I cannot say for certain with horses as thankfully I have never come across a horse with osteoarthritis but with dogs cartrophen injections are an absolute wonder drug and after a bit of research I have found that a similar product is on the market in the US (not sure about else where) called Adequan.
In dogs this is an injectable treatment administered by the vet usually once a week for the first four weeks and then further injections given at varying frequencies depending on the response to the drug ~ for most dogs this probably averages at maybe once a month but it is very much variable.
However in horses there is a rather large downside - the cost. In dogs it is more manageable as obviously they are smaller and therefore require less of the drug but with horses you basically end up with pretty big doses pretty often. It would seem that cartrophen with horses is often given for the 4 week period only because of the cost - this does seem to have some effect but obviously not permanent given that arth... is a degenerative disorder.
A clinical study: http://www.arthritis.au.com/horse/hrs_01.htm
This shows that cartrophen is safe to use up to 3mg/kg.
Oh and BTW in case any of you are really interested (
) the substance itself (cartrophen is the brand name) is 'Pentosan polysulphate' (http://en.wikipedia.org/wiki/Pentosan_polysulphate
) and is a very interesting looking molecule indeed! Curiously also used in humans to treat interstitial cystitis...