Yes, canker sores typically grow during their first few days before they stop expanding and begin to heal. This is a normal part of the ulcer’s life cycle, not a sign that something is wrong. Most minor canker sores reach their full size within about three to five days, then gradually shrink and close over the following week or so.
The Growth Phase Is Normal
A canker sore doesn’t appear at full size. It usually starts as a small, tender spot or slight swelling on the inner cheek, tongue, or gum. Over the next one to three days, the tissue breaks down further and the sore widens. During this early stage, inflammation is actively destroying the surface layer of tissue, which is why the sore feels like it’s getting worse, not better. The pain tends to peak around the same time the sore reaches its maximum size.
For the most common type (minor canker sores), peak size is usually somewhere between 2 and 5 mm, though they can reach up to 10 mm. Once the sore stops expanding, it enters a plateau phase before healing begins. The entire process from first tingle to full resolution takes about 4 to 14 days for minor sores.
How to Tell It’s Started Healing
The clearest visual signal is color. An active canker sore looks white or yellowish in the center with a bright red border. As healing begins, the sore shifts to a gray tone. That grayish appearance indicates new tissue is forming underneath. Pain also drops noticeably once the sore stops growing, often a day or two before you can see visible shrinkage.
The red ring around the edge will gradually fade, and the sore will flatten and shrink from the outside in. You may still feel mild tenderness during this stage, especially when eating, but the sharp, stinging pain of the first few days should be gone.
What Makes a Sore Grow Larger Than Expected
Several things can irritate an existing canker sore and cause it to expand more than it normally would. Acidic foods like citrus, tomatoes, and vinegar are common culprits. Spicy foods, coffee, and even certain nuts (particularly peanuts and almonds) can aggravate the tissue. Repeated mechanical trauma matters too: accidentally biting the sore, scraping it with a rough tooth edge, or irritating it with braces or ill-fitting dental appliances can reset the healing process and make the sore larger.
Toothpastes containing sodium lauryl sulfate (SLS), a common foaming agent, have also been linked to canker sore irritation. Switching to an SLS-free toothpaste while a sore is active can reduce discomfort and may help it heal faster.
Minor vs. Major Canker Sores
Not all canker sores follow the same timeline. There are three types, and the difference in size and healing time is significant.
- Minor canker sores are the most common. They stay under 10 mm (usually 2 to 5 mm), and heal on their own within 4 to 14 days without scarring.
- Major canker sores (sometimes called Sutton ulcers) grow larger than 10 mm, often reaching 1 to 3 cm. They dig deeper into the tissue, feel firm and raised around the edges, and can take anywhere from 10 days to 6 weeks to heal. These often leave scars.
- Herpetiform canker sores appear as clusters of very small sores (1 to 2 mm each), sometimes dozens at once. Despite their small individual size, the clusters can merge into larger irregular ulcers. They’re intensely painful and typically last 7 to 10 days.
If your sore keeps growing past 10 mm or has been present for more than two weeks, it’s more likely a major type. These benefit from treatment rather than waiting it out.
How to Speed Up Healing
Topical corticosteroid pastes are the most effective option for reducing pain and shortening healing time. These come in adhesive gels or pastes that stick to the wet tissue inside your mouth and deliver anti-inflammatory medication directly to the sore. They work best when applied early, ideally as soon as you notice the sore forming, because they can limit how much the tissue breaks down during the growth phase.
Hyaluronic acid gels, available over the counter in many oral wound products, have also shown benefits for reducing pain and shrinking ulcer size faster than leaving sores untreated. They create a protective film over the sore that shields it from food and saliva irritation.
Beyond topical treatments, the simplest thing you can do is stop re-injuring the sore. Avoid crunchy, acidic, and spicy foods. Use a soft-bristled toothbrush and be careful when brushing near the sore. Rinse with warm salt water a few times a day to keep the area clean without using alcohol-based mouthwashes, which tend to sting and can slow healing.
When a Growing Sore Isn’t a Canker Sore
A canker sore that keeps getting bigger after two weeks, or one that never fully heals, is worth getting checked. Persistent, non-healing ulcers in the mouth are considered a red flag for oral cancer and other conditions that can mimic canker sores. This is especially true if the sore is painless (canker sores are almost always painful), if it feels hard or has irregular borders, or if you notice unexplained lumps, numbness, or difficulty opening your mouth.
A single canker sore that follows the normal pattern of growing for a few days, plateauing, then shrinking is not cause for concern. But a sore that only grows, or one that heals and immediately returns in the same spot, warrants a closer look from a dentist or doctor who can determine whether a biopsy is needed.