Glucosamine supplements have been given to dogs on a wide spread basis and studied in detail. Very few side effects have been observed, with most dogs experiencing no adverse reactions.
The side effects that have been reported are typically vomiting and diarrhea. These are usually mild and subside within a few days of adding glucosamine supplements. In the rare cases where these side effects persist, they can often be alleviated by giving the dog the glucosamine in its food, or by splitting the dose between morning and night. No long term adverse effects have been reported in dogs taking glucosamine.
As with all dietary supplements and medications, consult your vet before introducing something new to your dog.
Glucosamine & Chondroitin for Hip Dysplasia & Arthritis in Dogs, Doctors Foster and Smith, (accessed 8/2011)Is it Safe to Use Glucosamine Sulfate on Dogs?, eHow (accessed 8/2011)