Canker Sores: Why You Keep Getting Them

It starts with a tiny sting — the inside of your cheek or lip catches on your tooth, or your morning orange juice suddenly tastes like battery acid on one spot. You look in the mirror and see it: a small round white or yellowish crater with a red border, about the size of a pencil eraser, delivering pain completely out of proportion to its size.

That's a canker sore (medically, an aphthous ulcer), and roughly one in five Americans develops them at some point. They aren't dangerous, they aren't contagious, and they'll heal on their own — but they can make eating, talking, and even brushing your teeth miserable for a week or more. Here's what sets them off, and what actually helps.

First: it's not a cold sore

This is the most common confusion in American medicine cabinets, and it matters — the treatments are completely different.

Cold sores (fever blisters) are caused by the herpes simplex virus. They appear on the outside of the mouth — usually on or around the lips — they're contagious, and they're treated with antivirals like Abreva.

Canker sores appear inside the mouth — on the inner cheeks, lips, tongue, gums, or the floor of the mouth. They are not caused by a virus, they are not contagious, and antivirals do nothing for them. You cannot give one to your partner by kissing, and you cannot catch one from a shared cup.

Inside your mouth: canker sore. On the lip border: cold sore.

Why you keep getting them

No one knows the single cause of canker sores. But researchers have a clear roster of suspects — and most recurrent sufferers can point to at least two:

Minor mouth injuries. Biting your cheek (usually in the exact same spot, infuriatingly), aggressive brushing, a sharp tortilla chip, dental work, or braces rubbing the same area. The sore shows up a day or two after the injury, as if on a timer.

Stress. This is the trigger most people recognize in hindsight — the sore appears during a brutal work week, right before a wedding, or in the middle of exam season. Stress doesn't directly cause canker sores, but it shifts your immune response in ways that make outbreaks more likely.

Acidic, salty, or spicy foods. Oranges, grapefruit, pineapple, tomatoes, vinegar-based dressings, and chips don't cause canker sores — but they irritate an existing one so badly that you notice it for the first time and blame the food. There's an exception: if a sore appears within a day of eating a specific food, pay attention. A small number of people have a true sensitivity to particular foods — chocolate, coffee, and wheat are the classic examples.

Toothpaste ingredient: sodium lauryl sulfate (SLS). SLS is the foaming agent in most mainstream American toothpastes (Colgate, Crest, and store brands). Small studies found that switching to SLS-free toothpaste reduced both the frequency and pain of canker sores. If you get them monthly, it's one of the easiest experiments you can run — SLS-free options are in every U.S. pharmacy.

Nutrient gaps. Low vitamin B12, iron, folate, or zinc is associated with recurrent sores. If you get them constantly and feel tired or run down, a basic blood panel can rule a deficiency in or out.

Hormonal shifts and immune quirks. Some women notice sores arriving on a predictable monthly schedule. And people with celiac disease, Crohn's disease, or other autoimmune conditions get them more often — which is another reason recurring sores are worth mentioning to a doctor, not just enduring.

What actually speeds healing

A canker sore typically lasts 7 to 10 days, and nothing will make it vanish overnight. But you can meaningfully shorten the misery.

1. Salt-water rinse. Half a teaspoon of salt in 8 ounces of warm water, swished for 30 seconds a few times a day. It reduces inflammation and keeps the sore clean.
2. Baking soda rinse. One teaspoon in half a cup of warm water. It neutralizes mouth acidity, which eases the stinging.
3. Numbing gels. Benzocaine (Orajel) or lidocaine (Kank-A) gels numb the spot so you can eat. They don't heal faster, but they buy comfort.
4. Protective barriers. Anbesol or old-school alum powder (still in the spice aisle) shield the sore. Alum stings sharply for about 30 seconds — then the soreness noticeably drops.
5. Leave it alone. Poking it or trying to "pop" it will only make it bigger and last longer.

What to eat while it heals:

Stick to soft, cool, bland foods — yogurt, smoothies, oatmeal, scrambled eggs — and avoid acidic, spicy, salty, or crunchy foods until the sore is gone. Keep brushing normally; a dirty mouth heals slower, not faster. Just brush gently around the sore.

What doesn't work:

Antibiotics (not a bacterial infection), antivirals (not a virus), toothpaste dabbed on the sore (burns and irritates), and full-strength hydrogen peroxide (too harsh — the bubbling "deep clean" feeling is damage, not healing).

How to get them less often

If you get canker sores every month, you can usually break the cycle:

• Switch to an SLS-free toothpaste for two months and see what happens. This single change has the best evidence of any preventive step.
• Fix your sleep. Chronic sleep deprivation and stress are the two lifestyle factors most consistently linked to outbreaks. Seven to nine hours is the target.
• Eat a balanced diet with enough B12, iron, and folate. Supplements aren't needed unless blood work shows a deficiency.
• Stop the self-inflicted injuries. If you bite the same cheek repeatedly, you may be grinding your teeth at night (bruxism). A dentist can check your bite and, if needed, fit a night guard.
• Protect your mouth during dental work. If you always get a sore after a cleaning, ask your dentist to be gentle around soft tissue, or use a lip balm beforehand so the corners of your mouth don't crack.

When to see a doctor or dentist

Most canker sores are a nuisance, not a medical problem. Get them checked if:

• A sore lasts longer than two weeks or doesn't heal at all
• You get multiple sores at once, or they keep coming back in waves
• The sores are unusually large — bigger than half an inch across
• You have trouble swallowing, a high fever, or severe pain that doesn't respond to OTC remedies
• The sore is in a spot that never seems to heal, which should be evaluated to rule out something else entirely

A dentist or primary care doctor can prescribe a steroid mouth rinse or a dab-on corticosteroid paste (like triamcinolone dental paste) that calms large or stubborn sores much faster than anything on the drugstore shelf.

The bottom line

Canker sores show up uninvited, hurt wildly out of proportion to their size, and leave on their own schedule. But they're one of the most manageable recurring annoyances in medicine: switch the toothpaste, calm the stress, rinse with salt water, and leave the sore alone. And if they're showing up constantly, that's your mouth asking for a conversation with your doctor. Listen to it.