Could you tell me what's the best way to treat a canker sore?

Not sure: Some find that applying table salt on the sores may help (painful when applied), others have tried topical antibiotic with success. Still others have been treated with a topical cortisone paste. All of which work for some people. There is no sure cure since we do not know what causes canker sores yet.

Licorice: Licorice has been used since ancient times to treat canker sores in the mouth. Also, small studies show it can be effective when used as a mouthwash -- 50-75% improvement in 1 day. http://tinyurl.com/kr9aeau sucking on dgl lozenges may work also. Other studies showed positive results with a dissolving licorice disk placed on the sore. http://tinyurl.com/mdvde3o.