A cold sore usually announces itself with a tingle or itch at the edge of the lip, hours before anything is visible. It's one of the most common skin infections there is — most adults carry the virus responsible, whether or not they've ever had a visible outbreak — yet the practical questions still trip people up: what actually causes it, why does it keep coming back, and does the cream from the pharmacy really make a difference if you use it early enough? This guide covers what a cold sore is, how it progresses, over-the-counter and prescription treatment, and when it's worth seeing a doctor rather than waiting it out.

What causes a cold sore

Cold sores are caused by herpes simplex virus type 1 (HSV-1). Most people acquire the virus in childhood, commonly through close contact such as a kiss from a family member, and often without ever noticing a first outbreak. Once acquired, HSV-1 doesn't leave the body — it travels along a nerve and stays dormant in nerve tissue near the site of infection, usually around the mouth, between flare-ups.

What brings it back varies from person to person, but recognised triggers include sunlight and UV exposure (a classic cause of "sun-triggered" cold sores), a cold, flu or other illness that stresses the immune system, tiredness and poor sleep, emotional stress, and hormonal changes such as those around a menstrual cycle. Identifying your own personal triggers — even loosely — can help you catch an outbreak earlier or take sensible precautions, like using a lip balm with sun protection.

The stages of a cold sore and how long they last

A typical cold sore moves through a fairly predictable sequence: a tingling or itching sensation at the site (usually the first sign, lasting hours to a day or so), a small cluster of fluid-filled blisters, the blisters bursting and weeping, and finally a crust or scab forming as the area heals. Left untreated, the whole cycle usually resolves within 7 to 10 days without leaving a scar.

Treatment doesn't dramatically shorten a fully developed blister overnight, but starting early — right at that first tingle — gives antiviral treatment the best chance of shortening the outbreak and reducing how uncomfortable it feels.

Over-the-counter treatment options

For most otherwise healthy adults, a cold sore can be managed with products available from a pharmacy without a prescription:

  • Topical aciclovir cream — applied as soon as tingling starts, several times a day for the course length on the packaging. It works by interfering with how the virus replicates in skin cells, and is most useful in that early window before a blister has fully formed.
  • Cold sore patches — hydrocolloid patches that cover the sore, may reduce the urge to pick at it, and can make the area less visible while it heals.
  • Simple pain relief — paracetamol or ibuprofen (following the product licence and any conditions that make them unsuitable for you) if the area is sore.

Cream applied late — once the blister is fully formed and weeping — has less to offer, since the virus has already done most of its replicating in that area. That's why the timing of application matters more than which specific cream you choose.

When a prescription antiviral is used

Most cold sores don't need anything beyond topical treatment. A clinician may consider oral antiviral tablets — such as aciclovir or valaciclovir — in situations where outbreaks are frequent, unusually severe or slow to heal, or where a person has a condition or medication that weakens their immune system. Oral tablets work throughout the body rather than just at the surface, which can make them more effective for these harder-to-manage cases.

Some people with very frequent outbreaks are offered a low daily dose of an oral antiviral on an ongoing basis, known as suppressive therapy, to reduce how often flare-ups happen. This is a clinical decision based on how much cold sores are affecting someone, not something to start without a consultation.

Reducing how often cold sores come back

You can't remove HSV-1 from the body once it's there, but a few sensible habits can lower the chances of triggering an outbreak or spreading it further:

  • Use a lip balm containing sun protection if sunlight is a trigger for you.
  • Avoid touching or picking at a cold sore, and wash your hands after applying cream.
  • Don't share cups, cutlery, towels or lip products while a sore is present.
  • Avoid kissing and, if you perform oral sex, avoid it while a sore is visible — HSV-1 can be passed to the genital area.
  • Prioritise sleep and manage stress where practical, since both are commonly reported triggers.

If outbreaks are becoming frequent enough to disrupt your life, it's worth raising this with a clinician rather than simply treating each one as it comes — suppressive therapy or a review of your triggers may help.

When to see a doctor

Most cold sores are straightforward and don't need medical attention beyond a pharmacy consultation. See a doctor if a sore hasn't started healing after 10 days, is unusually large, painful or keeps spreading, or if you have a condition or treatment that weakens your immune system. Cold sores near or affecting the eyes need urgent assessment, since HSV infection of the eye can threaten sight if left untreated. Anyone in close contact with a newborn baby should also be cautious and seek advice, as HSV-1 can cause serious illness in very young infants.

If you're unsure whether a mark around your mouth is a cold sore or something else — such as impetigo or a reaction to a product — a pharmacist or GP can help confirm what you're dealing with.

It's also worth flagging your first-ever cold sore to a pharmacist or GP if it's accompanied by a fever, sore throat or widespread mouth ulcers, particularly in a young child. A first HSV-1 infection can occasionally be more extensive than later recurrences, and a clinician may want to check nothing else is going on before treating it as a straightforward cold sore.

Cold sores vs other mouth or lip problems

Not every mark near the mouth is a cold sore. Angular cheilitis (cracking at the corners of the mouth), a mild allergic reaction to a lip product, or a minor injury can all look similar at a glance. A cold sore is usually distinguished by that early tingling sensation followed by a cluster of small fluid-filled blisters in roughly the same spot each time — most people who get recurrent cold sores notice they tend to reappear in a similar area, since the virus reactivates from the same nerve pathway. If a sore doesn't follow this pattern, looks different from previous episodes, or isn't responding to the usual approach, it's worth getting it checked rather than assuming it's the same thing as before.

Getting started with Farmeci

If your cold sores are frequent, severe, or over-the-counter cream isn't giving you enough relief, a Farmeci clinician can review your history and discuss whether a prescription antiviral — used episodically or as ongoing suppressive therapy — is appropriate for you.