Hm, I never heard that you could give Adequan any way but IM! That's the method my vet uses (he's a stellar vet!). Several of our older horses at the barn are on it and most of them respond very well. When you start off you need to do a series of loading doses (several injections over the course of a few weeks) which ends up being a little expensive (like $300 total) but then you go down to an injection every other month or so (which for us is $70, which includes vet fees). I think it's definitely one of the more effective maintenance products out there and it's not unheard of for younger horses to be on it as a preventative for the future.
There's another fairly common product called Legend. It's a shot of Hyaluronic acid given directly into the joint (it can be given IV but it's not as effective that way). It doesn't last as long as Adequan does so people usually don't use it as a maitenance thing, they usually give it maybe a week or so before a show. (I think it only lasts a few weeks, if that long) It's more expensive and more risky since the injection is given right into the joint so I'm not a huge fan but it's pretty common.
I know some people who swear by Cosqeuin (a joint supplement in their feed) but I haven't personally seen any results (positive or negative) from using it. In fact, my vet said that in research done there is no proof that any of it gets absorbed into your horse's system at all!