I looked into the GLM this past fall; a family member sells a human supplement with it as the main ingredient and gets rave reviews about its efficacy for arthritis and joint support, so I figured I’d look into it for horses.
The very few studies I could find suggested that horses dosed with 10,000mg+ per day showed signs of improvement on it, but I had zero luck finding a product that provides 10,000mg of GLM! I went with Cox Veterinary’s Acti-Flex Senior from Riding Warehouse, since it was easily accessible to me in Canada, but even that only has 1000mg per scoop and it was one of the higher concentrations I could find. I never noticed a difference on it.
I considered importing some pure GLM from NZ via the family member’s supplier, but decided against it as the cost was prohibitive with how much I would have to give my horse daily to come anywhere near the doses used in the studies. I can’t imagine my picky horse being willing to eat 10 (large) scoops of fishy-tasting powder in order to receive the full benefit of the product, so I gave up the hunt!
If a local, cost-effective option became available that provided a higher concentration of the GLM, I would be very interested to try it again, simply because of the good things I have heard about it from people. That being said, you’ll also read just as many rave reviews about glucosamine, Back on Track products, etc. Much of it is just anecdotal experience, so grain of salt and all that. 