Hyperammonemia produced by an infusion of glycine in dogs leads to death within four hours. Concurrent treatment with a total dose of 15 or 20 mg/kg of diphenhydramine hydrochloride (Benadryl) significantly reduces the mortality to 12%. Although the surviving animals have lower average peak ammonia levels, there is no dose-specific relationship between the dose of diphenhydramine and the level of hyperammonemia. The mechanism of action of diphenhydramine remains unanswered.
Darin JC, Rood JE, Lewis JD, Ycasas AY. Effect of Diphenhydramine on Glycine-Produced Hyperammonemia in Dogs. Arch Surg. 1972;104(1):81–82. doi:10.1001/archsurg.1972.04180010075019