Cold sores: what triggers them and how to stop one

Medically reviewed by Muhammad A Jalil, Superintendent Pharmacist (GPhC Reg No: 2236161)

Published 2026-08-22 · Clinically reviewed 2026-08-22 · Updated 2026-08-23

Why cold sores keep coming back, the triggers that wake the virus, and how to stop an outbreak fast — including when antiviral tablets beat the creams.

The moment to act on a cold sore is the tingle — that itching, burning warning in the lip before anything is visible. Antiviral treatment started in the first 24 hours, ideally within hours of the tingle, can shorten an outbreak and sometimes stop the blister forming at all; started once the blister is up, it gains you little. That is the single most useful fact about cold sores. The rest is knowing your triggers, understanding when tablets beat creams, and keeping the virus to yourself while it is active.

Why cold sores come back

Cold sores are caused by the herpes simplex virus, almost always type 1 (HSV-1). Most UK adults carry it — commonly picked up in childhood from a relative's kiss — but only some get recurrent sores, roughly one in five or six adults. After the first infection the virus retreats into the nerve that supplies the lip and goes dormant. It never leaves; outbreaks happen when something nudges it awake and it travels back down the nerve to the same patch of lip, which is why your cold sores keep returning to a favourite spot.

An outbreak follows a familiar script: tingling and itching (a few hours to a day), a cluster of small fluid-filled blisters, weeping and crusting, then healing — seven to ten days start to finish untreated.

The triggers that wake the virus

Recurrences cluster around anything that stresses the body or the lip itself:

Sunlight on the lips — one of the most reliable triggers; a UV-blocking lip balm (SPF 30+) is genuine prevention, not cosmetic advice.
Illness and fever — the "cold" in cold sore is earned; colds and flu commonly precede outbreaks.
Stress and exhaustion.
Cold wind and chapped lips in winter.
Menstrual cycle hormone shifts.
Local trauma — dental work, cosmetic lip procedures, even vigorous drying.

If you can name your pattern, you can pre-empt it: balm before the ski trip, extra sleep during crunch weeks, and treatment standing by for the moments you know are risky.

Stopping an outbreak: creams vs tablets

Antiviral creams (aciclovir 5%, applied five times a day for five days) are the familiar pharmacy option. Started at the tingle, they shave roughly half a day to a day off healing. Started late, they achieve little — worth knowing before you pay for a third tube in a winter.

Antiviral tablets are the stronger tool. A short course — aciclovir or valaciclovir, typically over one to five days at treatment doses — started at the first sign can meaningfully shorten an outbreak and, in some people, abort it before blistering. UK clinicians reserve tablets for people whose cold sores are frequent, large, slow to heal or reliably triggered (that pre-holiday sun outbreak), and for immunosuppressed people, in whom outbreaks need treating properly. Tablets in the cupboard, started at the tingle, beat any cream started at the blister.

For people with six or more outbreaks a year, daily preventive (suppressive) antiviral dosing over several months is an option worth discussing — it substantially cuts outbreak frequency for most who take it.

Pain relief is not trivial either: paracetamol or ibuprofen, and a bland lip barrier to stop cracking. Patches (hydrocolloid) hide the sore and stop you touching it, though they do not treat the virus.

If your cold sores arrive often enough that you can feel one coming as you read this, you can check your eligibility for antiviral tablets online — a UK clinician reviews your pattern and, where appropriate, prescribes a course to keep at home so treatment starts at the tingle, not after the pharmacy run.

Keeping it to yourself — and the exceptions that matter

Cold sores shed virus from tingle until fully healed. While one is active: no kissing (especially babies — neonatal herpes is a medical emergency), no sharing lip products, cutlery or towels, no oral sex (HSV-1 transmits genitally), wash hands after any touch, and leave contact lenses alone until hands are washed — HSV in the eye is serious.

See a clinician rather than self-treating if a sore lasts beyond ten to fourteen days, recurs very frequently, appears near the eye, if you are immunosuppressed or pregnant near delivery, or if a first-ever outbreak comes with fever and mouth ulcers.

Frequently asked questions

Can I get rid of the virus permanently?

No — once acquired, HSV-1 stays in the nerve for life. What you can change is expression: trigger management reduces how often it wakes, early treatment blunts each outbreak, and suppressive therapy can push frequent recurrences down to rare ones.

Are cold sores dangerous?

For healthy adults they are a nuisance, not a danger. The exceptions deserve respect: newborns, people with weakened immune systems, anyone with eczema (where the virus can spread across broken skin), and the eye. In those situations, seek medical advice promptly rather than waiting it out.

Why do I get them and my partner doesn't?

Carriage is near-universal but reactivation is not — genetics, immune differences and trigger exposure decide who gets recurrences. Your partner may carry the same virus silently. That also means they may already be immune to catching it from you, but caution during outbreaks remains the rule.

Do lysine, ice or tea tree oil work?

Ice genuinely eases pain and swelling early on. Evidence for lysine supplements is weak and for essential oils weaker still — none reliably shortens healing the way early antivirals do. By all means soothe, but do not let home remedies replace the treatment window.