Canker sores occur in up to 50% of people, tend to run in families, and are more common during the winter and spring. Previously thought to be due to the herpes virus, that is not the case -- the cause is still unknown. They are not contagious like cold sores, which are caused by the herpes simplex virus. L-lysine seems to help prevent cold sores, but I haven't heard or read of any supplement that prevents canker sores.
Stress, in the form of exhaustion and lack of sleep, is probably the biggest trigger. I just saw a patient a few weeks ago who had a large, painful, ulcerated canker sore. When I asked if she was getting enough rest, she admitted to recently taking on a second job. So, my advice for prevention is go to bed early and take a nap, too, if you need it.
If the sores are so painful you can't eat, I ask the pharmacist to make a concoction of tetracycline, benadryl, mylanta, and xylocaine for the patient to swish several times a day. The sores usually heal in 7-10 days.
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Dr. Rebekah Wang-Cheng is a former Professor of Medicine at the Medical College of Wisconsin.