Will Benadryl act as a sedative on your 9 month old 28kg golden retriever puppy?

If your puppy has a medical condition requiring sedation (e.g. broken bone needs time to knit), then talk to your vet about options and choose an option that is appropriate for your dog's health.

If your puppy is simply too hyper for you, it most likely needs more exercise. In this case, sedation is animal cruelty.

The rule of thumb is 1mg per pound....not recommended though .I would try "Rescue remedy" first.