It’s always a good idea to make sure of what you are treating. What are the symptoms? Are there cheesy yellow patches inside the beak? Does it smell rotten? Have you seen any fowl pox scabs on your chickens recently? Birds with canker should be separated from the others, and waterers and feeders should be disinfected. Some people treat canker with acidified copper sulfate 1/4 tsp per gallon of water for 5-7 days. Here is a good price on it:
https://www.mcmurrayhatchery.com/acidified_copper_sulfate.html
