I've been taking the horse one for 3 years now, since I dislocated my elbow in a fall - about a third of a teaspoon in a drop of ribena to disguise the taste! It's miracle stuff - if I don't take it for a couple of weeks (well, it does excite comment when you're on holiday!), my elbow joint starts to click.....
I told my Dad about it for his arthritic knee, and he asked his (Australian) doctor about it who said go for it, the equine version is more quickly absorbed because it is in powder form that the human tablet versions.
I just scaled down the horse measures, and it came in as a tip of a teaspoon per day! If my Dad's Aussie doctor thinks it's okay, I reckon it won't do me any harm, and I haven't suffered any ill effects from it over the past 3 years (apart from a tendency to whinny!)