Dentures are removable false teeth that fit snugly over the gums to replace missing teeth. They can be complete (a full set of upper and/or lower teeth) or partial (replacing one tooth or a few missing teeth).
What are the main causes?
Mouth ulcers can develop when you are fitted with new dentures, or when old dentures are worn or no longer fit you properly. Over time, your gums and jawbone may become smaller and your dentures can become loose.
If you don’t replace badly fitting or worn dentures, you can find yourself in pain and have mouth sores, infections and problems with eating and speech. 
What are the symptoms?
Sore spots and mouth ulcers can occur where the denture rubs against your gums. You may also find that you have cracked or continually sore corners of your mouth.
What are my treatment options?
The best thing to do is make sure that you see your dentist regularly, so that they can keep an eye out for any problems with your dentures. If you do get mouth ulcers or sore spots, you will need to check with the dentist whether they need to adjust the fit of your dentures, including if they are new and you have had teeth removed within the last six months, as this is when your bones and gums are likely to shrink the most.  Otherwise, you can use a mouth ulcer gel to relieve pain and inflammation and help fight infection.
What can I do about it?
The main goals for treating your mouth ulcer and sore spots are to relieve your pain, reduce inflammation and help fight infection. There are two products in the bonjela range, designed to help you feel more comfortable while wearing your dentures and help fight infection. Both are suitable for use in adults.
Brush your dentures with toothpaste or soap and water twice a day and soak them in denture cleaner.
Keeping your mouth clean is as important as making sure your dentures are clean. If you have a partial denture, you should also brush your remaining teeth twice a day with fluoride toothpaste to prevent tooth decay, gum disease and other dental problems.
You can leave your dentures out at night, but keep them moist with a denture-cleanser soaking solution or water, so they do not dry out or lose their shape.
Visit your dentist regularly so that they can check any remaining teeth are healthy as well as your gums and dentures.