Complex sentence: The relationship to vitamin B12 deficiency has been the subject of many studies; although these studies found that 0-42% of those with recurrent ulcers suffer from vitamin B12 deficiency, an association with deficiency is rare.
Split into simpler sentences: The relationship to vitamin B12 deficiency has been the subject of many studies. Although these studies found that 0-42% of those with recurrent ulcers suffer from vitamin B12 deficiency, an association with deficiency is rare.