What causes mouth ulcers?

Most of us have experienced the irritation of a mouth ulcer at some point.

Maybe you are prone to ulcers in the mouth and seem to get them all the time – or maybe you’ve just got your first one. They can be pretty painful – but are almost always harmless.

Read on to find out what causes mouth ulcers, and who might be particularly prone to developing them.

What is a mouth ulcer?

Mouth ulcers – also known as aphthous ulcers, or canker sores – are small round lesions which appear white in colour with a faintly inflamed red border.

Mouth ulcers develop on the mucous membranes or soft tissues of the mouth. This includes anywhere on the inside of the cheeks, inner lips, tongue and gums.

Mouth ulcers can be irritating, and are sometimes painful – especially when eating, drinking and brushing the teeth.

Mouth ulcers are not contagious. They usually last around seven to 10 days, usually healing without special treatment. Treatment can help relieve the symptoms and hasten the healing process.¹

Are there different types of mouth ulcers?

There are four main types of mouth ulcers.^2,3,4

1. Traumatic ulcer

These mouth ulcers are caused by an injury to the inside of the mouth, gums or tongue.

These injuries can be caused by biting the inside of the cheek, burning your mouth on hot food or drink, or even dental wear such as a brace or retainer.

These ulcers heal by themselves in around 10 – 14 days.

2. Minor aphthous ulcer

The most common type of mouth ulcer, with around 80% of mouth ulcers falling into this category.

Around 2 – 4 mm in diameter, they can be caused by a variety of factors (see list below) and heal by themselves in around 10 – 14 days.

3. Major aphthous ulcer

These are a slightly larger mouth ulcer, from 0.5cm to 1cm in diameter. This type can take a little longer to heal – at least 14 days and sometimes taking several weeks.

This type can be more painful than the first two types. They also reoccur frequently.

They can be caused by a variety of factors (see list below).

4. Herpetiform ulcer

These are a rarer form of mouth ulcer. They occur in clusters of small, pinhead-sized ulcers which appear close together. This can give the illusion of one large granular ulcer which can be very painful.

These heal in around 10 days but are known to reoccur frequently.

Summary

• Mouth ulcers are irritating, sometimes painful circular lesions on the inside on the mouth and tongue
• They are not contagious and usually go away without treatment
• There are four main types of mouth ulcer – the most common is a minor aphthous ulcer

What causes ulcers?

Anyone can experience a mouth ulcer at any time, and some people get frequent mouth ulcers. There is not always a clear reason as to why they appear.

Check out these 10 reasons which could be causing mouth ulcers.

What causes mouth ulcers?

1. Injury

The tissues inside your mouth are sensitive. Something as simple as over-zealous brushing can cause minor damage to the oral mucosa (the wet skin layer lining your mouth).

Other things which cause injury to the mouth, such as accidentally biting the inside of your cheek or tongue or a blow to the jaw, disrupt these delicate tissues and can lead to a traumatic mouth ulcer.

Another common cause of mouth ulcers is food that’s too hot, causing scalding to the mouth or tongue. Food that’s particularly acidic or spicy can also trigger them.

Crisp-lovers should watch out, too, as spiky, sharp foods are also known causes of mouth ulcers due to the micro-injuries they can inflict on the oral mucosa as we crunch them.

2. Irritation

Wearing dental braces, retainers or false teeth are all common causes of traumatic mouth ulcers especially along the gum line. Speak to your dentist or orthodontist if you experience recurrent mouth ulcers, as they can be a sign your dental device doesn’t fit properly and is rubbing.

Similarly, oral piercing such as to the lip or tongue, can be a risk factor for recurring mouth ulcers due to the jewellery causing irritation to the delicate oral tissues.

Anxious habits such as chewing the inside of your cheek could also be at the root of mouth ulcers in the absence of any clear medical explanation.

3. Nutrient deficiency

Being short of certain vitamins and minerals can cause issues in and around the mouth.

Low levels of iron (anaemia), and a deficiency of zinc and B vitamins, including vitamin B12 have been linked to mouth ulcers.⁵

4. Stress

Stress is a contributory factor in a whole range of ailments and is a leading mouth ulcer cause. A high-stress lifestyle, which can include lack of sleep and no time for relaxation, can trigger an outbreak of mouth ulcers especially if you’re already prone to them.

A 2017 study in Iran on 54 volunteers found that recurrent aphthous stomatitis (RAS) – the scientific name for mouth ulcers which keep coming back – was linked to high levels of both anxiety and depression in sufferers.⁶

5. Gut inflammation

Your mouth can mirror what’s going on in your gut. This is why bad breath is often a side effect of gut imbalances and acid reflux. Yes - your gut and mouth are directly linked, and the latest research shows that they share some of the same microbial species.⁷

Disorders such as Coeliac disease (an intolerance to gluten) and inflammatory bowel diseases such as ulcerative colitis and Crohn’s disease are some of the more serious causes behind mouth ulcers.⁸

This is because they cause inflammation in your digestive system – of which your mouth is a part. Remember, inflammatory bowel diseases can only be diagnosed by a doctor.

However, these conditions are not usually the cause of mouth ulcers in absence of other symptoms – so don’t worry.

6. Viral infection

Mouth ulcers aren’t usually caused by an infection. However, if you get multiple mouth ulcers at once and they keep coming back (more than a couple of times a year) it might be classed as recurrent aphthous stomatitis (RAS). In these cases, the mouth ulcer might be a secondary symptom of a viral or bacterial infection.⁹

Infections which can trigger mouth ulcers include herpes simplex, or the cold sore virus, chickenpox, and hand, foot and mouth disease.¹⁰

Mouth ulcers are just one symptom of these infections, and mouth ulcers alone are no cause for alarm. Consult your doctor if you’re unsure.

7. Medication 

Various drugs have been suggested as possible triggers for mouth ulcers. These include:^11,12

• Nicorandil - drug used to treat heart conditions
• Beta-blockers
• Ibuprofen and other anti-inflammatory medicines in the NSAID family
• Oral nicotine replacement therapy
• Medicines dissolved in the mouth such as aspirin
• Cytotoxic drugs which treat conditions such as cancer and rheumatoid arthritis
• Illegal drugs such as cocaine

8. Certain foods

For some people, certain foods can be a trigger for mouth ulcers due to an underlying sensitivity to that food. These foods can include spicy foods, coffee, peanuts, almonds, strawberries, cheese, tomatoes and wheat flour.¹³

9. Smoking

Smoking or using tobacco products like chewing tobacco and snuff can be triggers for mouth ulcers.

Interestingly, people who stop smoking often report an increase in mouth ulcers during the months after they’ve given up. According to a study, this affects up to 2 in 5 quitters.¹⁴

10. Genetics

It seems that certain people are susceptible to getting mouth ulcers. The answer could be in your genes.

A recent study identified genetic variants associated with mouth ulcers and found 97 genetic variations that make a person more likely to get mouth ulcers.¹⁵

Summary

• There are various reasons why people develop mouth ulcers
• Causes of mouth ulcers include certain foods, a nutrient deficiency, smoking, inflammation, stress and oral trauma
• A recurrent mouth ulcer might be due to viral infection, genetics or certain medications

How to treat mouth ulcers 

• Salt rinse

Salt water has mildly antiseptic properties and can help soothe the symptoms and hasten the exit of an irritating mouth ulcer.¹⁶

Simply dissolve a teaspoon of salt in warm water and rinse the mouth thoroughly. You can do this several times a day.

• Protective gel

Over-the-counter oral gels which form a protective layer over the mouth ulcer can be effective in soothing pain. They also prevent further irritation from food and drink.¹⁷

These gels can contain active ingredients like salicylic acid to help aid healing. Ask your pharmacist to recommend one that’s best for you.

• Antiseptic mouthwash

An antimicrobial mouthwash containing chlorhexidine gluconate 2mg/mL (0.2%) can help speed up the healing process.

• Corticosteroid buccal tablets

These tablets can be placed on the ulcer inside the mouth. They release hydrocortisone as they dissolve, which reduces pain and speeds up healing.¹⁸

• Lidocaine spray

Lidocaine is a local anaesthetic, which is combined with an antiseptic in a spray to treat oral soreness such as that associated with mouth ulcers.

What is the best vitamin for mouth ulcers?

Folic acid – studies have shown that a deficiency in folate (the natural form of folic acid) is commonly seen in patients with frequent mouth ulcers, so get lots of folate-rich foods or take a supplement.¹⁹

Vitamin B12 – similarly, B12 deficiency is common among cases of recurring mouth ulcers, so top up your levels or take a supplement.

Iron – people with recurring oral ulcers have been found to have lower levels of iron than people without mouth ulcers. A diet rich in iron, or an iron supplement, can help restore your levels. ²⁰

Zinc - in one study, supplementing with zinc helped prevent recurrence of mouth ulcers in patients over 12 months.²¹

When should I see a doctor for mouth ulcers?

See your GP or dentist if:²²

• your mouth ulcer lasts more than three weeks
• your mouth ulcers keep recurring
• your mouth ulcer is especially painful and red – this could be a sign of a bacterial infection, which may need treatment with antibiotics

Last updated: 29 April 2021