Well, the old racetrack guys are not known for their dosage calculations, since popping "a bute" for the aching joints (which I have also heard of) means taking the same dose (1 gram) that you would give to a horse!
Starlene, I think they put that on the label because they don't want people accidentally absorbing it when they did not mean to use it on themselves. Some people might be allergic to diclofenac or one of the carriers, or extremely sensitive to NSAID's in general (a negligible amount is absorbed into the systemic circulation, but I would guess if you are very sensitive you would not want to absorb any at all). Further, if you are a professional horse person who is handling these drugs every day (Surpass, DMSO, Furacin), I think the potential risks from long-term, even low-level, use (not to mention the interaction of substances!) is greater than for your average horse owner who may only use Surpass occasionally. In general, IMHO, it's probably better to minimize occupational exposure to drugs because of potential effects of long-term use and haphazard dosing.
I did not know that there was a human version in the U.S. of Surpass, either, until I started talking to a girl at the barn who works for the FDA as a pharmacy geek. I knew that there was a similar product licensed in Canada, and diclofenac pills are available by prescription in the U.S. -- I think one of the trade names is Voltaren.
My doctor has given me a refillable augmentin scrip. 
