I've read that to really do a bunch of good, there needs to be 9,000mg of glucosamine, daily, in the diet.
However, Lacey's on a supplement ( Majesty's Flex Wafers - Horse Treats from SmartPak Equine
) that's only 2,500mg of glucosamine and there has been improvement with that (she's also barely arthritic). I'm thinking of bumping her up to two "wafers" a day though, just to see it she improves even more.
I don't know why your pills wouldn't work, but the only thing I can think of is that there may be differences in the way horses and humans absorb things internally, and since the pills are for humans... But I don't know, that's just a guess.