Yes, they are 100% poisonous to dogs & often they are fatal. I am speaking from experience. Do not let your dog run around loose in areas where you have spotted these mushrooms or it may be their last run. If you know of a dog who has ingested one, seek medical help immediately. Maybe the local vet can give them something quick that will cause them to vomit it up, and have a chance of saving them. Check with your local veterinarian, and notify them immediately. Do not waste any time. Get them to the vet immediately, and explain what has happened, and hopefully, they can save your dog in time. Do not let your dog run around in pine or coniferous forest off their leash. That is where you find most Fly agaric mushrooms. Fly agaric mushrooms are very attractive to many dogs because of their aroma, and they are liable to ingest it. Thanks for asking this important question that needed to be asked, and I hope I have helped save many dog owners the grief of losing their precious dog.