Phenoxybenzamine for the treatment of severe nonresponsive diarrhea in the horse

Phenoxybenzamine, an alpha adrenergic antagonist, was administered IV to 6 clinically normal horses, 5 horses with experimentally induced diarrhea, and 7 horses with naturally-occurring diarrhea. It was established that a total of 2 mg of phenoxybenzamine/kg of body weight given in divided doses resulted in alpha adrenergic blockage of approximately 72 hours' duration, tranquilization, and mild constipation in the normal horse. The 5 experimental cases of diarrhea were involved in a laminitis research protocol in which laminitis was induced by oral carbohydrate overload. In all 5 of those cases, the severity of the diarrhea that accompanied the carbohydrate overload was reduced. The 7 clinical cases consisted of 2 foals and 5 adults with diarrhea, some of which were nonresponsive to conventional treatment. One of the foals did not respond to the phenoxybenzamine, and the other had a questionable response, whereas the 5 adults all responded favorably to the agent. It was concluded that phenoxybenzamine has a potential place in the therapy of some cases of diarrhea that are nonresponsive to conventional therapy. The mechanism of action of phenoxybenzamine on diarrhea is not known at this time.