Summary
There are numerous equine oral joint supplements available. Joint supplements may be used preventatively to ensure optimal joint health, and may also be part of a treatment plan for a particular condition. There is disagreement in the industry about the effectiveness of these products.
The most common ingredients in these supplements are glucosamine (building block of cartilage) and chondroitin sulfate (essential structural component of connective tissue and cartilage). Other common ingredients are sodium hyaluronate, MSM, ASU, Omega-3 fatty acids, and a variety of herbal compounds. Each of these ingredients has a different proposed mode of action with respect to its effect on joint health, and has different scientific evidence (or lack thereof) regarding effectiveness.
Many of the components in oral joint supplements on the market are not supported by rigorous scientific evidence. Many supplement brands do not even contain the amount of ingredients claimed on the label.
That said, there is anecdotal evidence (and some indirect scientific evidence) that some of these ingredients can be helpful in certain circumstances. Your vet may have an opinion on the science behind a particular product, and its clinical effectiveness.
Given the wide variety of choices and the cost of these supplements, you should consult with your vet before purchasing a joint supplement for your horse. It makes sense to choose a product made by a reputable company, that has more supporting research for its use, and that has been on the market for a longer period.
For more information about how vet's use this product as part of a treatment plan, please see the related Vet Treatment, Oral Supplements, Joint.