I just looked at, it contains 'glucosamine' and some 'chrondoitine', the former is recommended for humans and dogs, it lubricates the joints, chrondroitine is thought to help the joints but theres apparently less anecdotal evidence compared to glucosamine.
Anyway after looking at your Riaflex product i think its just typical of the way in which pet owners are ripped of all over the place on the selling point 'its for dogs' 'cats' etc, looking at the adds it does not even give the mg quantity per tablet.
I use glocosamine with my own dog, she's been on it since around 5 years, but i use tesco (asda etc the same) glucosamine with 500mg glucosamine sulphate, the sulphate (my vet said) just holds it all together, I buy them in tubs of 240, at 13 years she gets 3 of those per day, looking at todays prices they are 8 quid a tub or 3 for 2.
Just to add, glucosamine does not repair damage done prior to its use, it is a food not a medicine.
Tesco Add Below
http://www.mysupermarket.co.uk/tesco-price-comparison/Vitamins_And_Supplements/Tesco_Glucosamine_Sulphate_500mg_240.html.