Oral chondroprotective agents such as glucosamine and chondroitin have been used for many years in animals and humans for the management of degenerative joint disease, and multiple veterinary products exist now with a large variety of ingredients in addition to those two.
Historically, oral chondroprotective agents have been thought to have wide safety. LD50s for oral glucosamine in animals are approximately 8000 mg/kg with no adverse effects at 2700 mg/kg for 12 months.
A study done with cats with an oral chondroprotective agent at twice a therapeutic dose for 30 days showed no clinically significant hematologic, biochemical or hemostatic changes. Another study in dogs showed minor but not clinically significant hematologic and hemostatic changes during a 30-day oral chondroprotective study.
In 2010 ASPCA Animal Poison Control published a letter to the editor regarding 21 cases of hepatic damage in canines after oral overdoses of various joint supplements. Significant elevations in ALT followed by elevations in ALP were the most noted clinical finding. In these supplements, there were multiple ingredients noted beside just glucosamine and chondroitin.
If you have a patient who overdoses on an oral chondroprotective agent, precaution should be taken in minimizing exposure (emesis, charcoal) as well as monitoring liver enzymes and considering hepatoprotective agents.