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What Causes Cold Sores?

Cold sores are painful blisters that form on or near the lips and inside the mouth and are often grouped together in patches. The blisters are painful and can cause itching before they break open. Once the blisters pop, they form a scab that lasts two to three weeks. The blisters heal without leaving scars. Herpes simplex virus type 1 (HSV-1) more commonly causes cold sores, and herpes simplex virus type 2 (HSV-2) more commonly causes genital herpes.

Cold sores, frequently called fever blisters, are commonly confused with canker sores, another type of blister found in the mouth.

Are Cold Sores Herpes?

Herpes simplex virus type 1 (HSV-1) causes cold sores. This viral infection is highly contagious and spreads via direct contact. HSV-1 is typically acquired during childhood and lies dormant in the nervous system. Infection is lifelong. Some people never develop symptoms from an HSV-1 infection; others will have outbreaks.

The World Health Organization estimates that 67% of people under the age of 50 have been exposed to HSV-1 and therefore have the infection.1 In a 2016 population study, Centers for Disease Control and Prevention researchers found that approximately 48% of adults between 14 and 49 were positive for HSV-1.

How Do You Get a Cold Sore?

The herpes simplex virus is spread when a person comes into direct contact with someone who has it. Sharing lip balm, toothbrushes, razors, towels, and utensils can transfer the virus from person to person. Herpes can also be spread by kissing and by performing oral sex.

Herpes simplex virus is contagious even if you do not see cold sore blisters. The first thing many people with cold sores notice is a tingling or burning sensation that develops a couple of days before the blister develops. Even at this early stage of a cold sore, it is contagious. The cold sore then progresses through stages of forming a blister, crusting, and then healing.

If you are exposed to herpes simplex virus type 1 (HSV-1) for the first time and have never been exposed before, it may take up to 20 days to notice any symptoms. Usually, the first outbreak is the worst, and symptoms can last two to three weeks.

Typical symptoms of a first-time cold sore outbreak include:

  • Fever
  • Pain with chewing, swallowing, or talking
  • Headache
  • Sore throat
  • Muscle aches
  • Swollen glands

The virus then remains dormant in your nervous system, similar to how the chickenpox virus (varicella) causes chicken pox on the first outbreak and then remains dormant. If the varicella virus reactivates, it causes a more localized vesicular rash called shingles.

Risk Factors for Cold Sores

Most people with a healthy immune system can recover from cold sores without problems. However, infants, older adults, and people with a compromised immune system may have more difficulty. People with a compromised immune system are more likely to have outbreaks of cold sores, more severe symptoms, and more frequent recurrences than people with healthy immune systems.

Cold sores are more likely to break out in the following circumstances, but it is not clear why:2

  • Stress: either physical or emotional
  • Temperature: exposure to cold wind and sunlight
  • Fluctuating hormone levels: pregnancy and the menstrual cycle
  • Fatigue: lack of sleep
  • Other viral infections: lower resistance
  • Injury to the skin: can trigger an outbreak

If you have a weakened immune system, severe cold sore symptoms, frequent outbreaks, symptoms lasting more than two weeks, or any redness or swelling around your eyes, contact an online doctor for a diagnosis and treatment. 

Preventing Cold Sores

Since most of the population has HSV-1 and most became infected in early childhood, preventing exposure to HSV-1 infections is difficult. However, if you are unsure whether you have been exposed to HSV-1 and have not had cold sores, avoid the following:

  • Physical contact with a person with a cold sore
  • Kissing or performing oral sex
  • Sharing drinking glasses, toothbrushes, utensils, lip products, and towels
  • Touching their face or yours

If you have cold sores or feel a tingling sensation that indicates a potential outbreak, talk to an online doctor at Telegra MD to get a prescription online. Antiviral medications can help reduce the severity and duration of cold sores.

How To Treat Cold Sores

Prescription antiviral medications are available to reduce the symptoms and duration of cold sore outbreaks. They are most commonly prescribed to people who have recurrent outbreaks. There are also over-the-counter and lifestyle treatments that can help reduce symptoms and recurrences.3,4

  • Antivirals: Antiviral medications block the virus from replicating
  • Cold: Try a cold compress or ice pack to reduce pain
  • Pain and symptom-relieving creams: Docosanol, benzocaine, prilocaine, lidocaine, glycine, zinc oxide, and zinc sulfate creams may reduce symptoms and speed healing
  • Sunscreen: Sunscreen can help if ultraviolet light exposure triggers your outbreaks

Treatments for cold sores are generally most effective when started as soon as you notice the tingling sensation that is associated with an upcoming outbreak. Call an online doctor and get help fast because most scientific studies show that taking medication quickly can reduce symptom severity and the duration of cold sores.

Treatment can help reduce the risk of spreading the HSV-1 virus to others, especially to small children who may not have been exposed to the virus and people who are immunocompromised and may have more serious infections.4,5


While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice.


1. James C, Harfouche M, Welton NJ, et al. Herpes simplex virus: global infection prevalence and incidence estimates, 2016. Bull World Health Organ. May 1 2020;98(5):315-329. doi:10.2471/blt.19.237149

2. Stock C, Guillén-Grima F, de Mendoza JH, Marin-Fernandez B, Aguinaga-Ontoso I, Krämer A. Risk factors of herpes simplex type 1 (HSV-1) infection and lifestyle factors associated with HSV-1 manifestations. European Journal of Epidemiology. 2001/09/01 2001;17(9):885-890. doi:10.1023/A:1015652713971

3.  Godfrey HR, Godfrey NJ, Godfrey JC, Riley D. A randomized clinical trial on the treatment of oral herpes with topical zinc oxide/glycine. Altern Ther Health Med. May-Jun 2001;7(3):49-56.

4. Opstelten W, Neven AK, Eekhof J. Treatment and prevention of herpes labialis. Can Fam Physician. Dec 2008;54(12):1683-7.

5. Spruance SL, Stewart JC, Rowe NH, McKeough MB, Wenerstrom G, Freeman DJ. Treatment of recurrent herpes simplex labialis with oral acyclovir. J Infect Dis. Feb 1990;161(2):185-90. doi:10.1093/infdis/161.2.185

 McQuillan G, Kruszon-Moran D, Flagg EW, Paulose-Ram R. Prevalence of herpes simplex virus type 1 and type 2 in persons aged 14–49: United States, 2015–2016. NCHS Data Brief, no 304. Hyattsville, MD: National Center for Health Statistics. 2018.

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