The term "Canker Rash" is not commonly used in English language or medical terminology, but if we were to search for antonyms of the two separate words "Canker" and "Rash," we could explore some potential options.
For "Canker", words that could serve as antonyms may include "Healthy", "Sound", or "Strong".
For "Rash", antonyms could include "Clear", "Smooth", or "Unblemished".
Using these antonyms in context, we could describe a person's mouth as being healthy and strong, with clear and smooth skin free from any blemishes or irritation.