This randomized, double-blind, placebo-controlled study investigated the effects of glucosamine/chondroitin sulfate, chondroitin sulfate, and diacerein on the gastric mucosa of dogs, and on clinical signs of vomiting and diarrhea. Twenty-one healthy adult dogs were included in the study, and were randomly assigned to seven treatment groups, 3 dogs per group. The effect of those medicines on the gastric mucosa was evaluated by endoscopy on day 3 and day 14, and compared with pre-treatment. Clinical signs, including vomiting and diarrhea, were recorded every day during the study period. The results showed a non-significant effect of glucosamine/chondroitin sulfate, chondroitin sulfate, and diacerein on gastric mucosal lesions in healthy dogs. One dog vomited after receiving glucosamine/chondroitin sulfate, but only for the first 2 days. All dogs receiving diacerein showed symptoms of diarrhea during the 14-day trial. In conclusion, glucosamine/chondroitin sulfate, chondroitin sulfate, and diacerein at doses of 1,500 and 3,000 mg/day are gastric-safe for use in dogs.