The best and worst foods for mouth ulcers – and what really causes them

By Lauren Potts

When I’m stressed, run-down or eating a less-than-ideal diet, it’s only a matter of time before a mouth ulcer makes an appearance.

This is wildly common: around one in four of us will get a mouth ulcer (also known as an aphthous ulcer or canker sore) at some point, says Dr Lucy Hooper, private GP and co-founder of Coyne Medical in Fulham.

“The vast majority are harmless and should heal completely,” she reassures me, usually within seven to 10 days.

But it’s possible to send them packing sooner – or perhaps even prevent them altogether. From eating the right nutrients and having good oral hygiene to being mindful of certain conditions, medications and ingredients, there are lots of things you can try.

What causes mouth ulcers?

These round or oval-shaped sores are often caused by mild trauma, like accidentally biting the inside of your mouth and friction caused by braces.

Genes, hormonal changes and viral infections can also play a role, and patients with autoimmune conditions like Crohn’s and coeliac disease, or those undergoing chemotherapy can be more susceptible, says Dr Hooper. There’s even a link with some everyday medications, like ibuprofen.

Then there’s perhaps the most common cause of all: stress.

Stress causes inflammation, points out Dr Hooper, and “any time you’ve got inflammation going on in the body, it can affect your mouth.”

Can diet cause mouth ulcers?

Studies show that not getting enough vitamin B12, iron, folate and zinc are common causes of mouth ulcers.

Anyone avoiding meat, eggs, seafood and dairy has an increased risk of becoming deficient in those nutrients, Dr Hooper warns.

“It doesn’t mean you can’t have a wonderful diet as a vegan or vegetarian, but when you exclude some [food] groups, you need to be careful you’re getting all of these vitamins in your diet.”

To lower your risk of deficiency as a vegan or vegetarian, she advises:

• Choosing plant-based milks fortified with B12.
• Eating plenty of chickpeas, peanuts and quinoa for zinc.
• Including dark green veg and beans in meals as sources of folate
• Making sure you’re eating iron-rich vegetables like spinach.

Foods and drinks that cause mouth ulcers – or make them worse

Alcohol, fizzy drinks and citrus fruits top the list of culprits for worsening mouth ulcers. No surprise there, since frequent exposure to acidic foods and drinks lowers the pH of the mouth, explains Dr Hooper. This in turn disrupts the mouth's protective layer, damaging key protective proteins that leaves tissue more vulnerable to mouth ulcers.

But then there are the less obvious suspects.

“Strawberries and tomatoes are two things people don’t associate with [acidity] but can be a trigger for mouth ulcers,” says Dr Hooper.

She also recommends avoiding spicy and salty foods.

“These will irritate the lining of the mouth and the delicate mucosa, which can be enough to trigger a mouth ulcer.”

If you already have an ulcer, it’s worth considering the texture of foods too, says Dr Hooper.

“Avoid anything that’s hard, crunchy or sharp. Just the action of trying to chew through a crunchy baguette or something like that is going to be uncomfortable.”

There are some foods you might want to add to your diet, though.

Dr Hooper points to one study that found participants who ate a plant-rich diet had a lower recurrence of mouth ulcers. Certain probiotics have also been identified as able to help heal them and reduce pain – although evidence here is limited.

Hygiene matters

Good dental hygiene in general, like flossing and regularly going to the dentist, can help keep ulcers at bay, says Dr Hooper.

If your toothpaste contains sodium lauryl sulfate (SLS) – which is often included to help the paste foam and clean – it might be worth switching brands. One study found that using SLS-free toothpaste reduced the number, pain and recurrence of mouth ulcers.

“SLS is essentially a detergent – it helps your toothpaste clean your teeth, but unfortunately it may be quite disruptive to the inside of the mouth,” says Dr Hooper.

When to see a doctor

Most mouth ulcers might be small – but they can have a big impact. For the vast majority, a painkilling gel or antimicrobial mouthwash will see us through the worst of it.

But if they get larger than a centimetre, hang around for more than two weeks or come with other symptoms, it’s best to get them checked, she adds. A blood test might be needed to check for deficiencies.

“Any of us can get them once or twice. But if you’re getting them on a regular basis, or with severe pain or other signs of illness like a fever or pain in your tummy or joints, we should be looking for underlying causes.”

Published January 2026

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