Cold Sores

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What Are Cold Sores?

Cold sores are small, painful, fluid-filled blisters or sores that appear on the lips, mouth, or nose that are caused by a virus. The sores can be painful and usually last a few days. Unlike most viral infections, the cold sore virus is not completely eliminated by the body defenses. For this reason, cold sores often recur.

Picture of cold sores
Picture of cold sores; SOURCE: CDC/Dr. Hermann

What Causes Cold Sores?

The virus that causes cold sores is known as the herpes simplex virus (HSV). There are two types of HSV, type I and type II. Cold sores are usually caused by type I.

Herpes simplex is a contagious oral virus. The virus is spread from person to person by kissing or other close contact with sores or even from contact with apparently normal skin that is shedding the virus. Infected saliva is also a means of spreading the virus. The most contagious period is when a person has active blister-like sores. Once the blisters have dried and crusted over (within a few days), the risk of contagion is significantly lessened. However, a person infected with HSV can pass it on to another person even when a cold sore is not present. This is because the virus is sometimes shed in saliva even when sores are not present. Despite popular myth, it is almost impossible to catch herpes (cold sores) from contaminated surfaces, towels, or washcloths.

After the first infection, the virus enters the nerve cells and travels up the nerve until it comes to a place called a ganglion, which is a collection of nerve cells. There, it resides quietly in a stage that is called "dormant" or "latent." In more active stages, the virus starts multiplying again and travels down the nerve to the skin, causing blisters on the lips known as cold sores. The exact way this happens is not clear, but it is known that some conditions seem to be associated with recurrences, including

Sometimes there is no apparent cause of the recurrence.

Cold sores have a tendency to recur in more or less the same place each time. Such recurrences may happen often (for example, once a month) or only occasionally (for example, once or twice a year).

Cold Sores Causes, Remedies, Diagnosis

Cold Sore Symptoms and Signs

  • Some patients have a "prodrome," which is when certain symptoms occur before the actual sores appear. The prodrome to herpes infections typically involves a burning or tingling sensation that precedes the appearance of blisters by a few hours or a day or two. As the cold sore forms, the area may become reddened and develop small fluid-filled blisters. Several of these small blisters may even come together and form one large blister. Cold sores are mildly to moderately painful.
  • When cold sores recur, the blisters stage is usually short. Blisters dry up rapidly and leave scabs that last anywhere from a day to several days, depending on the severity of the infection.
Cold Sore on the mouth

Herpes of the Mouth

Oral Herpes (HSV-1) Symptoms and Signs

  • Incubation period: For HSV-1, the amount of time between contact with the virus and the appearance of symptoms, the incubation period, is two to 12 days. Most people average about four days.
  • Duration of illness: Signs and symptoms will last two to three weeks. Fever, tiredness, muscle aches, and irritability may occur.

When to Seek Medical Care

The very first time someone gets a cold sore (known as the "primary" attack), the symptoms can be severe. In some people, the first attack of herpes is associated with fever, swollen glands, bleeding gums, and many painful sore(s) around the mouth (gingivostomatitis) and nose. These signs and symptoms may last several days. The sores heal completely in two to six weeks, usually without scarring. Virus can be recovered from the saliva for days after the lesions heal. Because many people acquire the virus early in life, primary herpes usually happens during childhood. If the attack is severe, a doctor should be contacted. The doctor may prescribe medications that can shorten the attack, preferably during the prodrome. These medications are most effective if taken early in the attack. Difficulty in eating and drinking may lead to dehydration, which may also require medical attention.

Recurrent cold sores usually do not require medical care. A few people may have cold sores that come so frequently that a doctor will prescribe a daily medication to reduce the number of attacks. It is not possible to predict for how long the treatment should continue, because the virus continues to live in the ganglion. Thus, stopping suppressive treatment is largely a trial and error procedure.

People who have very weak immune systems from chemotherapy or other causes may have very severe outbreaks of cold sores. These look like the primary attacks described above. Medical care should be sought promptly to avoid complications.

Rarely, herpes simplex may infect the brain. People with this condition usually have fever and confusion. This infection requires hospitalization and intravenous antiviral medications.

In a few people, cold sores will be associated with painful skin lumps on the front of the legs known as "erythema nodosum." Erythema nodosum can be self-limited and usually goes away on its own in three to six weeks. If this happens, the person should be seen by a doctor to find out if prescription medications might make the condition go away faster.

Cold Sores Causes, Remedies, Diagnosis

Cold Sores Diagnosis

The diagnosis of cold sores is usually based on the appearance of the lesions. Usually, no laboratory test is needed because most sores that look like cold sores are cold sores. Occasionally, mouth sores known as canker sores may be mistaken for cold sores. However, canker sores occur inside the mouth whereas recurrent cold sores usually occur on the lips. If there is a question about the diagnosis, a variety of tests are available, including viral culture and polymerase chain reaction (PCR). To do these tests, a swab is rubbed over an active blister. Swabbing the blister in an attempt to culture the virus in the lab works best in the first 24-48 hours before the blister has crusted over. There is also a way to test for herpes DNA in a swab that has been rubbed on the sore. This type of testing is called a "polymerase chain reaction" or PCR for short. PCR testing is very good at detecting herpes virus, but it is not as readily available as culture.

Blood tests for antibodies are not very useful, because finding antibodies to herpes just means that the body has been exposed to this virus at some point in the past. It does not tell if the current sore is due to herpes.

If the diagnosis is in doubt, the best approach is to encourage the person to see a doctor at the first sign of a sore. That will allow the clinician to see active lesions that can be tested by culture or PCR. If a delay is anticipated in being seen, a mobile phone or camera can be used to take pictures of the lesion to show to the physician.

Self-Care at Home for Cold Sores

Cold sores contain the HSV-1 virus. People with cold sores should wash their hands often, especially after touching the face. Cups and eating utensils should not be shared with another person. A home remedy like cold compresses may relieve pain temporarily. Lysine, an amino acid, was once thought to be a potential treatment, but the facts supporting this are unconvincing. Other home remedies, including vitamin E oil, vitamin supplements, nail polish remover, and changes in diet, have not been well studied.

Cold Sores Causes, Remedies, Diagnosis

Medical Treatment for Cold Sores

  • There are several medications to reduce the duration or symptoms of cold sores. Some are available without a prescription (over the counter), and others require a prescription from a doctor. Some are topical (meaning that they are creams or ointments rubbed directly on the sore), and others are taken in pill form.
  • Over-the-counter (OTC) topical medications: Most topical OTC products provide symptomatic relief only. This means that they make people feel better but they do not decrease healing time. Using topical anesthetics that contain benzocaine (5%-20%), lidocaine (0.5%-4%), tetracaine (2%) or dibucaine (0.25%-1%) will help relieve burning, itching, and pain. Examples are Lipactin gel and Zilactin. It is important to keep in mind that these topical agents have a short duration of action, usually only lasting 20-30 minutes. Skin protectants, such as allantoin, petrolatum, and dimethicone-containing products, help keep the lesion moist and prevent cracking of the lesion. Sunscreen-containing lip balms may also help to prevent additional outbreaks if the sun is a precipitating factor. For additional pain relief, using ibuprofen (Advil) or acetaminophen (Tylenol) may be beneficial. Do not apply any topical steroids, such as hydrocortisone, to the lesions.
  • Docosanol 10% cream (Abreva) is the only over-the-counter product that has been shown to decrease healing time when applied at the first sign of recurrence (for example, a tingling sensation). Docosanol is applied five times per day until the lesion is healed. Possible side effects include rash and itching at the site of application.
  • Prescription-strength topical medications: Treatment with topical acyclovir (Zovirax 5% cream) or penciclovir (Denavir 1% cream) will reduce healing time by approximately half a day and decrease pain associated with the lesion. Topical treatment is limited in its effectiveness because it has poor penetration into the site of replication of the virus and therefore is restricted in its healing ability. Acyclovir cream should be applied five times per day for four days, and penciclovir cream should be applied every two hours while awake for four days.
  • Prescription-strength pills: The current FDA-approved medications used in the treatment of herpes simplex virus in adults are acyclovir (Zovirax), valacyclovir (Valtrex), and Famciclovir (Famvir). These oral medications have been shown to decrease the duration of the outbreak, especially when started during the "prodrome" (symptom onset before the actual condition becomes fully evident). The medications are generally well tolerated with few side effects. Headache, nausea, and diarrhea may occur in some people. For simple, recurrent cold sores in adults, valacyclovir is given as 2 grams orally every 12 hours for one day, and famciclovir is given as 1,500 milligrams orally for one dose. Acyclovir is given as 400 mg orally five times per day for five days. Pregnant women and nursing mothers should contact their physicians or pharmacists prior to using any medication. Famciclovir and valacyclovir are not FDA approved for use in children under 12 years of age with cold sores.

Follow-up for Cold Sores

Cold sores usually go away on their own and do not require follow-up. Unusual symptoms such as fever, confusion, or eye involvement should prompt a visit to a doctor.

How to Prevent Cold Sores

It is not always possible to prevent an outbreak of cold sores. In people who have attacks triggered by sunburn, using sunscreen or avoiding heavy sun exposure may reduce the number of attacks. Taking daily prescription antiviral medications (see above) can significantly reduce the frequency of cold sores. However, the vast majority of people do not have enough attacks to justify taking a daily medicine.

What Is the Prognosis for Cold Sores?

There is no cure for cold sores. The herpes simplex virus hides deep in nerve roots and cannot be wiped out by medicines that are currently on the market. The U.S. Food and Drug Administration warns that there are products that falsely advertise that they are getting rid of herpes for good, but these claims are fraudulent. The virus will continue to live in the nerve root despite treatment, and most people will continue to get cold sores off and on throughout their life.

Herpes can be spread from a cold sore to another area of the body, which is called "autoinoculation." For example, touching a cold sore on the lip can cause herpes of the finger (herpetic whitlow). Autoinoculation occurs most commonly at the time of primary infection when viral shedding is high and the immune system is still gearing up to contain it. The antibodies that are made after primary infection are usually -- but not always -- successful in preventing autoinoculation during recurrent attack. To prevent spreading the virus to other parts of the body, it is important to wash hands after touching the sore. One question that is sometimes asked is Do cold sores cause genital herpes or are they contagious to genital areas? Although it is possible to autoinoculate the genital area, most cases of genital herpes are acquired through sexual transmission.

A more serious complication is ocular herpes, which causes sores and severe pain around the eye. Ocular herpes is also caused by autoinoculation. If untreated, ocular herpes can lead to serious eye damage or even blindness.

Reviewed on 11/21/2017

Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care


Cernik, C., K. Gallina, and R.T. Brodell. "The Treatment of Herpes Simplex Infections: An Evidence-Based Review." Arch Intern Med 168.11 June 9, 2008: 1137-1144.

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