Honestly, I don't think it matters. Unless it is absorbed in the mouth. (and even then, it would be exposed to the mucas membranes longer mixed with food) When I had my wild mustangs I had to squirt the paste wormer into their food. There was NO WAY they were going to let me put it into their mouths, and I never had any problems. They actually get more if mixed into the food, assuming they eat all of the food because they won't be spitting it out. (like mine try to do.) Actually, now that I think about it, generally I shove an alfalfa cube into thier mouth right after I squirt the paste wormer in, to keep them from spitting it out.