TL;DR
A typical canker sore heals in one to two weeks on its own and doesn't need a doctor. You should get seen the same day if you can't swallow your own saliva, can't open your mouth fully, have one very swollen tonsil, or (for a young child) can't drink. Get seen within a week or two if a mouth sore hasn't clearly improved after three weeks, doesn't hurt, has a hard or raised edge, sits on the side of the tongue or floor of the mouth, or comes with genital sores or eye symptoms. UK cancer guidelines recommend an urgent referral for any unexplained mouth ulcer lasting more than three weeks (NICE NG12). And make a routine appointment if you get canker sores often, even if each one looks normal, because recurring sores are often linked to deficiencies or conditions that are easy to test for. Bring a record of your sores and ask for specific blood tests by name.
Most Canker Sores Don't Need a Doctor
A typical canker sore:
- Shows up on the soft, movable lining of the mouth: inside the lips and cheeks, under or on the sides of the tongue, or on the soft palate
- Is round or oval, with a yellow-white center and a red rim
- Hurts more than you'd expect for its size
- Starts getting better within about a week and is gone within two
- Looks like the ones you've had before
If that describes yours, home care is fine. See The Fastest Way to Heal a Canker Sore for what helps.
Same Day: Go Now
These aren't canker sores, or they're complications that need treatment fast. Go to urgent care or the emergency room if you can't reach your dentist or doctor today.
Signs of a throat abscess
A peritonsillar abscess is a pocket of infection next to a tonsil. It can look like a sore throat or a sore in the back of the mouth at first. Warning signs:
- One tonsil much bigger than the other
- The uvula (the small hanging tissue at the back of the throat) pushed off to one side
- Trouble opening your mouth
- A muffled, "hot potato" voice
- Drooling because swallowing hurts too much
- High fever with severe throat pain
It needs drainage and antibiotics. Don't wait it out.
Signs of a spreading neck infection
Severe throat or neck pain with a stiff neck, swelling or redness spreading into the neck, trouble breathing, or high fever that's getting worse quickly. This is rare but dangerous.
A young child who won't drink
A toddler or young child with many sores throughout the mouth, swollen and bleeding gums, fever, and drooling usually has a first herpes infection (primary herpetic gingivostomatitis), not canker sores. The danger is dehydration. If your child isn't drinking or has very few wet diapers, they need to be seen the same day. Antiviral medication helps most if it's started early. See Canker Sores in Kids.
Eye pain or vision changes with mouth sores
Mouth sores together with a red, painful eye, light sensitivity, blurred vision, or new floaters can mean inflammation inside the eye, which can happen in Behçet's disease. Eye inflammation can damage vision quickly. See an eye doctor urgently.
Within a Week or Two: Book It Now
These don't need an emergency room, but they shouldn't wait months for a routine checkup.
A sore that hasn't improved in three weeks
This is the most important rule in this article. Any mouth ulcer that has been there three weeks without clearly getting better (smaller, less painful, edges filling in) needs to be looked at. The UK's national cancer guideline recommends that people with unexplained mouth ulceration lasting more than three weeks be considered for a referral to be seen within two weeks (NICE NG12).
Most sores that last this long aren't cancer. They're usually injuries from a sharp tooth or appliance, a slow-healing large canker sore, or a condition like lichen planus. But three weeks is the line where something other than a canker sore becomes likely enough to rule out. See Canker Sore Won't Heal for what a non-healing sore can be and what happens at the appointment.
A sore that doesn't hurt
Canker sores hurt. A mouth ulcer that's painless or barely sore, especially if it's been there a while, isn't typical. Painless ulcers can be early cancers, long-standing injuries you've gotten used to, or a primary syphilis sore (a chancre), which is usually a single, firm, painless ulcer.
A hard or raised edge
Run your tongue around the edge. A canker sore's border is soft and flat. If the edge feels firm, thick, or raised, like there's something solid under it, that needs to be checked.
A sore in a higher-risk spot
Canker sores can show up almost anywhere on the soft lining of the mouth, but cancers favor certain places. The side of the tongue toward the back and the floor of the mouth (under the tongue) are the most common sites for mouth cancer. A sore there that doesn't follow the usual canker sore pattern deserves a lower threshold for getting it checked. The same goes for a sore on the lower lip that's crusting, bleeding, or not healing, especially if you've had a lot of sun exposure.
A red or white patch that won't go away
Not every concerning spot is an ulcer. A flat red patch, a white patch that doesn't wipe off, or a mixed red-and-white patch that lasts more than a couple of weeks should be looked at by a dentist.
Mouth sores plus genital sores
Recurring mouth ulcers together with sores on the genitals can be a sign of Behçet's disease, an inflammatory condition that can also affect the eyes, skin, joints, and blood vessels. It's worth a referral to a rheumatologist. See Canker Sores and Behçet's Disease.
Your first really big ulcer
A canker sore wider than about 1 cm, particularly the first time you've had one, is worth getting looked at. These "major" ulcers take longer to heal, can scar, and are more often linked to an underlying condition. See Giant Canker Sores.
Routine Appointment: Get a Proper Workup
You don't need an urgent visit for these, but they're worth scheduling.
You get canker sores often
If you're getting sores most months, several at a time, or they're interfering with eating, that's worth investigating even if each one looks ordinary. Recurring canker sores are linked to a number of things that are simple to test for and treat, and many people go years without anyone checking. See Why Doctors Dismiss Canker Sores.
You want something stronger than over-the-counter treatments
A dentist or doctor can prescribe a steroid gel or paste that reduces pain and helps sores heal faster, and some dentists offer in-office treatments. See How to Get Rid of a Canker Sore Fast.
Mouth sores plus other symptoms
Canker sores alongside any of the following are worth mentioning, because together they can point to an underlying condition:
- Diarrhea, belly pain, or weight loss (celiac disease, Crohn's disease)
- Fatigue, pale skin, or shortness of breath (iron, B12, or folate deficiency)
- Joint pain, rashes, or sun sensitivity (lupus, Behçet's)
- A child with fever that comes back on a regular schedule along with mouth sores (PFAPA syndrome)
Which Kind of Doctor to See
Dentist. The best first stop for most mouth sores. Dentists look inside mouths all day, can spot a sharp tooth or appliance causing an injury, can prescribe steroid gels, and can biopsy or refer if something looks wrong.
Primary care doctor. Good for the blood work, and for looking at the rest of your health if there are other symptoms.
Oral medicine specialist. A dental specialty focused on diseases of the mouth lining. This is the person to see for frequent or severe canker sores that aren't responding to treatment, unusual sores, or sores linked to a wider condition. There aren't many of them, so you may need a referral to a dental school or hospital clinic.
Oral surgeon. For biopsies of suspicious spots.
Gastroenterologist. If you have gut symptoms or a positive celiac test.
Rheumatologist. If Behçet's disease, lupus, or another inflammatory condition is suspected.
Eye doctor. Urgently, for eye pain, redness, or vision changes with mouth sores.
Pediatrician or pediatric dentist. For children. A pediatric rheumatologist if PFAPA is suspected.
Use our directory to find dentists and oral medicine specialists who treat recurrent canker sores.
Get connected with local help →What to Bring to the Appointment
Canker sores have a way of healing right before the appointment. You'll get much more out of the visit if you show up with evidence.
- Photos. Take a clear photo of each sore when it's at its worst, with the date. Use your phone's flash and pull your lip or cheek out of the way.
- A log. For a couple of months, write down when each sore appeared, where it was, roughly how big, and how long it took to heal. Even a note in your phone works. This turns "I get them a lot" into "I've had nine in the last two months, three at a time."
- Anything that seems connected. Your period, stressful weeks, new foods, new toothpaste, new medications, illness.
- A list of your medications and supplements, including over-the-counter ones and how long you've taken each.
- What you've already tried, and whether it helped.
- Other symptoms, even ones that seem unrelated: digestive problems, fatigue, joint pain, skin rashes, genital sores, eye problems.
Tests Worth Asking For
If you get canker sores often, these are reasonable to ask about:
- Complete blood count (CBC) to look for anemia and check white blood cell counts
- Ferritin, which measures iron stores. A normal CBC doesn't rule out low iron. See Iron Deficiency and Canker Sores.
- Vitamin B12, and methylmalonic acid if B12 comes back borderline. See B12 for Canker Sores.
- Folate. See Folate and Canker Sores.
- Zinc. See Zinc for Canker Sores.
- Celiac screen (tissue transglutaminase IgA plus total IgA). Keep eating gluten until you're tested, or the test can come back falsely negative. See Gluten and Canker Sores.
- Vitamin D, if your doctor is open to it. See Vitamin D and Canker Sores.
It helps to ask for these by name and say why: "I get recurring canker sores, and I've read that low ferritin, B12, folate, and zinc are linked to them. Could we check those, along with a celiac screen?" Most doctors will order them when asked directly. If you're told it's "just stress," it's reasonable to say you'd still like to rule out the deficiencies first.