What Causes Canker Sores? Triggers, Relief and When to Worry

Canker sores are small, shallow ulcers inside the mouth, and they’re usually triggered by minor injury, stress, certain foods, or a sensitivity to ingredients in toothpaste. They’re very common, not contagious, and most heal on their own in 7 to 14 days. A sore that lasts longer than 2 weeks, keeps coming back in the same spot, or comes with fever should be checked by a dentist, and the team at Hummingbird Dental sees these questions often.

I know how distracting one can be. A sore the size of a lentil can make coffee, toast, and talking feel like a project. If the pain is intense, you have swelling in your face, or you can’t eat or drink, call an emergency dentist rather than waiting it out.

Children get canker sores too, often after biting their cheek or scraping their gum with a toothbrush. Our pediatric dentistry team can tell the difference between a simple ulcer and something like hand, foot, and mouth disease.

Sometimes the cause is mechanical. A rough filling edge, a chipped tooth, or a loose denture can rub the same spot over and over, which is something restorative dentistry can smooth out in one short visit. If you can’t tell what’s causing yours, a quick look from a dentist usually settles it.

Canker sore at a glance

What it looks like: a round or oval sore with a white or yellow centre and a red border.
Where: inside the cheeks or lips, under the tongue, on the soft gum.
How long: minor sores heal in 1 to 2 weeks without scarring.
Contagious? No.

What exactly is a canker sore?

The medical name is aphthous ulcer, or recurrent aphthous stomatitis when they keep coming back. It’s a break in the soft lining of the mouth, where the immune system seems to over-react to a trigger and damages a small patch of tissue.

Roughly 1 in 5 people get them at some point. They often start in childhood or the teen years and tend to become less frequent with age.

What are the 3 types of canker sores?

Minor: the most common type. Under 1 cm, heals in 1 to 2 weeks.
Major: larger and deeper, can take up to 6 weeks to heal and may leave a scar.
Herpetiform: clusters of many tiny sores that can merge. Despite the name, they’re not caused by the herpes virus.

Man having a dental checkup and mouth exam — A dental exam includes a check of the cheeks, tongue and gums.

Canker sore or cold sore: how do you tell?

People mix these up all the time, and they’re quite different.

	Canker sore	Cold sore
Location	Inside the mouth, on soft tissue	Outside, on or around the lips
Cause	Immune reaction to a trigger	Herpes simplex virus
Contagious	No	Yes, especially when blistering
Appearance	Flat ulcer, white or yellow centre	Fluid-filled blisters that crust
Early warning	Tingling or burning for a day	Tingling or itching on the lip

What causes canker sores?

There’s no single cause. Most people have 1 or 2 personal triggers, and figuring out yours is the best way to get fewer sores.

Common triggers

Minor injury: biting your cheek, a sharp chip of a tooth, aggressive brushing, a dental appliance rubbing.
Sodium lauryl sulfate (SLS): a foaming agent in many toothpastes. Some studies show fewer sores when people switch to SLS-free paste.
Acidic or spicy foods: citrus, tomatoes, pineapple, vinegar, hot sauce.
Stress and poor sleep: exam weeks and work deadlines are classic.
Hormonal changes: some people notice sores around their period.
Nutritional gaps: low iron, vitamin B12, folate, or zinc.
Food sensitivities: chocolate, coffee, nuts, and strawberries come up often.

Health conditions linked to frequent sores

Recurring or severe ulcers can sometimes point to a condition elsewhere in the body, like celiac disease, inflammatory bowel disease (Crohn’s or ulcerative colitis), Behçet’s disease, or a weakened immune system. Most canker sores have nothing to do with these. If yours are frequent, large, or come with other symptoms, though, it’s worth looking a bit deeper with your dentist and family doctor.

How can you make a canker sore heal faster?

Most minor sores heal on their own. These steps ease the pain and can shorten the timeline a little.

Rinse with salt water 3 or 4 times a day: half a teaspoon of salt in a cup of warm water. Swish gently and spit.
Try a baking soda rinse as an alternative: a teaspoon in half a cup of water.
Use an over-the-counter numbing gel with benzocaine for short-term relief. Follow the label, and don’t use benzocaine on children under 2.
Cover the sore with a protective paste or patch from the pharmacy before meals.
Avoid triggers while it heals: spicy, salty, acidic, and crunchy foods.
Switch to an SLS-free toothpaste and a soft brush.
Ask about prescription options if sores are large or frequent. Your dentist may suggest a steroid paste or a medicated rinse.

Things to skip

Putting aspirin directly on the sore. It can burn the tissue.
Rubbing alcohol or hydrogen peroxide straight from the bottle.
Picking at or scraping the sore.

How can you prevent canker sores from coming back?

You can’t always stop them, but you can usually cut down how often they show up.

Keep a simple log. Write down what you ate, how you slept, and anything that bumped your mouth in the 2 days before a sore. Patterns show up within a couple of months.
Use a soft-bristled brush and gentle pressure. Hard scrubbing scrapes the gums.
Try an SLS-free toothpaste for at least 8 weeks before judging it.
Chew slowly. Most cheek bites happen when you’re eating fast or talking while chewing.
Get rough edges fixed. Chipped teeth, worn fillings, and poorly fitting dentures are easy to repair.
Ask your doctor about blood work if sores are frequent. Correcting low iron or B12 can make a big difference.
Manage stress where you can. Easier said than done, I know. Even better sleep during busy weeks helps some people.

Are canker sores different in children?

Kids get the same type of ulcers as adults, often after a fall, a toothbrush slip, or biting their lip while numb after a filling. They usually heal in the same 1 to 2 weeks.

The bigger job for parents is telling a canker sore apart from a viral infection. Hand, foot, and mouth disease causes many small sores in the mouth along with a rash on the hands and feet and a fever. Primary herpes infections in young children can cause swollen, bleeding gums and lots of sores at once. Both of these are contagious and need a call to your doctor.

For a simple canker sore, keep your child drinking fluids, offer soft, cool foods like yogurt and smoothies, and skip citrus and salty snacks until it heals. Children’s acetaminophen or ibuprofen can help with pain if it’s appropriate for their age and weight. Check the dosing with your pharmacist.

When is a mouth sore a reason to see a dentist?

This is the most important section of the article, so I’ll be direct.

The sore hasn’t healed in 2 weeks
It’s larger than 1 cm, or keeps getting bigger
It’s painless but won’t go away (this one matters)
You have a white or red patch that doesn’t wipe off
It comes with fever, swollen glands, or feeling unwell
You get new sores before the old ones heal
You can’t eat or drink enough
A sore keeps showing up in the same spot next to a tooth or appliance

Most of these will turn out to be harmless. But a non-healing ulcer can occasionally be an early sign of oral cancer, which is why dentists check your cheeks, tongue, gums, and throat at every exam. Early detection makes treatment much simpler.

What happens at the appointment?

Your dentist asks how long the sore has been there and whether you’ve had them before.
They look at the sore and the tissue around it, and check nearby teeth, fillings, and appliances for sharp edges.
They feel the lymph nodes in your neck.
If something rough is causing it, they’ll smooth or repair it.
For sores that don’t fit the usual pattern, they may refer you for a biopsy or blood work.

Three situations we see often

The cheek biter. A university student had sores in the same spot every month. A broken edge on a back tooth was catching her cheek when she chewed. A small repair ended the cycle.

The toothpaste switch. A man in his 30s had 2 or 3 sores a month. After 8 weeks on an SLS-free toothpaste, he had 1.

The slow healer. A patient in her 60s had a painless ulcer that lasted 3 weeks. It turned out to be benign after a biopsy, and she was glad she didn’t keep waiting.

Finding a dentist who checks more than teeth

Hummingbird Dental is one of the best and most trusted choices if you’re looking for a top dentist in Richmond Hill. The clinic won the Top Choice Award for Richmond Hill Dentist in 2022, 2023, 2024, and 2025, and every checkup includes an oral cancer screening.

The office is at 10376 Yonge St #202, Richmond Hill, ON L4C 3B8. You can call +1 647-370-2024 or email info@hummingbirddental.ca. New patients and CDCP patients are welcome.

Care is available in English, Persian, Russian, Portuguese, Hindi, and Urdu, and the clinic is open 6 days a week with Saturday appointments and extended weekday evenings. If a sore is affecting your ability to eat or drink, ask about same-day emergency appointments at this emergency dental clinic in Richmond Hill.

Safety note: This article is general information and isn’t a diagnosis. Any mouth sore lasting more than 2 weeks should be examined by a licensed dentist registered with the Royal College of Dental Surgeons of Ontario (RCDSO) or by your physician.

Frequently asked questions

Are canker sores contagious?

No. You can’t spread a canker sore through kissing, sharing utensils, or any other contact. Cold sores, which form outside the lips, are contagious.

Can vitamin deficiencies cause canker sores?

Yes, in some people. Low iron, vitamin B12, folate, or zinc have all been linked to recurring sores. A simple blood test from your doctor can check, and correcting a deficiency often reduces outbreaks.

Why do I get canker sores after a dental cleaning?

Minor irritation to the gums or cheeks during cleaning can trigger a sore in people who are prone to them. Let your hygienist know, and they can be extra gentle in sensitive areas.

Do braces cause canker sores?

Brackets and wires often rub the cheeks and lips, especially in the first few weeks. Orthodontic wax over the rough spots helps a lot while the tissue toughens up.