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Cold Sores

Alternate Names: Oral Herpes, Fever Blister

Featuring content from MRI Clinic

  • About
  • The Facts

    Cold sores, also known as fever blisters, are caused by a virus. They usually appear around the mouth and on the lips. They are highly contagious but not dangerous.

    About 60% of the population have suffered cold sores at some point in their lives. On average, people who get cold sores have 2 or 3 episodes a year, but this figure can vary significantly from person to person.


    The virus that most commonly causes cold sores is herpes simplex 1, a cousin of herpes simplex 2. However, in a minority of cases, herpes simplex 2 can also cause cold sores. About 80% of the people in North America have dormant (inactive) herpes 1 virus living permanently in their body.

    The virus typically resides in a dormant state within the body's nerve cells. The body's immune system is normally able to keep the virus in its inactive state. When an infected person is exposed to a "trigger," or if the immune system is weakened, then the virus quickly multiplies and spreads down the nerve cell and out onto the skin, usually on the lips. This produces the characteristic tingling sensation and subsequent clusters of blisters.

    Specific triggers include:

    • cold weather
    • fatigue
    • fever, such as from stomach flu or other infections
    • menstrual periods
    • mental or physical stress
    • physical irritation of the lips (e.g., following a visit to the dentist)
    • sunlight or sunburn

    You can catch the virus if you come into direct contact with the cold sore blisters or the fluid inside them, which contains a high number of the viruses. This can easily happen through touching the hands of someone who has touched their blisters. It can also occur through sharing toothbrushes, cups, cutlery, face cloths, towels, lipstick, or other personal items that have been contaminated with fluid from the blisters. Once the blisters have stopped oozing or have crusted over, the person is no longer contagious.

    Symptoms and Complications

    People who get cold sores may feel some unusual sensations around the lips in the 24 hours before the blisters appear, including tingling, burning, pain, or numbness. This is called a prodrome or warning sign that cold sores will appear at these spots. The skin turns red and blisters form. They ooze a clear liquid for a few days that dries to a yellow crust over a period of about 3 to 5 days. There is usually some pain in the first few days after the cold sores break out, but this often disappears as the cold sore crusts over. Complete healing takes from 10 to 14 days.

    The condition typically causes a cluster of lesions or blisters at a site around the lips. Areas other than the lips such as the inside of the mouth, around the nostrils, or even the surface of the eyes, can also be affected. It is possible to spread the virus to other parts of your body if you touch the blisters and then touch yourself elsewhere. Cold sores inside the mouth can be problematic, interfering with talking and eating. If the virus infects the eye, it can damage the surface leading to vision loss. Very rarely, it can get into the brain, causing viral meningitis or encephalitis. The virus that causes cold sores, herpes simplex 1, can also be spread to the genitals during oral sex, leading to genital herpes.

    Herpes simplex 1 never goes away completely, so cold sores can return later on if they are triggered again. Most cold sores don't leave scars; however, if an open blister becomes infected with bacteria or the lesions tend to return at the same site, scarring may result. People with weakened immune systems tend to get more cold sores and heal slower.

    Making the Diagnosis

    Cold sores have clear and obvious symptoms, so there's no need for a battery of tests. In any case, they're rarely a matter for the doctor. However, you should see a doctor about any redness and pain in the eye, any fever over 38°C (100°F), or if there is thick yellow-white fluid coming from the blisters. This may be a sign of bacteria in the wound. Your doctor can check the fluid from the blisters for the presence of bacteria.

    A doctor should also be seen if lesions are present for more than 14 days, if the lesions occur more than 6 times a year, or if you have a cold sore in addition to a condition that weakens your immune system.

    Treatment and Prevention

    There is no known cure for cold sores. Cold sores go away on their own, but some medications may be helpful for preventing the progression of cold sores or for treating cold sore pain.

    Medications for treating cold sores:

    • Acyclovir*, a topical (applied on the skin) antiviral medication, can be applied in an ointment form to the cold sore 4 to 5 times daily.
    • Your doctor may prescribe a combination of topical acyclovir and hydrocortisone cream. This medication may stop the progression (ulceration) of the disease if used at the prodrome stage (at the first sign of symptoms, before the actual sores have appeared). It can reduce the ulceration of lesions, speed up healing by 1.4 days and reduce the duration of pain by 1 day.
    • Acyclovir, famciclovir, or valacyclovir medications can be taken by mouth to prevent the development of a cold sore. These antiviral medications can be helpful when you start taking them within one hour of when your symptoms start, or if you take them when you are exposed to known cold sore triggers (e.g., sunlight). In these cases, these medications can shorten the amount of time it takes for a cold sore to heal by 1 to 2 days.
    • Some cold sore remedies are available without a prescription. These products are usually liquids, ointments, gels, or balms. They can contain lip moisturizers and protectants to prevent cracking and excessive drying of the lips, or anesthetics that help to reduce the pain of cold sores. These products may help in relieving some symptoms, but will not stop the progression of the sores nor stop the virus replication.
    • Docosanol is an antiviral blocking agent which, if applied when prodrome symptoms arise, may help prevent the cold sore virus from spreading into healthy tissue, limiting the growth of the cold sore. It shortens healing time and the duration of cold sore symptoms including pain, burning, tingling, and itching.

    If possible, try to limit exposure to the triggers described in "Causes." For example, if sunlight tends to give you cold sores, apply a sunscreen with an SPF of 30 or higher to your lips and face 30 minutes before going out into the sun. You should also avoid tanning beds. Reducing stress by eating properly and getting enough sleep, exercise, and relaxation may also help prevent cold sores.

    If you experience frequent, severe cold sores or have a weak immune system, medication may be prescribed by your doctor on a continuous basis to suppress the virus.

    To keep from getting infected with the cold sore virus, it's important to avoid contact with it. Never touch active lesions in other people (through kissing or oral sex).

    When cold sores are active, wash your hands frequently to avoid passing the virus on to others. Try to keep away from newborns or people who have weakened immune systems, since they are more likely to develop severe cold sores.

    *All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.

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