Cold Sore – Diagnosis, Symptoms, and Treatment

Doctor Nathan LeDeaux, MD

Medically reviewed by Dr. Nathan LeDeaux, MD

Medical Professional

Updated - December 22, 2020

Nathan LeDeaux is an emergency medicine physician at the University of Wisconsin and got his M.D. from Northwestern University in Chicago Illinois.

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What is a Cold Sore?

Herpes simplex virus is responsible for oral and genital herpes. Oral herpes is responsible for the formation of cold sores  These conditions are characterized by the development of vesicular (blister-like) rashes that overlie an erythematous (red) base.  The vesicles tend to form in clusters then combine to form one large sore that lasts for around a week.  These sores are painful and highly contagious.  They can also lead to significant psychological discomfort and self-consciousness due to their cosmetic appearance.

Herpes simplex virus type 1 (HSV 1) tends to cause oral herpes, also known as cold sores, whereas herpes simplex virus type 2 (HSV 2) is more likely to cause genital herpes – however, either of these viruses can cause oral or genital infection.  HSV1 infection is extremely common– a large portion of infected individuals are asymptomatic carriers who never develop any signs of infection. HSV 2 is much more likely to result in symptoms and is rarely asymptomatic.  These viruses are spread via oral or genital contact.

HSV 1 and 2 are so common as they can be spread without any sores being present. Open lesions do not need to necessarily be present as there may be asymptomatic viral sheddingLesions that are crusted over are less likely to be contagious.

What Causes Cold Sores?

Both the HSV 1 and HSV 2 viruses that lead to cold sores are spread by direct contact. This can occur regardless of if lesions are present on the skin but is much more likely to occur if there are open sores present. 

Oral herpes (herpes labialis) is typically acquired by oral contact with the virus in the oral or genital secretions of an infected individual.  This type of infection is also commonly known as a “cold sore.”  Genital infections are similarly transmitted by contact with an HSV infected area in another individual. 

In rare cases, it is possible for an individual to spread the infection from one area of themselves to another area. This almost always occurs when viral particles from an active sore contact an area on the body where the skin is broken. 

The reason that cold sores and genital herpes results in repeated breakouts of sores is not due to constant reinfections. After the initial infection, the HSV virus remains inactive in the nerve cells near the area. It can occasionally reactivate and cause sores to form before being fought back by the immune system and returning to the nerve cells. When these reactivations occur they result in the classic painful vesicular and erythematous lesions of herpes.  These form on the lips and mouth in oral herpes and in the genital region in genital herpes.

How Common is a Cold Sore?

Cold sores are extremely common in the United States. It is estimated that over 90% of individuals are infected with the HSV1 virus by age 50 and 16% with HSV2. The exact number of people that experience cold sores due to these viruses is more difficult to estimate as they may be mistaken for other conditions, only occur once or twice, or occur in other areas of the mouth.

Signs and Symptoms

The initial sign that a cold sore is developing is a sensation of tingling, numbness, pain, or discomfort in the region of the lip where they usually develop. After several days painful and itchy vesicles (fluid-filled blisters) will develop in the area. This is similar to the process that occurs in genital herpes. However, in genital herpes, the vesicles appear over a red and inflamed area of skin. 

Occasionally, patients will develop fever, fatigue, or malaise in association with their symptoms.  Patients that are immunosuppressed such as those with HIV/AIDS, organ transplants, or taking steroid medications may have more severe presentations of herpes.


The diagnosis of herpes simplex virus is based on your description of your symptoms and the presence of vesicles on physical exam.  Your doctor will occasionally confirm the diagnosis with PCR (polymerase chain reaction) testing or viral culture. Rarely, your doctor will check for HSV 1 and HSV 2 antibodies in your serum – this does not usually change management of the condition.

Cold Sore Medication and Treatment

Some herpes infections that result in cold sores self resolve, the patient may get one or two cold sores separated by several months and then never present with symptoms again. In other cases, the disease remains chronic and will result in regular cold sores every few months. 

If cold sores occur regularly they may be treated with antiviral medications that can reduce the number of times that cold sores that appear and the severity of those sores when they do appear. The most common antiviral agents are:

These agents may be taken on a daily basis in the rare cases where people experience cold sores on a weekly or monthly basis. In the majority of situations, these medications are given on an “as needed” basis. Starting these medications within 24 hours after the first cold sore symptoms begin can reduce the severity, size, and duration of the cold sore. These medications have still been shown to be effective up to 72 hours after symptoms begin. 

The best way to prevent infection is to avoid contact with the virus.  Partners of patients with HSV infection are typically encouraged to use barrier devices (eg, condoms) during oral or sexual intercourse. 


  1. Tyring SK, Baker D, Snowden W. Valacyclovir for herpes simplex virus infection: long-term safety and sustained efficacy after 20 years' experience with acyclovir. J Infect Dis 2002; 186 Suppl 1:S40. -
  2. Workowski KA, Bolan GA, Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2015. MMWR Recomm Rep 2015; 64:1. -

The above information is an educational aid only. It is not intended as medical advice for individual conditions or treatments. Talk to your doctor, nurse or pharmacist before following any medical regimen to see if it is safe and effective for you.