What causes cold sores?

DoktorABC editorial team

Accurate, up-to-date health information written by our editorial team and reviewed by UK-registered prescribers.

What causes cold sores?

Cold sores are caused by herpes simplex virus type 1 (HSV-1). Most people are exposed in childhood through close contact with a family member or friend who carries the virus, often without anyone noticing at the time. Once acquired, HSV-1 stays in the body for life, though it does not cause symptoms continuously.

How do you get cold sores?

HSV-1 spreads through direct skin contact, most commonly kissing, and through sharing items that touch the mouth such as cups, cutlery, lip balm, or towels.

Many adults have no memory of "catching" the virus because it can spread even without a visible sore, through asymptomatic shedding. This means transmission can happen from someone with no symptoms at all, which is why the point of infection is rarely identifiable.

Why do cold sores keep coming back?

After the initial infection, HSV-1 travels along nerve fibres and becomes dormant in a nerve cell cluster near the site of entry. It can remain inactive for long periods before reactivating and travelling back to the skin, producing a new outbreak at the same site or nearby. This cycle of dormancy and reactivation is why cold sores tend to recur in the same general area throughout life.

What triggers cold sores?

A flare-up happens when something disrupts the balance that normally keeps the virus dormant. Common triggers include:

• Stress: one of the most frequently reported triggers
• Illness or fever: cold sores are sometimes called "fever blisters" for this reason
• Sun and UV exposure: a well-recognised trigger for outbreaks on the lips
• Hormonal changes: some women notice a pattern linked to their cycle
• Tiredness: being run down can lower the body's ability to suppress the virus
• A weakened immune system: illness or certain medications can increase reactivation
• Cold weather and wind: can irritate the lips and prompt an outbreak

Can stress cause cold sores?

Yes. Stress is one of the most commonly reported triggers. The exact mechanism is not fully understood, but stress is thought to affect immune function in ways that allow the dormant virus to reactivate. Not every outbreak has an identifiable trigger, but managing stress is a reasonable part of reducing flare-ups.

Do cold sores mean you have an STI?

This is an understandable worry, and it deserves a clear, reassuring answer. Cold sores are not evidence of a sexually transmitted infection.

Cold sores are almost always caused by HSV-1, usually acquired through non-sexual contact in childhood, such as being kissed by a family member. Genital herpes is more often caused by herpes simplex virus type 2 (HSV-2), a related but distinct virus that is sexually transmitted.

The picture is not entirely black and white. HSV-1 can also cause genital herpes, usually through oral-genital contact, and HSV-2 can occasionally cause cold sores in the same way. It is the location of an outbreak, rather than the virus type alone, that determines whether a case has a sexual transmission route.

For most people, a cold sore on the lip reflects childhood exposure to HSV-1 through ordinary social contact and has no connection to sexual activity. If you have specific concerns about genital symptoms, a prescriber or sexual health clinic can offer proper assessment.

Why do some people get recurring cold sores and others don't?

Most adults carry HSV-1, yet outbreak frequency varies enormously. Some people have frequent recurrences, others have only one or two in a lifetime, and many never develop a visible sore despite carrying the virus.

The reasons are not fully understood, but individual differences in immune response, and how effectively the immune system suppresses the virus between reactivations, appear to play a part, alongside personal exposure to triggers such as sun or stress. Carrying the virus does not guarantee frequent outbreaks.

How can you reduce cold sore outbreaks?

HSV-1 cannot be cleared from the body once acquired, but reducing exposure to known triggers can help lower how often outbreaks occur:

• Use a lip balm with SPF, particularly in strong sun or wind
• Manage stress where practical, through rest, exercise, or relaxation
• Prioritise sleep and general immune health, especially during illness
• Protect lips in cold or windy weather

Good hygiene also limits spread to others or to other parts of your own body during an outbreak. Avoid kissing and sharing cups, cutlery, towels, or lip products, and wash your hands after touching the affected area.

Warning Avoid touching your eyes after touching a cold sore, as HSV-1 can cause a serious eye infection. Seek prompt medical advice if you develop eye redness, pain, or vision changes alongside a cold sore.

What are the treatment options for cold sores?

This article focuses on what causes cold sores rather than how to treat them. For guidance on managing a current outbreak, our article on how to get rid of cold sores covers the available options, and our Cold Sores page has a full overview of treatments available through DoktorABC.

Final thoughts

Cold sores are caused by a common virus, HSV-1, that most people acquire in childhood through everyday contact rather than anything unusual or concerning. The virus can lie dormant for long periods and reactivate in response to triggers such as stress, illness, sun exposure, or tiredness. Recurrence varies widely between individuals, and having cold sores says nothing about sexual history. Understanding your own triggers is often the most useful step towards reducing how often outbreaks occur.

FAQ

The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board