What is a cold sore, and why does it come back?
A cold sore is the visible part of an infection with the herpes simplex virus. The NHS describes the pattern: tingling, itching or burning first, then one or more fluid-filled blisters over the next 48 hours, which burst, crust over and start to heal within 10 days. Most people meet the virus in childhood, and once you have it, it stays in the skin for life.
Between attacks it rests in the nerves that supply the skin, as DermNet NZ explains, and travels back along them during a recurrence. Illness, sunshine, a period and stress are the usual triggers. DermNet also lists a different one that matters here: minor trauma, surgery or procedures to the affected area. A person with no cold sore today can still carry the virus.
Why can filler, a peel or a laser set one off?
Each of them is a controlled injury to the skin around the mouth or face, and trauma is the trigger the literature recognises. A 2021 guideline from the Complications in Medical Aesthetics Collaborative says the risk of reactivation due to trauma is acknowledged, that reactivation after a cosmetic procedure is considered rare, and that there are no reliable data on how often it happens. So no honest percentage can be given here.
The reports are specific. A 2020 case report describes a 24-year-old woman with redness, crusted spots, pain and swelling on the nose after a hyaluronic acid injection, where a swab grew the virus. For resurfacing, the introduction to a 2000 trial states that reactivation after facial resurfacing has led to severe outbreaks, delayed healing and scarring. That is why the articles on lip filler, chemical peels and fractional CO2 laser each mention cold sores, and why this one sets out the detail.
Why does the history matter if there is no cold sore today?
Because it changes the plan. The same guideline is built around screening the history first, then preventing a flare and managing one if it comes. For some procedures a doctor may advise antiviral tablets before and after, and that is a prescription decision made for the individual, not something to start or skip on your own.
The main evidence for this is in resurfacing. In a randomised trial of 84 people having facial resurfacing with laser, chemical peeling, dermabrasion or a combination, an antiviral tablet started the morning before or the morning of the procedure was followed by no herpes simplex reactivation in either group over 21 days. It is a small trial of resurfacing and not of filler, so it supports the principle and does not promise an outcome. Even so, it explains why a doctor asks how often cold sores come, when the last one was, and which part of the face they appear on.
Why is treatment put off while a cold sore is active?
An active cold sore is contagious from the first tingle until it has healed completely, according to the NHS, and it can spread to other parts of the body. DermNet NZ adds that during an attack the virus can be inoculated into new areas of skin, and that minor injury helps it do so. A needle, a peel or a laser passing through or near an active sore can carry it further.
So the sensible step is to wait until the skin is clear and rebook. It is a delay of days, and it protects the result. The same waiting rule appears on the pages for chemical peels and lip filler.
What does a flare after treatment look like?
It looks like a cold sore, sometimes in an unexpected place. The NHS pattern is tingling, itching or burning, then small blisters over about two days that crust. DermNet describes small, closely grouped blisters on a red base that normally heal in 7 to 10 days without scarring, with fever, pain and tender glands in some people. In the filler case report the first signs were redness, crusted spots, pain and swelling on the treated nose.
When the picture is not clear, a swab for culture or PCR confirms it, which is how the case report was diagnosed. In that report the antiviral tablets were given for seven days and the signs improved after a week, which is why prompt review is advised. See a doctor promptly if blisters are spreading, the pain is increasing, there is a fever, a sore has not started to heal within 10 days, or the immune system is weakened (the NHS lists chemotherapy and diabetes as examples).
When is an eye involved, and what then?
DermNet NZ lists eye infection among the complications of herpes simplex: swollen eyelids, conjunctivitis, and an ulcer on the cornea. It is uncommon, and it is the reason for one firm rule. If an eye becomes red and painful, or vision blurs, while a cold sore or blisters are present on the face, a doctor needs to see you the same day. Do not wait for it to settle.
How is a cold sore different from a filler reaction or a bruise?
A bruise is a flat patch of colour that follows the needle and fades over days, and it does not blister. Filler swelling is smooth and firm and does not come with a tingle first, and the late lumps and swelling in the filler problems article need different treatment. The common side effects of toxin are covered in the side effects article. A cold sore announces itself with tingling or burning, then clusters of small fluid-filled blisters that crust.
The NHS also lists what else a sore can be: a spot or boil, a mouth ulcer, or impetigo. The 2021 guideline also covers accurate diagnosis and the conditions a flare can be mistaken for. Skin that turns white, grey or blue after filler, unusual pain, or any change in vision is a separate emergency, set out in what a doctor weighs before agreeing to filler. If you are unsure which one you have, call the clinic that treated you and describe it, with a photograph if you can.
What to tell the doctor, in one list of questions
Whether you have ever had a cold sore or fever blister, how often, when the last one was, and where on the face. What brings them on for you. Whether you have taken antiviral tablets before, and whether you have a condition or medicine that weakens your immune system. And whether you can feel a tingle or have a blister now. None of this counts against you, and all of it changes the plan.
A doctor who asks these questions is following the guideline, which also says clinicians should document the steps they take to avoid reactivation. Giving the history in full is the one step that is in your hands.
