the virus can cause genital herpes in a partner. Wash your hands after touching the sore, and avoid sharing items that touch saliva. Never kiss a newborn baby if you have a cold sore.

Common triggers

Illness or fever, sun, periods, injury, stress and surgery can all trigger cold sores. If sun is a trigger, a sunblock lip balm (SPF 15 or above) can help. Skin procedures like laser resurfacing and peels can also trigger them, so tell your provider beforehand.

Self-care and treatment

A pharmacist can suggest creams to ease pain and antiviral creams, which work best at the first tingle. Pain relievers and plenty of fluids help. A doctor may prescribe antiviral tablets if cold sores are large, painful or keep coming back.

When to see a doctor

See a doctor if a cold sore hasn’t started to heal within 10 days, is very large or painful, is near your eye, if a child has swollen, painful gums and mouth sores, or if you have a weakened immune system.

All doctor answers on this topic (34)

Questions readers have sent in over the years, answered by herpes.com’s contributing doctors. Answers reflect each doctor’s view at the time they were written; for current guidance, see the sources below and talk to your own provider.

• Are all Mouth Sores Caused by the Herpes 1 Virus?
• Are Symptoms for Herpes 1 Milder than Herpes 2?
• Burning in Lip
• Can genital herpes be caught from oral sex and cold sores?
• Can Herpes ever cause Tonsillitis?
• Can herpes symptoms show up inside of the mouth?
• Can I get Herpes Going Down on Her?
• Can I give a man genital herpes when I have a cold sore?
• Can one develop multiple cold sores ?
• Can oral sex be safe since I have herpes?
• Can we still kiss with Herpes?
• Can you get Genital Herpes from Herpes in your Mouth?
• Can you get Herpes from a Glass?
• Cold Sore Type Lesion on Penis
• Did I get Herpes from Kissing?
• Girl says she got Herpes from Oral Sex
• How can I have sex with my husband when I have HSV1 herpes?
• How is Herpes going to affect us when having Oral Sex?
• HSV-1 Genital Herpes
• I have had HSV1 cold sores on my lips all my life
• I Assume I have Herpes 1 Genitally
• I got Herpes from Having Oral Sex with my Boyfriend
• I have Herpes Labialis
• If You Have a Cold Sore, is that Automatically Herpes?
• If you have herpes 1 will you get herpes 2?
• Is it Difficult to Pass Genital Herpes to the Mouth?
• Is Herpes Simplex I, airborne?
• Is HSV 1 on the Genitals?
• Is it safe to have sex when I have a cold sore?
• Positive for type 1 Herpes and Negative for type 2 Herpes
• Positive Herpes Type 1
• She has genital herpes, Can we still have oral sex?
• Sun and Stress Causes Herpes Symptoms
• Unprotected Oral Sex

More guides

• Herpes Guide: HSV-1 and HSV-2 Basics
• Herpes Testing Guide: Which Test, When, and What Results Mean
• Dating With Herpes: Relationships and Telling a Partner
• Nutrition and Daily Wellness Guide

Sources

1. NHS – Cold sores
2. MedlinePlus (NIH) – Herpes – oral
3. CDC – About Genital Herpes
4. World Health Organization – Herpes simplex virus fact sheet
5. Mayo Clinic – Laser resurfacing
6. MedlinePlus (NIH) – Cold Sores

Medical disclaimer: The information on herpes.com is for general education only and is not a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your doctor with any questions about a medical condition. Read our full disclaimer. Guide published October 2026.