Update! I've been treating my hen with fish zole for a few days. She's very skinny but still alive, so I can only assume she's been eating. She doesn't go after her grain, but will peck at bread and other scraps I toss to the chickens. So her appetite's barely increased and the yellow lumps have receded just a little bit. She doesn't seem well enough for me to stop giving her medicine. How long should I keep treating her for?
Urgent - hen with swollen mouth - Page 2
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You might take a small amount of her feed, mix a little raw egg and plain yogurt or buttermilk, and add water to make a runny paste. She should take some of that (my hens love it) if she can eat. Metronidazole is usually prescribed for 5-7 days. Here is a good thread with info from Dawg53 and Speckled Hen who are very experienced: http://www.backyardchickens.com/t/690374/abscess-in-chickens-cheek-and-mouth-canker-remedies