Canker sores get worse when they’re exposed to acidic foods, physical irritation, or ongoing stress, all of which increase pain and slow healing. Most minor canker sores are 2 to 5 mm across and heal on their own in 4 to 14 days, but certain everyday habits and conditions can drag that timeline out or make the pain significantly more intense.
Acidic and Spicy Foods
The single biggest day-to-day factor that makes a canker sore hurt more is what you eat. Acidic foods irritate the already-exposed tissue inside the ulcer, intensifying pain and potentially slowing healing. The worst offenders include citrus fruits and juices (oranges, lemons, grapefruits, limes), tomatoes and tomato-based sauces, strawberries, coffee, and carbonated drinks including sparkling water. Even foods you might think of as healthy can be a problem when you have an open sore in your mouth.
Spicy foods work through a different mechanism but produce the same result. Hot peppers, salsa, curry, and spicy seasonings inflame sensitive oral tissue and can aggravate an existing sore while also raising your risk of developing new ones. If you’re prone to canker sores, these foods are worth avoiding during a flare-up and possibly reducing in general.
The practical move is straightforward: while you have an active sore, stick to bland, soft, room-temperature foods. Hot beverages like coffee hit you with both acidity and heat, a combination that’s especially irritating to exposed mouth tissue.
Physical Irritation and Trauma
Anything that repeatedly rubs or presses against the lining of your mouth can worsen an existing canker sore or trigger new ones. Orthodontic hardware is a common culprit. In studies of patients with braces, 15 to 25% developed lesions on the lips, and 15 to 25% developed lesions on the inner cheeks, depending on whether they had upper, lower, or both sets of brackets. Palate expanders caused reversible palatal lesions in 35% of patients.
Beyond braces, other sources of mechanical irritation include biting the inside of your cheek, brushing too aggressively with a hard-bristled toothbrush, ill-fitting dentures, and sharp edges on chipped teeth. If you notice canker sores repeatedly forming in the same spot, look for a source of friction. Switching to a soft-bristled toothbrush and using orthodontic wax over sharp brackets can reduce the ongoing trauma that keeps sores from healing.
Stress and Your Immune System
Psychological stress is one of the most well-documented triggers for canker sore flare-ups, and it can also make existing sores heal more slowly. When you’re under sustained stress, your body’s main stress-response system (the pathway connecting your brain’s hypothalamus, pituitary gland, and adrenal glands) becomes overactive. This disrupts normal immune function, tipping the balance toward inflammation in vulnerable tissues like the lining of your mouth.
This isn’t just a vague connection. The immune disruption from chronic stress shifts your body’s inflammatory signaling in ways that make oral tissue more prone to ulceration and less efficient at repair. If you notice canker sores appearing during exam periods, work deadlines, or emotionally difficult stretches, the pattern is real and has a biological basis.
Nutritional Gaps That Fuel Recurrence
Low levels of certain nutrients don’t just raise your risk of getting canker sores; they make it harder for your body to heal the ones you have. The two most significant deficiencies are vitamin B12 and folate. People with recurrent canker sores consume roughly 7% less B12 and 20% less folate per day than those who don’t get them, based on comparisons with national dietary survey data.
Iron deficiency is another established contributor. All three nutrients play roles in maintaining healthy oral tissue and supporting the immune processes involved in wound repair. If you get canker sores frequently (the condition affects roughly 20% of the general population at any given time), it’s worth looking at whether your diet provides enough leafy greens, legumes, eggs, meat, and fortified grains, which are the primary sources of these nutrients.
Hormonal Shifts
Many women notice canker sores appearing or worsening right before or during their period. This timing isn’t coincidental. Estrogen levels drop sharply in the days leading up to menstruation, and research links this low-estrogen phase to increased oral ulcer recurrence. The drop appears to shift immune cell activity toward a more inflammatory state, making the mouth lining more vulnerable.
The relationship is complex. Some evidence suggests that elevated salivary estrogen during other phases of the cycle or during pregnancy can also increase shedding of the oral lining, potentially contributing to ulceration. But the premenstrual pattern is the one most women recognize, and tracking your cycle alongside flare-ups can help you anticipate and prepare for them.
Certain Medications
Several common medications can either trigger canker sores or make existing ones harder to resolve. Nonsteroidal anti-inflammatory drugs (like ibuprofen and naproxen), beta blockers used for blood pressure and heart conditions, nicorandil (a heart medication), and alendronate (used for osteoporosis) have all been linked to aphthous-like oral ulcers. Beta blockers in particular can cause dry mouth alongside the ulcers, compounding the problem since saliva plays a protective role in oral healing.
If you started a new medication and noticed canker sores appearing or worsening shortly after, the drug could be contributing. In documented cases, stopping the offending medication resolved the recurrent sores.
Home Remedies That Backfire
One of the most common mistakes people make is putting plain salt directly on a canker sore. This causes intense pain and does nothing to promote healing. Salt works only when dissolved in water: a teaspoon of salt in a cup of warm water, swished for about 30 seconds, can ease discomfort and support the healing process. The concentration matters because a dilute saltwater rinse gently draws fluid from swollen tissue, while undiluted salt crystals are abrasive and dehydrating to the wound surface.
Other irritating home remedies include applying undiluted hydrogen peroxide, rubbing alcohol, or crushed aspirin directly to the sore. All of these can damage the fragile tissue that’s trying to regenerate, turning a sore that might heal in a week into one that lingers for two or three.
When a Sore Becomes a Bigger Problem
Most canker sores are the minor type, under 1 cm across, and resolve within two weeks without scarring. Major canker sores, which range from 1 to 3 cm, are deeply set into the tissue and can persist for 6 weeks or longer. If a sore has grown unusually large, hasn’t started improving after two weeks, or keeps coming back in the same location, that’s a sign something beyond normal irritation is at play. Persistent mechanical trauma, an underlying deficiency, or a medication side effect could be keeping the cycle going.