Canker sores are lesions that can develop on the the soft tissues in your mouth or at the base of your gums. Most canker sores are small, measuring less than 1 millimeter in diameter, and they usually appear white or yellowish in color with a red border. (2)

These lesions are common. But don’t assume that every sore in or around your mouth is a canker sore, especially since a number of conditions can look like a canker sore but actually represent something more serious, says Chirag Shah, MD, cofounder of Accesa Labs, a website that helps improve access to affordable medical lab testing.

Not sure what that white spot in your mouth is? Here’s a look at conditions that could be confused with a canker sore.

Cold Sores

What you believe to be a canker sore could actually be a cold sore. Also known as fever blisters, cold sores are painful blisters with similar characteristics to canker sores. (3) But canker sores differ from cold sores, which are caused by the herpes simplex virus (HSV-1), in that they are not contagious and don’t form on the keratinized tissues of the oral area (such as your lips), explains Samantha Rawdin, DMD, a prosthodontist based in New York City.

Some people get a cold sore after coming into contact with the virus through a kiss or after sharing a drink or food with someone who’s been infected. (2)

Cold sores can be unsightly, but the good news is that they usually heal in about two to four weeks, although sores may recur. (3)

Leukoplakia

These lesions can also be mistaken for canker sores because they produce white spots or patches inside the mouth.

Unlike a canker sore, leukoplakia patches aren’t sensitive or painful. (4) This condition can be harmless like a canker sore, but some patches may become cancerous. In fact, roughly 3 to 17.5 percent of people who have leukoplakia eventually develop squamous cell carcinoma, with the range dependent on the size, shape, and appearance of abnormal cells. (4) So if you have this condition, your doctor may recommend a biopsy to rule out cancer, since early treatment and diagnosis can reduce the risk.

Erythroplakia

Most canker sores go away in a week or two. (2) If a canker sore persists after that period of time, it increases in size, or you’re concerned about a particular sore, a medical evaluation is warranted, says Dr. Shah.

A sore that doesn’t heal could be erythroplakia, a red lesion that causes a white or red spot inside the mouth. These spots, however, are more serious because some can be precancerous or cancerous. About half of these lesions develop into squamous cell carcinoma. (5) The cause of erythroplakia is unknown, but the condition has been linked to smoking and tobacco use.

Lichen Planus

If you develop a tender white spot that also itches, you may have lichen planus instead of a canker sore. The spots are sometimes accompanied by a lacelike white patch on the inside lining of the cheeks. What’s interesting about this condition is that it affects more than just the inside of the mouth. If you have lichen planus, you’re likely to notice itchy bumps or spots on different parts of your skin. (6)

This is an inflammatory condition that’s believed to result from an abnormal immune response, when the immune system attacks cells of the skin and the mucous membranes. (6)

Oral Thrush

Oral thrush, also called oral candidiasis, is a fungal infection that causes white patches on the inside of the mouth. This condition can be painful and is sometimes accompanied by a loss of taste, a burning sensation, bleeding gums, and difficulty swallowing. (7)

Oral thrush occurs when yeast reproduces rapidly inside the mouth, usually the result of a weak immune system. This might happen if you’ve been prescribed antibiotics or are taking a medication that suppresses your immune system. (7)

Oral Cancer

If you have a sore that doesn’t improve after a couple of weeks or worsens, see your doctor or dentist. What appears to be a canker sore could actually be oral cancer.

Oral cancer can also cause small white ulcerations inside the mouth. These lesions can be painful and gradually become thicker. (8) Early diagnosis is important to prevent oral cancer from spreading to other parts of the body, such as the lymph nodes.

Keep in mind that while canker sores usually develop on the soft palate inside the mouth, oral cancer can also form on the hard palate and tongue. (8)

A sore might be oral cancer if you have other symptoms as well — such as a fever, throat numbness, or hoarseness — or if you have a history of smoking and other tobacco use. A biopsy of the lesion can diagnose or rule out cancer. If testing confirms cancer, your doctor will use imaging tests (such as an MRI or CT scan) to see whether the cancer has spread to other parts of your body. (9)

Gingivostomatitis

This is a painful infection of the mouth and gums that can develop after the first exposure to HSV-1, the same herpes virus that causes cold sores. Gingivostomatitis can occur in anyone, but it is most common in children and is often their first exposure to HSV-1. (10) About 90 percent of people have been exposed to the HSV-1 virus before age 50. (11) But not everyone will develop gingivostomatitis, and not everyone develops cold sores as a result.

Similar to a canker sore, gingivostomatitis can cause a small red or white sore on the soft palate, on the gums, or inside the cheek. But unlike a canker sore, this condition can also cause a fever and bleeding gums. Bad breath is also a common sign.

These sores are temporary and often go away in about two to three weeks without treatment. But taking an oral antiviral such as acyclovir (Zovirax) may shorten the duration of the sores if taken within 72 to 96 hours of onset. (10)

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