I have developed a severe sensitivity to citric acid found in citrus fruits, as well as blackberries, cherries, pineapple, and tomatoes, which has led to frequent canker sores. In combination with increased stress due to covid-imposed isolation the canker sores have become chronic and severe, and nothing was helping until my Dr recommended taking high doses of vitamin C. He noted that Vit C is ascorbic acid, NOT citric acid, which is also difficult to avoid in most processed foods, and that Vit C is essential for Healing. FYI Current Research has confirmed that most cankers are Not caused by virus or bacteria, and are Not contagious, ie some people are hereditarily predisposed to suffer from them. After trial-error I began with 2000mg per day, and increased to 4000mg per day > Success! Since reaching the 4000mg daily dosage two months ago I have not experienced any chronic cankers. For the other type of cankers typically caused by a Trauma, such as accidentally biting your cheek or tongue, rinsing 3-4 times per day with Licorice extract (alcohol-free) will heal them within about 3 days. This is also confirmed by Research. However, it's important to remember to Swish but Don't Swallow! Excess Licorice can lead to other health issues including severe gastric distress