Benadryl blocks histamine, which the Mast Cell tumors produce. When I first started Andy on the Benadryl, there was a red ring around the tumor. After a few days on Benadryl, the redness was gone, and over time, the tumor even appears smaller. However, it is still there but my vet examined it and said it is just there, not getting larger or spreading, and she feels comfortable waiting until the thyroid and liver values are within more normal limits. Shelties have thyroid problems - when we first adopted Andy he had very little fur on his back and a nasty skin condition that required me to give him medicated baths twice a week. He was also taking a higher dose of Soloxine and when his thyroid values were normal, the dose was reduced. Now that he's getting older, he has low thyroid issues again and must take a larger dose. By the way, an injection of Benadryl is given before surgery to prevent complications during the Mast Cell removal. I have read that dogs can go into shock from a massive release of histamine when the tumor is removed. Andy will stay on Benadryl until the tumor is removed, and possible for a short time thereafter. Dr. Doug, I've read and read as much as I could about Mast Cell tumors on the Internet, and I hope I'm giving correct information here. Meanwhile, Andy is doing well - he still eats like a champ and is still active with his toys. Hopefully this situation will be successfully resolved in the new year. Thanks for your good wishes.
Rita
