Cold Sores vs Mouth Ulcers in Sydney: How to Tell the Difference (and When to See a Dentist)

Two of the most common patient questions a Sydney dentist hears every week are some version of "is this a cold sore or a mouth ulcer", and "do I need a prescription, or will it heal on its own". The two conditions look similar at a glance and both hurt for about the same length of time, but the treatment, the contagiousness, and the way you prevent the next one are completely different. Getting the diagnosis right at home saves a pharmacy trip and a lot of unnecessary worry.

This guide walks through how to tell a cold sore from a mouth ulcer at a glance, the Australian over-the-counter and prescription options that work for each, the cost ladder from a $7 pharmacy gel through to a dental appointment, and the red flags that warrant a same-week visit to your dentist. The decision rules are written so you can use them tonight, then come back for the deeper detail when you have time.

Key takeaways

• Cold sores appear on the outside of the lip and skin around the mouth. Mouth ulcers appear inside the mouth on the cheek, tongue, gum or palate.
• Cold sores are caused by the herpes simplex virus (HSV-1) and are contagious. Mouth ulcers are not caused by a virus and are not contagious.
• Both heal in 1 to 2 weeks. Anything that has not healed in 14 days needs a dental or medical review.
• Cold sores respond to antiviral creams (aciclovir, famciclovir) started in the tingle phase. Mouth ulcers respond to topical anaesthetics, antiseptic gels, and addressing nutritional or trauma triggers.
• Pharmacy options range from $7 (Bonjela, salt rinse) to $40+ (Famvir single-dose pack). Dental review for a non-healing sore is the highest-value step at around $80 to $130.
• See a dentist if any sore lasts more than 2 weeks, recurs more than 4 to 6 times a year, or feels indurated (firm) underneath.

The one rule that decides 90% of cases: where is it?

The single most reliable rule for telling a cold sore from a mouth ulcer is location. If the sore is on the outside of your lip, the vermilion border (the colour change between the lip and the skin), or the skin around the nose and chin, it is almost certainly a cold sore. If the sore is on the inside of your mouth (cheek lining, tongue, gum, palate, or floor of mouth), it is almost certainly a mouth ulcer (aphthous ulcer or canker sore).

There is one grey zone. The wet inner edge of the lip can host either, because the herpes virus does occasionally cross from the lip to the inside of the mouth during a primary (first ever) infection. After the primary infection, recurrent cold sores stay on the outside. So if you are an adult with a history of cold sores and a sore turns up on the inner cheek, it is far more likely to be a mouth ulcer than a herpes outbreak.

Side-by-side comparison

Use this table as a quick reference before reaching for the pharmacy aisle.

Feature	Cold sore	Mouth ulcer (canker sore)
Medical name	Herpes labialis, fever blister	Recurrent aphthous stomatitis (RAS)
Cause	Herpes simplex virus, usually HSV-1	Local trauma, immune trigger, nutritional deficiency, stress
Location	Outside lip, vermilion border, around nose and chin	Inside cheek, tongue, gums, palate, floor of mouth
Appearance	Cluster of small fluid-filled blisters that burst and crust	Single (or a few) round or oval white or yellow sore with red rim
Early warning	Tingling or burning 24 to 48 hours before the blister appears	Sudden onset, often discovered when eating something acidic
Pain pattern	Burning, itching, throbbing on the lip	Sharp pain on contact with food, toothbrush or speech
Healing time	7 to 10 days (with treatment) or up to 14 days (untreated)	7 to 14 days for minor ulcers, up to 4 weeks for major ulcers
Contagious	Yes, highly contagious through saliva and skin contact	No, not contagious
Recurs in same spot	Often, because the virus reactivates from a local nerve	Sometimes, but rarely in the exact same spot twice in a row
Crusts over	Yes, forms a yellow-brown crust before falling off	No, heals from the inside out without a crust

Cold sores explained

Cold sores are caused by the herpes simplex virus, type 1 (HSV-1) in the vast majority of cases and rarely by HSV-2. The Australian Department of Health and healthdirect both estimate that around 70 to 80% of Australian adults carry HSV-1 by age 30, although most never get visible cold sores. The virus is contracted through close contact, usually in childhood from a parent or sibling, then lives dormant in the nerve ganglion of the face for life. Periodic reactivation produces the recognisable blister cluster.

The four stages of a cold sore

Recognising the stage matters because antiviral treatment is most effective in the first 24 hours.

1. Tingle / prodrome (12 to 48 hours). A burning, itching or tight feeling on a small patch of lip skin, often the same spot as last time. Nothing is visible yet. This is the most important window for treatment.
2. Blister (1 to 2 days). Small fluid-filled vesicles form, often in a cluster. The lip swells. This is the most contagious stage.
3. Weep and crust (3 to 5 days). The blisters burst, weep clear or yellow fluid, then dry into a yellow-brown crust. Still contagious.
4. Healing (5 to 10 days). The crust falls off and pink new skin appears underneath. No longer contagious once the area is fully dry and skin-coloured.

What triggers a cold sore?

The virus reactivates when the immune system is briefly distracted or when local lip tissue is stressed. Common Sydney-specific triggers include:

• UV exposure. Sydney's summer sun is one of the most common cold sore triggers. A day at Bondi or Manly without lip sunscreen is a classic precipitant. Even reflected glare from concrete or water can be enough.
• Wind chill and dry air. Winter mornings, ski trips, and aeroplane cabins all dehydrate the lip and increase the chance of an outbreak.
• Physical or emotional stress. Exam season, work deadlines, family pressure.
• Other infections. A head cold, gastro, or COVID can trigger an outbreak, which is where the old name "fever blister" comes from.
• Hormonal cycles. Many women find premenstrual hormonal shifts trigger a sore.
• Dental work. Long appointments with the lip stretched (extractions, crown preparations, orthodontic adjustments) can trigger a sore in patients with a history. If you know you are prone, mention it to your dentist before the appointment.
• Trauma to the lip. A bitten lip, an accidental scratch, or a hot food burn can all wake the virus.

Australian treatment options for cold sores

Treatment falls into three tiers. The earlier you treat, the shorter the outbreak.

Tier	Product / approach	Approx. cost	Best for
Tier 1: OTC topical antiviral	Zovirax cream (aciclovir 5%), Vectavir (penciclovir 1%)	$13 to $20	Tingle phase or first day of blister. Apply every 4 hours for 5 days.
Tier 2: Pharmacist-only single-dose oral antiviral	Famvir Once (famciclovir 1500mg, single dose)	$35 to $45	Tingle phase, single tablet stops or shortens the outbreak. Pharmacist consultation required (Schedule 3).
Tier 3: Prescription oral antiviral (frequent or severe outbreaks)	Aciclovir 400mg tablets, valaciclovir 500mg tablets, suppressive dosing	$10 to $40 with PBS; up to $80 private	Patients with 6+ outbreaks per year, immunocompromised patients, or those with severe outbreaks.
Adjunct	Compeed Invisible Cold Sore Patch	$15 to $20	Once the blister has formed, patches reduce visibility, prevent picking, and act as a barrier.
Symptom relief	Paracetamol 1g every 4-6 hours, ibuprofen 200-400mg every 6-8 hours	$5 to $10	Throbbing pain or swelling. Use within standard OTC dose limits.

Aciclovir, famciclovir and valaciclovir are all in the same drug family and broadly equivalent in efficacy. The advantage of famciclovir as Famvir Once is the convenience of a single dose at the tingle phase. The disadvantage is cost. The Australian Commission on Safety and Quality in Health Care lists Famvir Once as a Schedule 3 medicine (pharmacist-only), so you can buy it without a doctor's appointment but the pharmacist will ask a few questions before handing it over.

Reducing contagion during an outbreak

A cold sore is most contagious between the blister and crust stages, but viral shedding can occur throughout. Reasonable precautions:

• No kissing or oral sex while a sore is active.
• No sharing of cups, cutlery, lip balm, towels or razors.
• Wash hands carefully after touching the sore. Herpes can spread to the eye (herpetic keratitis) which is genuinely dangerous.
• Do not put contact lenses in with hands that have touched the sore.
• Be especially careful around newborns. Neonatal herpes can be severe. If you have an active sore and a newborn, wear a surgical mask and wash hands rigorously.

Mouth ulcers explained

Mouth ulcers (recurrent aphthous stomatitis, RAS) are a different beast. The technical definition is a break in the mucous membrane lining the mouth, exposing the underlying tissue. The mechanism is not viral. The current best understanding is a local T-cell mediated immune reaction triggered by some combination of trauma, nutritional deficiency, hormonal shift, stress, or an underlying systemic condition.

Three forms by size

• Minor ulcers (80% of cases). Under 10mm, usually on movable mucosa (cheek lining, tongue edge, inner lip). Heal in 7 to 14 days without scarring.
• Major ulcers (10% of cases). Over 10mm, often deeper, can occur on the palate or pharynx. Take 2 to 6 weeks to heal and may scar.
• Herpetiform ulcers (10% of cases). Confusingly named, despite not being herpes. Multiple tiny ulcers (10 to 100) that may coalesce. Heal in 7 to 14 days.

Common triggers

Most patients can identify at least one of these factors in the week before an ulcer appears.

• Local trauma. Accidentally biting the cheek while eating, a hot chip burn, a sharp tooth edge, an orthodontic bracket, or a poorly-fitting denture. Trauma is the single most common cause of one-off ulcers.
• Nutritional deficiency. Low iron, vitamin B12, folate or zinc. Patients with recurrent ulcers should have these checked with a blood test through their GP.
• Sodium lauryl sulphate (SLS) in toothpaste. The foaming agent in most supermarket toothpastes irritates some patients' oral mucosa. Switching to an SLS-free toothpaste (Sensodyne Pronamel, Biotene, or Colgate Sensitive Pro-Relief) resolves recurrent ulcers in a meaningful subset of patients.
• Stress and sleep deprivation. Final exams week is famously a peak ulcer time.
• Hormonal cycles. Some women get ulcers premenstrually.
• Food triggers. Chocolate, citrus, tomatoes, walnuts, pineapple, and gluten are commonly reported triggers in susceptible patients.
• Systemic conditions. Coeliac disease, Crohn's disease, ulcerative colitis, Behcet's disease, and lupus can all present with recurrent mouth ulcers. Anyone with frequent ulcers plus gut symptoms, joint pain or skin rashes deserves a GP referral.
• Smoking cessation. Counterintuitively, quitting smoking can trigger a wave of ulcers for a few weeks.

Australian treatment options for mouth ulcers

There is no cure for recurrent mouth ulcers, but treatment can shorten an episode and reduce frequency.

Tier	Product / approach	Approx. cost	Best for
Tier 1: Topical anaesthetic and antiseptic	Bonjela, Ora-Sed, SM-33, Difflam mouth rinse	$7 to $15	Pain relief, single small ulcer. Apply 4 times daily.
Tier 1b: Salt water rinse	1 teaspoon salt in a glass of warm water, 3 to 4 times daily	Free	Reduces local inflammation, encourages healing. Use alongside or instead of OTC gels.
Tier 2: Antiseptic mouthwash	Curasept, Savacol (chlorhexidine 0.12%)	$15 to $25	Multiple ulcers or recurrent episodes. Use twice daily for 7 to 10 days.
Tier 3: Topical steroid (prescription)	Triamcinolone in Orabase, Kenalog in Orabase	$20 to $40	Major ulcers or stubborn cases. Apply 2 to 4 times daily directly to the ulcer.
Tier 4: Systemic steroid or immunomodulator (specialist)	Prednisolone, colchicine, pentoxifylline	Variable	Severe recurrent cases or underlying systemic disease. Specialist supervision required.
Adjunct	Vitamin B12, iron, folate replacement if deficient	$10 to $30	Patients with confirmed deficiency on blood test.
Adjunct	SLS-free toothpaste (Sensodyne Pronamel, Biotene, Tom's of Maine)	$8 to $14	Patients with recurrent ulcers triggered by foaming agents.

What to avoid while you have an ulcer

The pain itself is unavoidable, but you can stop it getting worse.

• Acidic foods (citrus, tomato, vinegar dressings) sting and slow healing.
• Spicy foods (chilli, hot sauce) irritate the open lesion.
• Crunchy textures (chips, crusty bread, hard crackers) can re-injure the ulcer.
• Toothpastes with SLS foam will sting and may extend the duration.
• Alcohol-based mouthwashes (Listerine) often sting too much to be useful during an active ulcer.

Cost-of-prevention vs cost-of-treatment ladder

For both conditions, prevention is dramatically cheaper than treatment. The maths makes the case for itself.

Step	Approx. cost	What it covers
SPF 30+ lip balm (Sun Bum, Cancer Council, Blistex Daily Lip Conditioner)	$5 to $12	Prevents UV-triggered cold sores. Reapply every 2 hours outdoors.
SLS-free toothpaste swap	$8 to $14	Reduces recurrent ulcers in susceptible patients.
Bonjela / SM-33 / salt rinse	$7 to $15	First-line treatment for a mouth ulcer.
Zovirax cream	$13 to $20	First-line treatment for a cold sore in the tingle phase.
Famvir Once	$35 to $45	Single-dose oral antiviral for an early cold sore.
GP consultation + blood test for recurrent ulcers	$45 to $90	Identifies B12, iron, folate or zinc deficiency. Often bulk-billed.
Dental review for a non-healing sore	$80 to $130	Item 011 consultation. Rules out anything sinister and gives a treatment plan.
Specialist oral medicine referral	$200 to $350	Severe recurrent cases or suspected systemic disease.
Biopsy of a non-healing sore	$300 to $600	Rarely needed, but the gold standard for ruling out oral cancer when a sore has not healed in 3 weeks.

Skipping the prevention rung and going straight to the dental review rung is a tenfold cost jump for the same patient. The lesson is to keep an SPF lip balm in your bag and an SLS-free toothpaste in your bathroom.

Red flags: when to see a dentist or doctor

Most cold sores and mouth ulcers heal on their own within two weeks. The two-week rule is the most important figure in this article. Any sore that has not healed in 14 days needs an in-person review, because oral cancer can look indistinguishable from a stubborn ulcer to the naked eye.

Book a same-week dental review if any of the following apply:

• The sore has not healed in 2 weeks.
• You are getting more than 4 to 6 mouth ulcers per year.
• The sore feels firm or indurated (hard) when you press the surrounding tissue.
• The sore is on the side or underside of the tongue, the floor of the mouth, or the soft palate (higher-risk anatomical sites for oral cancer).
• You have a single very large ulcer (over 10mm) that is not responding to treatment.
• You have a cold sore that is spreading rather than healing.
• You have eye involvement (redness, light sensitivity) with a cold sore. This needs same-day GP or eye care.
• You have a fever, swollen glands, and multiple sores together (could be primary herpes, hand foot and mouth, or another condition needing diagnosis).
• You have weight loss, night sweats, unusual fatigue, gut symptoms, or skin rashes alongside recurrent ulcers (could indicate a systemic cause).
• You are immunocompromised, pregnant, or undergoing chemotherapy and have any persistent sore.

Indurated firmness underneath an ulcer is the single most important physical sign. A benign ulcer feels soft. A worrying ulcer feels like there is a small pebble underneath it.

Prevention strategies that actually work

Preventing cold sores

• SPF 30+ lip balm every day, regardless of weather. The single highest-value intervention. Reapply every 2 hours when outdoors.
• Treat the tingle, not the blister. Keep Zovirax cream or Famvir Once at home if you are prone. Starting treatment in the prodrome can prevent the visible blister entirely.
• Suppressive antiviral therapy. Patients with 6 or more outbreaks per year benefit from low-dose daily aciclovir or valaciclovir under GP supervision. Reduces frequency by around 70 to 80%.
• Lysine supplements. Evidence is mixed, but some patients report fewer outbreaks on 1000mg daily of L-lysine. Low cost, low risk.
• Sleep, stress management, illness avoidance. Vague advice but consistent across the literature.
• Pre-dental appointment heads-up. If you know dental work triggers your cold sores, mention it to your dentist. A prophylactic dose of antiviral the day before a long appointment can prevent an outbreak.

Preventing mouth ulcers

• Switch to an SLS-free toothpaste. Sensodyne Pronamel, Biotene, Colgate Sensitive Pro-Relief, or Tom's of Maine. The most underrated intervention.
• Check for deficiencies if you get more than 4 to 6 ulcers a year. GP blood test for iron studies, B12, folate, and zinc. Often bulk-billed.
• Address sharp dental edges promptly. A chipped tooth or a rough filling that keeps catching your tongue or cheek will keep producing ulcers in the same spot. Get it smoothed at your next check-up.
• Wax over orthodontic brackets that rub. Most orthodontic patients have a tube of dental wax at home for this reason.
• Identify and avoid food triggers. If you suspect citrus, gluten, walnuts or chocolate, keep a 4-week food diary alongside ulcer dates and look for patterns.
• Manage stress. Vague but real. Sleep, exercise, and not running on cortisol are all protective.
• Brush carefully, especially with a new brush. A stiff-bristled toothbrush against the soft tissue at the back of the mouth can trigger ulcers in susceptible patients. Soft bristles only.

Common myths about cold sores and mouth ulcers

A few persistent misconceptions are worth addressing because they delay sensible treatment.

"Cold sores mean I have an STI." No. The vast majority of cold sores are HSV-1, contracted in childhood through innocent contact (a parent's kiss, a shared cup at preschool). HSV-1 and HSV-2 are related viruses but not the same condition.

"If I pick the crust off it will heal faster." The opposite. Picking re-opens the wound, spreads the virus, extends healing, and risks bacterial infection. Leave the crust alone.

"Vitamin C tablets cure mouth ulcers." No evidence supports this. In fact, the citric acid in chewable Vitamin C tablets often irritates the ulcer and slows healing.

"Mouth ulcers are caused by 'heat' or 'bad blood'." Cultural belief, not biology. The cause is local trauma plus an immune trigger, not body temperature or diet "heat".

"Burning the ulcer with hydrogen peroxide will cure it." Concentrated hydrogen peroxide is caustic and will cause a chemical burn on top of the ulcer. Diluted (1.5%) peroxide rinses are sometimes used clinically but are not first-line for routine ulcers.

"It's just a cold sore, I don't need to be careful." Cold sores are contagious. The virus can spread to a partner, a child, the eye (herpetic keratitis), or fingers (herpetic whitlow). Treat them respectfully.

How Lumi Dental approaches sores that won't heal

If a sore is still there after 2 weeks, the priority at a dental review is diagnosis, not just symptom relief. A typical Item 011 consultation at Lumi Dental for a non-healing sore involves a careful intra-oral and extra-oral examination, palpation of the lesion and surrounding lymph nodes, photography for monitoring, and a clear plan: either watchful waiting with a recheck in 7 to 10 days, a referral for blood work, a topical steroid prescription, or, where the lesion has features of concern, a same-week referral to an oral medicine specialist or oral surgeon for biopsy.

Patients with frequently recurring cold sores often benefit from a short conversation about pre-procedure antiviral prophylaxis before any longer appointment (a crown preparation, a wisdom tooth removal, or an orthodontic adjustment) because the mechanical stress of holding the lip retracted is a common trigger. Patients with recurrent mouth ulcers benefit from a sharp-edge audit of their existing teeth and restorations because small mechanical irritants are often the missing piece.

Frequently asked questions

Can a dentist tell the difference between a cold sore and a mouth ulcer just by looking?

In almost all cases, yes. Location, appearance, and the patient's history of similar episodes are usually enough for a confident diagnosis. The diagnostic uncertainty cases are typically large or atypical lesions, lesions in unusual sites, or non-healing lesions that need to be ruled out as oral cancer with a biopsy.

Are cold sores and genital herpes the same thing?

They are caused by closely related viruses (HSV-1 and HSV-2) and can occasionally cross between sites, especially during oral-genital contact. Most cold sores are HSV-1 acquired in childhood. Most genital herpes is HSV-2 acquired through sexual contact, although HSV-1 genital cases are becoming more common.

How long should I wait before going to the dentist for a sore?

Two weeks. If a sore (cold sore or mouth ulcer) has not visibly healed in 14 days, book a review. If the sore is large, indurated (feels firm underneath), or on the side of the tongue or floor of the mouth, do not wait the full two weeks.

Can I get a cold sore from sharing a water bottle?

Yes, although the risk is lower than from direct lip contact. Saliva from someone with an active sore can carry the virus, and shared water bottles, cups, lip balm, and cutlery are all possible transmission routes. Avoid sharing these during an active outbreak.

Why do I always get ulcers in the same spot?

Usually because there is a mechanical irritant in that spot, such as a sharp edge of a tooth, a rough filling, a wisdom tooth pressing against the inside of the cheek, or an orthodontic bracket. A dentist can spot the irritant in a few minutes and smooth or shield it.

Are mouth ulcers a sign of cancer?

The vast majority of mouth ulcers are not cancer. The features that make a sore worth investigating for oral cancer are: a non-healing duration of more than 2 to 3 weeks, induration (firmness underneath), a location on the side or underside of the tongue or floor of the mouth, no obvious cause or trigger, and a patient history of smoking or heavy alcohol use. A dental review easily distinguishes a routine ulcer from a concerning one.

Is Famvir Once worth the cost over Zovirax cream?

It depends on how reliably you catch the outbreak in the tingle phase. Famvir Once is one tablet, no repeat dosing, and can stop the outbreak entirely if taken early. Zovirax cream is cheaper but needs reapplication every 4 hours for 5 days. Patients who get cold sores often and recognise the tingle clearly often prefer Famvir. Occasional sufferers usually find Zovirax adequate.

Can I kiss my partner if I have a mouth ulcer (not a cold sore)?

Yes. Mouth ulcers are not contagious. Cold sores are.

Do over-the-counter cold sore patches work?

Patches such as Compeed Invisible Cold Sore Patch do not shorten the outbreak the way antivirals do, but they reduce visibility, prevent picking, and act as a physical barrier to reduce transmission. They are a useful adjunct, not a primary treatment.

Should I cancel my dental appointment if I have a cold sore?

Most dentists prefer to reschedule a routine appointment until a cold sore has fully crusted or healed, both for patient comfort and to reduce cross-contamination risk. Emergency or urgent appointments still go ahead with extra precautions. Ring the practice and tell them what you have.

The takeaway

Cold sores and mouth ulcers feel similar to the patient but behave very differently. Location alone identifies most cases. Cold sores are viral, contagious, and respond to early antivirals plus daily SPF lip protection. Mouth ulcers are immune and trauma-driven, not contagious, and respond to topical relief, nutritional checks, and identifying mechanical or product triggers. Both conditions heal in about two weeks, and any sore that has not healed in 14 days deserves a dental review, because the only way to confidently rule out anything more serious is an in-person examination.

At Lumi Dental we see this question constantly, often during routine check-ups when a patient finally mentions the recurring sore they have lived with for years. A 15-minute conversation about triggers, products, and prevention frequently solves what years of pharmacy-shelf trial-and-error has not. If you have a non-healing sore, a recurring pattern you cannot make sense of, or a fast-approaching dental appointment that you know tends to trigger a cold sore, book a consultation and we will take it from there.

Sources: healthdirect Australia — Cold sores; Australian Commission on Safety and Quality in Health Care — Famvir for cold sores; NPS MedicineWise — Famvir; Guideline for the Diagnosis and Treatment of Recurrent Aphthous Stomatitis for Dental Practitioners; Australian Dental Association fee guide.

Related reading from Lumi Dental: Mouth ulcers: causes, treatments and when to see a dentist, Oral cancer screening in Sydney, Dental anxiety and how to overcome fear of the dentist, Sydney dental emergency guide.