Hi.. Can understand your concern.. As per your complain a recurring mouth sore can be a canker sore and if it has reduced in size or it healed some what and recurred you need not to worry as a cancerous ulcer will be chronic in nature and will show no signs of healing rather it will keep on gradually increasing in size.. A canker sore can occur due to a large number of causes like stress, taking plenty of citrus foods, nutritional deficiency, allergic reactions,underlying digestive problems like acid reflux etc.. It can also be due to viral infection (Herpes) but it is mostly in the form of clusters of sores but in less common conditions a single sore can also occur.. I would suggest you to consult an oral pathologist and get evaluated and in case if it is a canker sore and as it is recurring steroid therapy is needed.. Topical application of steroid based creams and oral steroids can help in healing.. Salt water gargles can help.. Rinsing with 1:1 solution of Milk of magnesia and benadryl can be relieving.. Intake of multivitamin for 15 days. Avoid citrus,hard and spicy foods.. Sucking ice pops can be relieving.. Drink plenty of water to maintain hydration.. In case of viral infection antiviral oral and topical therapy like Acyclovir can help.. Hope this information helps.. If you find the answer helpful please write a positive review and click on find this answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora.