Canker Sores (Aphthous Ulcers)
If you have a small, painful, white or yellow sore with a red border on the inside of your lip or cheek, it's very likely a canker sore. They're one of the most common things people ask us about. They hurt more than their size would suggest, and they almost always heal on their own.
A canker sore is a shallow ulcer that forms on the soft, moveable lining of the mouth. Most are smaller than a pencil eraser and heal in 7 to 14 days without treatment. They are not contagious and they're not caused by the cold sore virus. Ice, a salt-water rinse, an over-the-counter numbing gel or protective paste, and skipping acidic or crunchy foods will get most people through it. See a dentist if a sore lasts longer than two weeks, is larger than about 1 cm (⅜ inch), keeps coming back every few weeks, or comes with fever or sores elsewhere on the body.
Not sure this is what you have? Answer five quick questions in our mouth spot checker. It compares what you see with more than 40 common conditions and tells you how soon to have it looked at.
What a canker sore looks like
Most people notice a canker sore by feel before they see it. A day or so ahead of time there's often a burning or tingling spot. Then a small, round or oval sore shows up with:
- A shallow center that's white, gray, or yellow. This is a thin layer of fibrin, not pus, so there's nothing to drain.
- A red, inflamed border around it.
- Pain out of proportion to its size, especially when you eat, talk, or brush near it.
Canker sores are open sores from the start. They don't begin as blisters, which is one of the easiest ways to tell them apart from cold sores.
Where canker sores show up (and where they usually don't)
Here's a trick dentists use. Canker sores almost always form on the loose, moveable tissue of the mouth: the inside of the lips and cheeks, the underside and sides of the tongue, the floor of the mouth, and the soft palate toward the back. They rarely form on tissue that's firmly attached to bone, like the gums right around your teeth or the hard roof of your mouth.
A cluster of small sores on your gums or hard palate points to something else, most often a flare of the cold sore virus inside the mouth. That's one reason we like to see sores that don't fit the usual pattern.
The three types of canker sores
Doctors split canker sores into three groups by size and pattern. Most people only ever get the first kind.
| Minor | Major | Herpetiform | |
|---|---|---|---|
| How common | Most cases (roughly 7 or 8 in 10) | About 1 in 10 | Up to 1 in 10 |
| Size | Under about 5 mm, smaller than a pencil eraser | Over 1 cm, sometimes much larger | Pinpoint, 2–3 mm each |
| How many | 1 to 5 at a time | Usually 1 or 2 | Clusters of 10 to 100 that can merge |
| Healing time | About 1–2 weeks | Several weeks, sometimes 6 or more | 1–2 weeks, sometimes longer |
| Scarring | No | Often | Occasionally, if they merge |
| What to do | Home care is usually enough | See a dentist. Prescription treatment helps a lot. | See a dentist |
Canker sore vs. cold sore: how to tell the difference
People mix these two up all the time, and it matters, because one is contagious and the other isn't. Our cold sore guide covers the virus side in detail.
| Canker sore | Cold sore (fever blister) | |
|---|---|---|
| Where | Inside the mouth, on soft moveable tissue | Usually outside, on or around the lips. Inside, on gums or hard palate. |
| Starts as | An open sore | A cluster of tiny fluid-filled blisters that break and crust |
| Cause | Not fully understood. Immune reaction plus triggers. | Herpes simplex virus (HSV-1, sometimes HSV-2) |
| Contagious? | No | Yes, especially while blisters are present |
| Crusts over? | No | Yes, on the lip |
| Heals in | 7–14 days (minor) | 7–10 days |
| Helpful treatment | Topical steroids, protective pastes, numbing gels | Antiviral medicine, best started at the first tingle |
Other things that can look like a canker sore
Most mouth sores really are canker sores. A few other conditions can look similar at first glance:
- Bite or injury ulcer. A sore right where you bit your cheek, scraped with a chip, or rub against a sharp tooth or broken filling. It heals once the source of irritation is fixed.
- Cold sores inside the mouth. Clusters of tiny blisters on the gums or hard palate that break into small sores. Read about cold sores.
- Oral thrush. White patches that wipe off and leave a red, sore spot. Read about oral thrush.
- Burns. "Pizza burn" on the roof of the mouth peels and stings, then heals in about a week.
- Hand, foot, and mouth disease or herpangina. Viral infections, mostly in young kids, with fever and many small sores toward the back of the mouth (and a rash on hands and feet with HFMD).
- Oral lichen planus. Lacy white lines, sometimes with raw or ulcerated areas that come and go for months. Read about oral lichen planus.
- Medication-related ulcers. Some drugs can cause mouth ulcers, including certain blood pressure and heart medicines, NSAIDs, some chemotherapy, and bisphosphonate tablets that dissolve in the mouth.
- A sore that doesn't heal. Rarely, a persistent ulcer turns out to be something more serious, including oral cancer. That's why we want to see any sore that hasn't healed in two weeks. Learn the signs of oral cancer.
What causes canker sores?
No one knows the exact cause. The best explanation is that the immune system overreacts to the lining of the mouth in people who are prone to it, and certain triggers set it off. They tend to run in families. Somewhere between a quarter and a half of people who get them have a relative who does too.
Common triggers
- Minor injury: biting your cheek, aggressive brushing, a sharp tooth edge, braces, dental work, or a rough chip.
- Stress and lack of sleep.
- Hormonal changes, such as around a menstrual period.
- Certain foods for some people: citrus, pineapple, tomatoes, chocolate, coffee, nuts, strawberries, and spicy or very salty foods.
- Toothpaste with sodium lauryl sulfate (SLS), a foaming agent. Switching to an SLS-free paste helps some people.
- Low iron, vitamin B12, folate, or zinc.
- Quitting smoking. Oddly, sores often flare for a while after someone stops. Don't let that talk you out of quitting. It settles down.
Medical conditions linked to canker sores
Most people with canker sores are otherwise healthy. Frequent, severe, or new-in-adulthood sores can sometimes go along with:
- Celiac disease
- Crohn's disease or ulcerative colitis
- Behçet's disease (mouth ulcers plus genital ulcers, eye inflammation, or skin problems)
- Anemia or vitamin deficiencies
- HIV or other immune system problems
- PFAPA syndrome in young children (repeating fevers every few weeks with mouth sores, sore throat, and swollen neck glands)
If your sores fit that description, we'll talk with you about bloodwork and coordinate with your physician.
How long do canker sores last?
A minor canker sore follows a fairly predictable course:
Pain usually peaks in the first three or four days and fades well before the sore is completely gone. Major canker sores are a different story and can hang around for weeks. If any sore is still there after two weeks, get it checked.
Canker sore home care, day by day
Nothing makes a canker sore vanish overnight. The goal is to protect it, keep it comfortable, and avoid making it worse.
At the first tingle (or the first day you see it)
- Cover or treat it early. Steroid gels and protective pastes work best when they go on early. If you have a prescription from a past outbreak, this is the time to use it.
- Ice it. Let an ice chip slowly melt against the spot. It numbs the area and cuts down swelling.
- Take a regular pain reliever if you need it, like acetaminophen or ibuprofen, following the label. Don't hold an aspirin against the sore. That causes a chemical burn.
Days 2 through 5 (the sore stage)
- Rinse 3–4 times a day. Use warm salt water (½ teaspoon salt in a cup of water) or a baking soda rinse (1 teaspoon in ½ cup water). Swish and spit. It keeps the area clean and calms it down.
- Numb it before meals. A dab of over-the-counter numbing gel 10–15 minutes before eating makes food much easier.
- Eat soft, bland, cool foods. Yogurt, eggs, oatmeal, smoothies without citrus, pasta, mashed potatoes. Use a straw for drinks.
- Skip the troublemakers. Citrus, tomato, pineapple, vinegar, spicy food, salty snacks, chips, crusty bread, and alcohol-based mouthwash.
- Keep brushing. Use a soft brush and be gentle around the sore. A clean mouth heals faster.
Until it's gone
- Keep up the rinses and gentle brushing.
- Switch to an SLS-free toothpaste if you get these often.
- Write down what you ate and what was going on in the day or two before it started. A pattern is often obvious after two or three episodes.
Myths worth skipping
- Don't try to pop or scrape it. There's no fluid inside. You'll only make it bigger and more painful.
- Don't put aspirin, straight hydrogen peroxide, rubbing alcohol, or salt directly on it. These can burn the tissue.
- Don't take leftover antibiotics. Canker sores aren't a bacterial infection.
Over-the-counter canker sore treatments
These are available at any pharmacy. None will cure a canker sore, but the right one can make the week a lot more pleasant.
| Type | What it does | Things to know |
|---|---|---|
| Numbing gels and liquids (benzocaine, lidocaine) | Temporary pain relief, good before meals | Wears off in 20–60 minutes. Do not use benzocaine in children under 2. The FDA warns it can cause a rare but dangerous blood condition (methemoglobinemia). |
| Protective pastes, patches, and films | Form a barrier over the sore so food and teeth don't rub it | Dry the area with a cotton swab first so it sticks. Great for sores on the lip or cheek. |
| Oxygenating rinses (diluted hydrogen peroxide, about 1.5%) | Cleanse the sore and may reduce soreness | Use as directed. Swish and spit, don't swallow. Not for long-term daily use. |
| Antiseptic rinses | Keep the area clean | Choose alcohol-free. Alcohol stings. |
| Pain relievers (acetaminophen, ibuprofen) | Overall pain control | Swallow them. Never hold a pill against the sore. |
What a dentist can do for canker sores
For a single minor sore, you probably don't need us. If your sores are large, frequent, or keeping you from eating, there's a lot we can do.
- Make sure it's actually a canker sore. We look at the size, location, and pattern, and check for a sharp tooth, worn filling, or appliance that might be causing it.
- Prescription topical steroids. A steroid gel or paste applied to the sore, or a steroid mouth rinse when there are several. This is the most effective treatment for most people, especially if it's started at the first tingle. Many of our patients keep a tube at home.
- Prescription rinses that reduce pain and help keep the sores clean.
- In-office treatment to seal or cauterize a painful sore. This often gives quick relief.
- Fix the source of irritation: smooth a rough tooth, repair a broken filling, or adjust a denture or night guard.
- Look for an underlying cause. If you get sores often, we may recommend bloodwork for iron, B12, folate, and zinc, and work with your physician to rule out conditions like celiac disease or inflammatory bowel disease.
- Biopsy a sore that doesn't heal or doesn't behave like a canker sore, so we know exactly what we're dealing with.
Severe, nonstop cases sometimes need medicine taken by mouth. When that happens, we coordinate with your physician or an oral medicine specialist.
How to prevent canker sores
If you're prone to them, you probably can't stop canker sores completely. Most people can cut down how often they show up.
- Try an SLS-free toothpaste for a couple of months. A 2019 review of small studies found people had fewer sores, shorter sores, and less pain after switching.
- Use a soft toothbrush and brush gently.
- Get sharp edges fixed. Chipped teeth, rough fillings, and poking wires are easy for us to smooth.
- Figure out your food triggers with a simple diary.
- Manage stress and sleep as best you can, especially around exams, deadlines, or travel.
- Ask about vitamin B12. In one placebo-controlled trial, people with frequent canker sores who took 1,000 mcg of B12 under the tongue each night had far fewer outbreaks after about five or six months, even if their B12 levels were normal to begin with. It's inexpensive and low-risk for most people. Check with your physician first.
- Keep a prescription on hand if your sores are frequent, so you can treat at the very first tingle.
Canker sores in children
Canker sores often start in childhood, and kids get them for the same reasons adults do. A child's mouth sores deserve a closer look if there's:
- Fever, drooling, or refusing to eat or drink. Young children can get dehydrated quickly.
- Many sores toward the back of the mouth, or a rash on the hands and feet (possibly hand, foot, and mouth disease).
- Swollen, bleeding gums with lots of sores, which can be a first herpes infection.
- Mouth sores that come back with a fever on a regular schedule every few weeks.
For comfort: cold foods like popsicles and yogurt, plenty of fluids, and children's acetaminophen or ibuprofen as directed. Avoid benzocaine numbing products in children under 2.
When to see a dentist about a canker sore
Make an appointment if:
- A sore hasn't healed in 2 weeks
- It's larger than about 1 cm (⅜ inch), or growing
- You get new sores before the old ones heal, or outbreaks are getting more frequent
- You started getting canker sores for the first time as an adult
- It sits right next to a sharp tooth, broken filling, or appliance
- The sore is on your gums or the roof of your mouth, or it looks different from your usual canker sores
Get care quickly if:
- You have a high fever, feel very sick, or have sores on the eyes, genitals, or skin
- Pain is so bad you can't drink enough fluids
- You also have diarrhea, belly pain, or unexplained weight loss
- Your immune system is weakened (chemotherapy, transplant medicines, HIV)
Canker sore FAQ
How long do canker sores last?
Most minor canker sores heal in 7 to 14 days. The first three or four days are usually the worst. Major canker sores (larger than about 1 cm) can last several weeks and sometimes leave a scar. Anything still there after two weeks should be checked.
Are canker sores contagious?
No. You can't catch a canker sore from kissing or from sharing a drink, and you can't give one to someone else. Cold sores, which are caused by the herpes simplex virus, are contagious, which is why it helps to know which one you have.
What's the fastest way to get rid of a canker sore?
There's no instant fix, but treating it early makes the biggest difference. A prescription steroid gel applied at the first tingle can shorten a sore and ease the pain. At home: protective pastes, numbing gel before meals, salt-water or baking soda rinses, and avoiding acidic, spicy, and crunchy foods. For a very painful sore, a dentist can sometimes treat it in the office for quicker relief.
Why do I keep getting canker sores?
Some people are simply prone to them, and it often runs in families. Common triggers include minor injuries, stress, hormonal changes, certain foods, SLS toothpaste, and low iron, B12, folate, or zinc. Less often, frequent sores are linked to celiac disease, inflammatory bowel disease, or another medical condition. If you get them every few weeks, it's worth a visit to look for a cause.
Can a canker sore be cancer?
A true canker sore isn't cancer and doesn't turn into cancer. It heals on its own. Oral cancer can sometimes show up as a sore that looks like a canker sore but doesn't heal. That's why the two-week rule matters: if a sore hasn't healed in two weeks, have it looked at. Most of the time it's something harmless, but it should be checked.
Should I pop a canker sore?
No. A canker sore isn't a blister or pimple. The yellow-white layer is part of the healing tissue, not pus. Picking or scraping it makes it bigger, more painful, and slower to heal.
What foods should I avoid with a canker sore?
Acidic foods (citrus, tomatoes, pineapple, vinegar), spicy foods, salty snacks, and sharp or crunchy foods like chips and crusty bread. Some people find chocolate, coffee, nuts, or strawberries set off new sores. Soft, cool, bland foods are easiest while it heals.
Are canker sores caused by stress?
Stress doesn't cause them on its own, but it's one of the most common triggers in people who are already prone to them. Many people notice sores during exams, busy stretches at work, or after a few nights of poor sleep.
Can a vitamin deficiency cause canker sores?
It can contribute. Low iron, vitamin B12, folate, and zinc are all linked to canker sores. If you get them often, simple bloodwork can check. In one study, nightly vitamin B12 cut down outbreaks even in people whose levels were normal, so it's worth discussing with your doctor.
Is a canker sore the same as a mouth ulcer?
A canker sore is one kind of mouth ulcer, and by far the most common kind. "Mouth ulcer" just means any open sore in the mouth. That also includes bite injuries, burns, viral sores, and sores from medical conditions.
Does salt water help canker sores?
Salt-water rinses keep the area clean and many people find them soothing. They won't make a sore disappear, but they're a cheap, safe part of home care. Rinse with warm water and dissolved salt. Don't put salt directly on the sore.
When should I see a dentist for a canker sore?
If it's been more than two weeks, if it's larger than about 1 cm, if you get them all the time, if it's right next to a sharp tooth or broken filling, or if you have fever or sores elsewhere on your body. Also come in if you just aren't sure what it is. We'd rather take a look.
Not sure what that spot is?
If you're in the Jacksonville area and a mouth sore isn't healing, keeps coming back, or just doesn't look right to you, we're glad to take a look. Dr. Henley sees patients with all kinds of mouth sores, from everyday canker sores to ones that are harder to pin down.
Related conditions
Sources
- Plewa MC, Chatterjee K. Aphthous Stomatitis. StatPearls. National Library of Medicine; updated 2023.
- Volkov I, et al. Effectiveness of vitamin B12 in treating recurrent aphthous stomatitis: a randomized, double-blind, placebo-controlled trial. J Am Board Fam Med. 2009;22(1):9–16.
- Alli BY, Erinoso OA, Olawuyi AB. Effect of sodium lauryl sulfate on recurrent aphthous stomatitis: a systematic review. J Oral Pathol Med. 2019;48(5):358–364.
- U.S. Food and Drug Administration. Risk of serious and potentially fatal blood disorder prompts FDA action on oral over-the-counter benzocaine products. 2018.
- Edgar NR, Saleh D, Miller RA. Recurrent aphthous stomatitis: a review. J Clin Aesthet Dermatol. 2017;10(3):26–36.
This page is for general education and isn't a diagnosis. Every mouth is different. If you have a sore you're worried about, see your dentist or physician. Photos marked Wikimedia Commons are used under their listed licenses and may have been cropped or resized.