Hi.. Thanks for the query.. Well, L-Methionine is an amino acid and is seen to help in healing mouth sores while S-methyl methionine is a derivative of Methionine and is researched to be helpful in healing stomach ulcers. However L-Methionins is commonly seen to heal mouth ulcers.. So my suggestion is to consult an Oral Physician and get evaluated and a thorough clinical examination should be done.. You can be advise proper management of your chemotherapy induced mouth sores.. It can include lysine and Vitamin B complex supplements and along with it you can also be advised application of gargling with specialised mouthwash like Magic mouthwash and avoiding spicy and hard foods.. Suck ice to reduce inflammation and irritation.. Hope this helps.. Regards..