I know that this is an old post but chickens can have gabpentin but only when it is formulated for chickens. I've had it compounded for my birds before. It will help with seizure activity especially if combined with phenobarbital it makes the pheno last longer so you dont need as many doses to control seizures, nerve pain, as well as other types of pain as in oviduct prolapse. I dont know kw if I would recommend it for a head injury due to the possibly of actual brain injury that may not be known. Seeing that gabpentin has a sedative type effect i would worry about that with a head injury. Meloxicam can be used as a pain reliever in chickens as well. Gabpentin is 50mg per ML for liquid or 25 mg tabs or 50mg tabs I have broke them in half for bantam sized birds. The liquid as I stated was an animal compounded formula for birds. Hope this may help someone who reads this post in the future.