A tingling sensation on your lip that turns into a small blister within a day or two is a familiar and unwelcome pattern for many people. Cold sores are extremely common, caused by a virus that stays in the body long-term, which is why they tend to return during certain triggers.
Quick Answer
Cold sores are caused by the herpes simplex virus, which remains dormant in the body after the first infection and reactivates during stress, illness, sun exposure, or fatigue. They typically heal within one to two weeks.
Why Cold Sores Keep Coming Back
Once you’re infected with the herpes simplex virus, usually type 1, it travels to nerve tissue and stays there permanently, even after the initial sore heals. Certain triggers can reactivate the virus, bringing on a new outbreak in the same general area.
Common Triggers
Stress
Physical or emotional stress can weaken the immune system’s ability to keep the dormant virus suppressed, making an outbreak more likely.
Illness or Fever
Being run down with a cold, flu, or fever is a classic trigger, which is why cold sores are sometimes called “fever blisters.”
Sun Exposure
UV light can trigger reactivation directly, which is why cold sores often appear after a day in strong sun, particularly without lip protection.
Fatigue and Poor Sleep
Being run down from lack of sleep reduces the immune system’s ability to keep the virus suppressed, increasing the chance of an outbreak.
Hormonal Changes
Some people notice outbreaks linked to their menstrual cycle, likely due to hormone-related shifts in immune function.
Weakened Immunity
Anything that suppresses the immune system, including certain illnesses or medications, can make reactivation more likely.
Minor Trauma to the Lip
Dental work, lip injury, or even chapped lips can occasionally trigger a nearby outbreak.
Stages of a Cold Sore
- Tingling stage: A day or two of itching or tingling before anything is visible
- Blister stage: Small, fluid-filled blisters form and can merge together
- Weeping stage: Blisters burst and ooze, which is when the sore is most contagious
- Crusting stage: A scab forms as healing begins
- Healing stage: The scab falls away and skin fully heals, usually within one to two weeks total
How to Manage a Cold Sore
- Apply an antiviral cream at the first sign of tingling for the best results
- Use a cold compress to ease discomfort and reduce swelling
- Avoid touching the sore to reduce the risk of spreading it to other areas or people
- Use SPF lip balm regularly if sun exposure is a known trigger
- Manage stress and prioritise sleep to reduce outbreak frequency
When to See a Doctor
- Cold sores are severe, very frequent, or slow to heal
- The sore spreads near the eye
- You have a weakened immune system
- Symptoms include fever or swollen glands alongside a large outbreak
FAQs
Are cold sores the same as canker sores?
No. Cold sores appear on the outer lip and are caused by a virus, while canker sores appear inside the mouth and are not contagious or virus-related.
Can I prevent cold sores completely?
Not entirely, since the virus stays in the body permanently, but managing known triggers like stress, sun exposure, and fatigue can reduce how often outbreaks happen.
When are cold sores most contagious?
They are most contagious during the blister and weeping stages, when fluid is present, so avoiding close contact during this time is important.
Conclusion
Cold sores are caused by a common virus that reactivates under certain triggers like stress, illness, and sun exposure. While the virus cannot be cured, managing your personal triggers can meaningfully reduce how often outbreaks occur.
Last reviewed: August 2026. This article is for general information only and does not replace professional medical advice. If outbreaks are severe or frequent, please speak to a GP.
