Canker sores are not contagious. Unlike cold sores, which are caused by the herpes simplex virus and spread through direct contact, canker sores have no viral or bacterial origin that can be transmitted from person to person. Early research confirmed this decades ago, when lab cultures of canker sore tissue showed zero viral activity, while herpes lesions readily produced virus in the same tests. The confusion between the two is one of the most persistent mix-ups in everyday health knowledge, and it matters because the causes of canker sores, and the ways to manage them, are entirely different from what you’d do for a viral infection.
Why People Confuse Canker Sores With Cold Sores
Both canker sores and cold sores are small, painful mouth-area lesions that come and go, which is where the confusion starts. But they are fundamentally different conditions. Cold sores (fever blisters) appear on the lips and outer mouth, are caused by herpes simplex virus, and absolutely can be passed to others through kissing, shared utensils, or skin-to-skin contact. Canker sores, by contrast, form inside the mouth on soft tissue like the inner cheeks, tongue, or the base of the gums. They are open ulcers, not fluid-filled blisters, and they are not caused by any known pathogen.
A classic study cultured tissue from both types of lesions: herpes simplex virus was isolated from herpetic lesions, but none of 100 canker sore samples showed any viral activity at all.1Archives of Oral Biology. Recurrent “fever blister” and “canker sore” tests for herpes simplex and other viruses with mammalian cell cultures You cannot “catch” a canker sore from someone else, and you cannot give yours to anyone. This distinction is worth remembering if you find yourself avoiding sharing a drink with a friend who has a mouth ulcer on the inside of their cheek.
What Actually Causes Canker Sores
The honest answer is that researchers still don’t fully understand what triggers canker sores in any given person, but the condition appears to arise from an overactive immune response in the lining of the mouth rather than from an outside invader. The medical term, recurrent aphthous stomatitis (RAS), hints at the chronic, recurring nature of the problem. It is one of the most common oral mucosal conditions worldwide, typically first appearing in childhood or adolescence.2PubMed Central. Oral mucosal disease: recurrent aphthous stomatitis
The current understanding is that canker sores involve a combination of genetic predisposition, local trauma, stress, hormonal shifts, and environmental factors.3PubMed Central. Management of oral aphthous ulcer: A review None of these alone is a complete explanation, and most people who get canker sores can point to more than one factor that seems to set them off. What’s clear is that the problem is internal, not infectious.
The Genetic Side
If your parents got canker sores frequently, your odds of getting them go up. This runs in families in a way that goes beyond just sharing the same household or diet. Research into the genetic basis of recurrent aphthous stomatitis has identified certain gene variants, particularly those involved in producing inflammatory signaling molecules called cytokines, that appear more often in people who get canker sores and in their relatives.4PubMed Central. Recurrent aphthous stomatitis: genetic aspects of etiology The higher prevalence of canker sores among family members points to inherited variations in how the immune system handles inflammation inside the mouth.
This genetic component also helps explain why some people never get a single canker sore in their life while others deal with them multiple times a year, even when lifestyle and diet are similar. Your immune system’s baseline tendency toward overreacting to minor oral irritation is, to a real degree, something you’re born with.
Everyday Triggers That Set Them Off
Genetics loads the gun, but specific triggers pull the trigger. Knowing your personal triggers is probably the most useful thing you can do if you get canker sores regularly, because avoiding them is more effective than treating ulcers after they appear.
Physical Trauma to the Mouth
Biting the inside of your cheek, jabbing yourself with a toothbrush, irritation from braces or dental work, or even eating something with a sharp edge like a tortilla chip can damage the delicate lining of the mouth enough to kick off a canker sore in someone who is predisposed. This is one of the most consistently reported triggers. The ulcer doesn’t form simply because of the injury itself but because the immune system overreacts to the minor wound.
Nutritional Deficiencies
Low levels of certain nutrients are linked to more frequent outbreaks. People with recurrent canker sores have been found to consume less vitamin B12 and folate compared to the general population, with folate intake about 20% below recommended daily levels.5PubMed Central. Reduced dietary intake of vitamin B12 and folate in patients with recurrent aphthous stomatitis Iron deficiency is another factor: screening canker sore patients for low ferritin levels (a marker of iron stores) has been recommended because iron supplementation alongside a diet rich in iron may help reduce recurrence.6PubMed Central. Significance of ferritin in recurrent oral ulceration
This doesn’t mean taking a multivitamin will cure canker sores. But if you get them often and your diet is low in leafy greens, meat, eggs, or fortified grains, a blood test to check B12, folate, and iron levels is worth asking your doctor about. Correcting an actual deficiency can make a noticeable difference for some people.
Stress and Hormonal Changes
Stress is one of the triggers patients report most frequently, and it makes biological sense: psychological stress alters immune function in ways that could amplify the inflammatory response in the oral lining. Hormonal fluctuations follow a similar logic. Some women notice canker sores appear around the time of their menstrual period. Research using genetic analysis methods has explored whether estradiol levels directly cause mouth ulcers; while no direct causal link from estradiol itself was found, the authors noted that rapid changes in estradiol, such as those occurring before or during menstruation, could disrupt the body’s inflammatory balance enough to contribute to outbreaks.7PubMed Central. Causal associations between estradiol and mouth ulcers: A Mendelian randomization study
Your Toothpaste Might Be Making It Worse
This is a trigger most people never think about. Sodium lauryl sulfate (SLS) is a foaming agent found in the majority of commercial toothpastes. It is also an irritant. A systematic review of clinical trials found that switching to an SLS-free toothpaste led to fewer ulcers, shorter episodes, and less pain compared to standard SLS-containing formulas.8PubMed. Effect of sodium lauryl sulfate on recurrent aphthous stomatitis: A systematic review The improvements were consistent across all four measures tracked: number of ulcers, duration, number of episodes, and pain levels.
SLS works as a detergent, and in the mouth it strips protective layers from the soft tissue. A clinical study comparing SLS toothpaste to a gentler alternative (using steareth 30 instead) found that SLS toothpaste caused significantly more mucosal lesions and higher severity scores within 30 minutes of use.9PubMed. A randomised clinical study comparing the effect of Steareth 30 and SLS containing toothpastes on oral epithelial integrity (desquamation) Separate research has shown that SLS kills bacteria in oral biofilms and disrupts microbial community structure, increasing the number of pathogenic bacteria while also damaging periodontal tissue and amplifying inflammation.10PubMed Central. Effects of sodium lauryl sulfate and postbiotic toothpaste on oral microecology
If you get canker sores regularly, switching to an SLS-free toothpaste is one of the easiest and cheapest experiments you can run. Several widely available brands sell SLS-free formulas, and you don’t lose any cleaning effectiveness by making the switch.
The Oral Microbiome Connection
The community of bacteria living in your mouth may play a role in whether canker sores develop. Research comparing the oral microbiota of canker sore patients with healthy controls found real differences in bacterial populations on the inner cheek lining. Certain bacterial types were more common in patients, while others were less common. The differences were most pronounced in patients who had active ulcers at the time of sampling, raising a chicken-or-egg question: does an abnormal bacterial community trigger the lesions, or do the lesions alter the community?11PubMed Central. The oral microbiota of patients with recurrent aphthous stomatitis Either way, maintaining good oral health and avoiding harsh products that disrupt the microbiome, like SLS, may help keep things in balance.
When Canker Sores Signal Something Bigger
For most people, canker sores are a nuisance and nothing more. But recurrent mouth ulcers can sometimes be a sign of an underlying systemic condition, and it’s worth knowing when the pattern warrants further investigation.
Celiac disease is one of the conditions most commonly linked to canker sore-like ulcers. People with undiagnosed celiac disease sometimes present with mouth ulcers as one of their earliest symptoms, before gastrointestinal issues become obvious. However, canker sore-like lesions are not unique to celiac disease and can appear in several other disorders.12PubMed Central. Oral manifestations of gastrointestinal disorders
Behçet’s disease is a rarer but more serious condition where recurrent oral ulcers are nearly universal, occurring in an estimated 97% to 100% of patients.13PubMed Central. Interventions for managing oral ulcers in Behçet’s disease Behçet’s is an inflammatory disorder that also causes genital ulcers, eye inflammation, and skin lesions. If you have frequent mouth ulcers along with any of those other symptoms, it’s something to bring up with your doctor. For severe or refractory oral ulcers associated with Behçet’s, systemic immunosuppressive treatments may be necessary.14PubMed Central. The treatment of chronic recurrent oral aphthous ulcers
A reasonable rule of thumb: if your canker sores are unusually large (bigger than about a centimeter), last longer than three weeks, come with a fever, or occur alongside sores in other areas of the body, see a healthcare provider rather than waiting them out.
What Actually Helps When You Have One
Most canker sores heal on their own within one to two weeks without any treatment. The goal of treatment is to reduce pain, speed up healing, and in some cases lower the odds of recurrence. Several approaches have decent evidence behind them.
Topical Corticosteroids
Prescription-strength steroid gels or pastes applied directly to the ulcer are one of the best-studied treatments. A systematic review found that topical corticosteroids shortened healing time and reduced pain compared to placebo.15Acta Otorrinolaringologica (English Edition). Topical Corticosteroids in Recurrent Aphthous Stomatitis. Systematic Review These are typically applied a few times a day directly to the sore. Over-the-counter hydrocortisone mouth preparations exist, but stronger formulations require a prescription. Starting treatment as early as possible in the ulcer’s development tends to produce better results.
Antiseptic and Medicated Rinses
Chlorhexidine mouthwash, available without a prescription in many countries, can help reduce pain and may speed healing. A clinical trial combining chlorhexidine rinse with a growth-factor gel found faster pain relief, smaller ulcer areas, and a lower recurrence rate compared to standard treatment alone.16PubMed Central. Clinical Efficacy of Chlorhexidine Gargle Combined with Recombinant Bovine Basic Fibroblast Growth Factor Gel in the Treatment of Recurrent Oral Ulcers and Its Effects on Inflammatory Factors, Immune Function, and Recurrence Rate Even without the gel component, chlorhexidine on its own is commonly recommended by dentists for managing canker sores because it reduces the bacterial load around the ulcer and keeps secondary infection at bay.
Barrier-Forming Products
Products containing hyaluronic acid, available as mouth rinses or gels, work by forming a protective film over the ulcer. This shields it from further irritation by food, drink, and saliva. In a clinical study comparing a hyaluronic acid gel with a barrier-forming rinse, both produced significant reduction in lesion size and pain within a week, with more than half of ulcers completely closed by day seven. The gel form showed faster early improvement, with about 72% of lesions improved by day three compared to 40% with the rinse.17PubMed Central. Treatment of recurrent aphthous stomatitis (RAS; aphthae; canker sores) with a barrier forming mouth rinse or topical gel formulation containing hyaluronic acid: a retrospective clinical study These products are available without a prescription and are a good option if you want relief without steroids.
Home and Natural Remedies
Saltwater rinses are the oldest and cheapest approach: dissolving about half a teaspoon of salt in a cup of warm water and swishing gently can soothe the area and promote healing. It won’t speed things up dramatically, but it keeps the area clean and is harmless.
Honey has shown more promising results than you might expect. A systematic review found that topical honey application reduced the severity or duration of oral ulcerative lesions in the majority of studies examined.18Exploratory Research and Hypothesis in Medicine. The Effect of Honey as a Treatment for Oral Ulcerative Lesions: A Systematic Review Animal research has further supported this, finding that honey gel was more effective than other honey delivery forms in shortening wound healing time for oral ulcers.19PubMed Central. Effect of two different delivery systems of honey on the healing of oral ulcer in an animal model Applying a small amount of raw honey directly to the sore a few times a day is a low-risk option. It stings for a moment on contact but many people find it soothing afterward.
What Doesn’t Help
Because canker sores aren’t caused by a virus or bacterium, antiviral medications like acyclovir (used for cold sores) and antibiotics won’t do anything for them. This is one of the practical reasons the canker-sore-versus-cold-sore distinction matters: if you go to a pharmacy asking for cold sore treatment when you actually have a canker sore, you’ll end up with a product designed for a completely different problem.
Aggressive mouthwashes containing high concentrations of alcohol can also make things worse by irritating the ulcer further. Stick with gentle, alcohol-free rinses if you want to use a mouthwash while a canker sore is healing.
Living With Frequent Outbreaks
For people who get canker sores several times a year, the focus shifts from treating individual sores to reducing the frequency of episodes. A practical approach combines several of the strategies covered above:
- Switch toothpaste: Use an SLS-free formula. This is the single change with the strongest evidence for reducing recurrence.
- Check nutrient levels: Ask your doctor to test B12, folate, and ferritin. Correct any deficiencies through diet or supplements.
- Protect your mouth: Use dental wax over braces or rough spots on dental work. Be mindful of hard or sharp-edged foods when the lining of your mouth feels sensitive.
- Track your triggers: Keep a simple log of outbreaks alongside your diet, stress levels, menstrual cycle, and any oral injuries. Patterns often emerge after a few months.
- Treat early: If you feel the tingling or sensitivity that signals an ulcer forming, applying a topical steroid or barrier gel right away can shorten the episode.
Canker sores tend to become less frequent with age. Many people who dealt with them regularly in their teens and twenties find that outbreaks taper off by middle age. The reasons for this aren’t entirely clear, but changes in immune function and hormonal patterns over the lifespan likely play a role. If you’re in your peak canker-sore years, the condition is annoying but manageable, and the strategies that work best are the boring, consistent ones: the right toothpaste, a balanced diet, and early treatment when a sore appears.