Mouth sores most often fall into two categories: canker sores (small ulcers inside the mouth) and cold sores (fluid-filled blisters on or around the lips). Canker sores have no single known cause, but they’re typically triggered by a combination of minor injury, stress, nutritional gaps, or food sensitivities. Cold sores are caused by the herpes simplex virus, usually type 1. Figuring out which type you’re dealing with, and what’s setting it off, is the first step toward fewer and less painful outbreaks.
Canker Sores vs. Cold Sores
These two get confused constantly, but they look different and behave differently. Canker sores appear inside the mouth, on the inner cheeks, lips, or tongue. They’re usually a single round sore, white or yellow in the center with a red border. They are not contagious and not caused by a virus.
Cold sores (fever blisters) show up outside the mouth, typically around the border of the lips. They look like clusters of small fluid-filled blisters and are caused by herpes simplex virus infection. Once you carry the virus, outbreaks can be triggered by sun exposure, stress, or illness. If your sores are inside your mouth and appear one at a time, you’re almost certainly dealing with canker sores, and the triggers below are where to focus.
Nutritional Deficiencies That Cause Sores
Low levels of iron, vitamin B12, or folate are among the most common and most overlooked reasons people develop recurrent mouth ulcers. A deficiency in B12 or folate can cause a sore, reddened tongue along with mouth ulcers, sometimes as part of a broader pattern of anaemia that also includes fatigue, pale skin, and breathlessness. Iron deficiency produces similar oral symptoms.
If your sores keep coming back without an obvious trigger, it’s worth checking these levels with a simple blood test. Correcting a deficiency often reduces or eliminates the sores entirely, which no amount of topical treatment can do on its own.
Foods and Drinks That Trigger Outbreaks
Acidic foods and beverages are a major trigger. Citrus fruits, fizzy drinks, and alcohol lower the pH inside your mouth, which disrupts the protective layer of proteins that shields your oral tissue. Once that barrier is weakened, ulcers form more easily. Strawberries and tomatoes are two foods people rarely suspect, but both are acidic enough to set off sores in sensitive individuals.
Spicy and salty foods irritate the delicate lining of the mouth directly, which can be enough to start an ulcer on its own. And if you already have a sore, hard or crunchy foods (chips, crusty bread, raw carrots) can reinjure the area and delay healing.
Physical Injury and Irritation
Biting your cheek, brushing too aggressively, or catching your gum on a sharp edge of a tooth or dental appliance is one of the most straightforward causes. These minor injuries create a break in the tissue that develops into a full canker sore within a day or two. Braces, ill-fitting dentures, and recent dental work are frequent culprits.
Your toothpaste may also play a role. Sodium lauryl sulfate (SLS), a foaming agent in many toothpastes, can irritate the oral lining and may promote ulcer formation in people who are prone to them. Switching to an SLS-free toothpaste is a low-cost experiment worth trying if you get sores regularly.
Stress and Hormonal Shifts
Stress is one of the most consistently reported triggers for canker sores. The mechanism isn’t fully understood, but immune function shifts during periods of high stress, and the mouth’s mucosal lining becomes more vulnerable. Many people notice sores appearing during exams, work deadlines, or emotionally difficult periods.
Hormonal changes can follow a similar pattern. Some women who are prone to canker sores develop a recognizable cycle where sores recur during every menstrual period. If you notice your sores arriving on a predictable monthly schedule, hormonal fluctuations are the likely explanation.
Medications That Cause Mouth Sores
Several common medications can trigger oral ulcers as a side effect. NSAIDs like aspirin and naproxen can cause chemical irritation of the mouth’s lining, producing white or yellow lesions and tissue sloughing. Some blood pressure medications, including certain ACE inhibitors and angiotensin-receptor blockers, carry the same risk.
Anti-seizure medications, certain antibiotics, and gout medications are associated with more serious drug reactions that can include severe mouth sores as part of a broader skin and mucosal response. If mouth sores appeared shortly after starting a new medication, that timing is worth flagging with whoever prescribed it.
Allergic Reactions
Allergic reactions can sometimes trigger canker sore outbreaks. This might involve a food allergy, a sensitivity to an ingredient in your toothpaste or mouthwash, or a reaction to a dental material. If your sores seem to correlate with exposure to a specific product or food, an elimination approach (removing one suspected trigger at a time) can help identify the cause.
How to Manage the Pain at Home
Most canker sores heal on their own within one to two weeks. In the meantime, a few approaches can cut the pain significantly. A saltwater or baking soda rinse is the simplest option: dissolve one teaspoon of baking soda in half a cup of warm water and swish several times a day. This raises the pH in your mouth, neutralizes acids, and keeps the area clean.
Over-the-counter numbing gels containing benzocaine provide direct pain relief when applied to the sore. Antiseptic mouth rinses with hydrogen peroxide can also help. For sores that are especially painful, your doctor or dentist can prescribe a topical steroid, which is the treatment with the strongest evidence for reducing symptoms. These don’t prevent future sores, but they make current ones less painful and may shorten healing time. Antimicrobial rinses containing chlorhexidine can also reduce the severity of an outbreak.
While you’re healing, stick to soft, cool, bland foods. Avoid anything acidic, spicy, salty, or abrasive until the sore has closed.
When Mouth Sores Signal Something Serious
A sore that doesn’t heal within two weeks needs medical attention. Oral cancer can present as a mouth sore that persists, a white or reddish patch inside the mouth, a lump, unexplained mouth pain, ear pain, or difficulty swallowing. These symptoms overlap with harmless conditions, which is exactly why a sore that won’t resolve deserves evaluation. A doctor or dentist will examine the area and, if anything looks abnormal, take a small tissue sample for testing.
Recurrent sores that are unusually large, appear in clusters, spread rapidly, or come with fever may also point to an underlying condition like an autoimmune disorder, celiac disease, or an immune system that’s compromised. Sores that keep returning despite addressing the common triggers listed above warrant a deeper workup, including blood tests for nutritional deficiencies and screening for systemic conditions.