Coffee is a well-known trigger of canker sores for some people. There are different factors, which can contribute to that:

Acidity

Coffee is acidic, and acidic foods or drinks can irritate the lining of the mouth. This irritation can make the mouth more susceptible to canker sores or exacerbate existing ones.

Caffeine's Drying Effect

Caffeine in coffee can cause dehydration if consumed in large quantities, leading to a dry mouth. A dry mouth can make the oral environment less protective, increasing the risk of irritation and sores.

Temperature

Hot coffee can burn or irritate the sensitive tissues inside the mouth, creating micro-injuries that might turn into canker sores.

Stress Connection

Coffee consumption is often linked with stress (e.g., pulling all-nighters, rushing to meet deadlines). Stress is a known trigger for canker sores.

Additives and Ingredients

Milk or Cream

Dairy products can sometimes trigger canker sores in sensitive individuals.

Sugar or Sweeteners

Sugar can feed bacteria in the mouth, potentially worsening oral irritation. Artificial sweeteners, like aspartame, are also suspected triggers for some people.