Does anyone have advice for getting rid of a canker? We are on day 6 of treatment with no improvement. I doubled her dose of metronidazole today. She has had 1/4 tsp copper sulfate per gallon of water, plus apple cider vinegar. I apply nystatin to the canker twice a day. She's happy and seems to be eating and drinking fine but it's getting bigger.. At what point do I have to euthanize her? What other treatments are available?