In humans at least, the half-life of adenosine in the bloodstream is seconds. I canāt imagine it would be a particularly effective calming agent anyway.
I tried googling what adenosine is used for in horses and I got a bunch of hits indicating that itās used IM to āimprove energy levelsā (probably because adenosine is a building block of ATP, an energy source for cellular metabolic processes ⦠but the body does a pretty good job of making its own ATP using just food and oxygen, so this seems like a sketchy claim) and dilate coronary arteries (a known effect of it which is why itās used to do cardiac stress tests in people ⦠but Iām not sure what the utility of that would be in horses, who presumably donāt have atherosclerotic coronary disease).
Regardless, it sounds sketchy and like something that is unlikely to be supported by any evidence. Any vets can feel free to correct me if Iām mistaken!