Having a sore in your mouth is no fun. They're easily irritated — and downright painful at those times.

Is it a mouth ulcer? They're certainly very common, and usually not something in need of a doctor's evaluation. A harmless annoyance that you just have to wait to clear up over the next week or two.

But what about a sore that doesn't seem to get better, that persists for weeks, even months. Or one that keeps getting bigger, more painful or starts to bleed?

Could it be mouth cancer?

"Early detection and early treatment of mouth cancer leads to drastically better outcomes, as well as less aggressive treatment, so we always want to catch these cancers early," says Dr. Joshua Kain, head and neck surgical oncologist at Houston Methodist.

It's why cancer experts stress that a mouth sore that won't go away should be evaluated sooner rather than later.

Still, there are some questions you might want answered first. Are you even at risk for mouth cancer? How long is too long for a sore to last in your mouth? What does mouth cancer look like — specifically, how can you tell a harmless mouth ulcer from cancer?

Dr. Kain answers these questions and more.

What causes mouth cancer?

Similar to other types of cancer, mouth cancer — also called oral cancer — happens when mutations accumulate and cause cells to grow out of control. In the case of mouth cancer, this happens most commonly in the thin, flat mucosal cells that line the inside of your mouth.

Anyone can develop mouth cancer, but certain factors increase a person's risk:

• Smoking or chewing tobacco
• Heavy alcohol use
• Gum disease (periodontitis)
  
  

"Historically, smoking is the most strongly related risk factor for oral cancers," says Dr. Kain. "A combination of tobacco and excessive alcohol can make a person's chance of developing it even greater. But these certainly aren't the only risk factors we see."

This explains why mouth cancer can occur in people who have never smoked a cigarette in their life. In these cases, gum disease is often the culprit.

"The chronic inflammation that happens around the teeth with gum disease is another very common reason we see this type of cancer develop," adds Dr. Kain.

Dr. Kain says the jury is still out on whether other forms of smoking, such as vaping and hookah, also might contribute to a person's risk.

"We know that chronic exposure to high heat in the mouth can be a risk factor for oral cancer, but whether these behaviors lead to oral cancer for that reason is still undetermined," adds Dr. Kain.

What are the signs of mouth cancer?

"There are two basic ways that oral cancers present in the mouth — as white patches or red patches," says Dr. Kain. "Red patches are a bit more concerning than white patches, but either needs to be evaluated if it doesn't go away after several weeks."

These patches can occur anywhere inside of your mouth, including:

• Inside the upper and lower lip
• Inside the cheeks
• Around the gum line and teeth
• Underneath the tongue
• On the tongue itself
• On the roof of the mouth
• At the back of the mouth (behind the molars)
  
  

Dr. Kain says it's unclear why red patches are more concerning than white ones.

"There's a lot of speculation about what's going on at the cellular level to make red patches more concerning, but no defined answer yet," Dr. Kain explains. "What we do know is that red patches have a higher likelihood of being cancerous or precancerous."

Mouth ulcer vs. cancer: What's the difference?

Mouth ulcers can look like red or white patches, too. So how can you tell if what you're seeing is really a harmless ulcer or a sore that needs evaluating?

"One major difference is that a mouth ulcer will come and go within one to two weeks," says Dr. Kain. "Oral cancer won't. Any patch in the mouth that lingers beyond four to six weeks needs to be evaluated."

Other mouth cancer symptoms that can help differentiate it from an ulcer include changes to the patch, such as:

• Pain
• Spontaneous bleeding
• Progressive growth
• Thickening
  
  

"If the patch is changing, that's a sign that it may not be benign and could be progressing into cancer," Dr. Kain points out.

Additionally, he says that unexplained bleeding is always a red flag for something more serious.

"Bleeding can happen if you bite an area in your mouth where a sore has developed, but a mouth ulcer typically won't bleed on its own spontaneously," explains Dr. Kain. "Whereas deeper infiltrating precancers and cancers of the mouth can have spontaneous bleeding."

What should you do if you have a mouth sore that won't go away?

Anytime you have a sore, ulcer or painful patch in your mouth that won't go away, Dr. Kain says it's important to be evaluated by an ENT specialist, a doctor who specializes in the ear, nose and throat.

"Often times, these patches are noticed by dentists who then refer the person to an ENT doctor," says Dr. Kain. "But if you notice a persistent issue between dental visits, don't wait to schedule an appointment with an ENT doctor to have it checked."

During the evaluation, your doctor will perform a visual exam and ask you about your risk factors.

"When there is a high enough level of concern based on the lesion's appearance and the person's risk factors, we proceed by performing a tissue biopsy," explains Dr. Kain. "For instance, if someone with poor dental health develops a fairly rapidly growing lesion inside their mouth, that's much more worrisome than someone with a sore that looks concerning but who has really impeccable dental hygiene."

Mouth tissue biopsy is a standard, straightforward procedure that can be performed safely and comfortably in your doctor's office — even at the first visit if the concern is great enough.

"We administer numbing spray and a numbing injection," explains Dr. Kain. "Then we biopsy the site. At times, a limited number of stitches may be needed."

If biopsy results indicate cancer, treatment varies based on how advanced the cancer is.

"With any mouth cancer, full surgical removal is the primary mode of treatment," says Dr. Kain. "Depending on cancer staging, additional therapies, like radiation or chemotherapy, may be needed. Ideally we find the cancer early enough to avoid having to use these additional therapies, which is another reason we stress having suspicious lesions evaluated as soon as they're noticed."