Twenty-one adult crossbreed dogs with chronic pain due to severe osteoarthrosis were enrolled in the study (placebo vs. treatment groups). The dogs in the experimental group received the dietary supplement (Pain ReliefTM, Giantec, Isernia, Italy) for 30 days to evaluate its effects on metabolism and pain relief. During the trial, the Helsinki Chronic Pain Index significantly decreased (p < 0.01) in the experimental group, indicating reduced pain and improved quality of life. Additionally, the treated group showed improvements in oxidative stress, demonstrated by a reduction in reactive oxygen metabolites, and an increase in biological antioxidant potential. Interleukins 6 levels decreased in the treated group, while interleukins 10 levels increased, thus suggesting an anti-inflammatory effect of the supplement. Importantly, no adverse effects were observed. Results suggest that Pain ReliefTM is effective in ameliorating osteoarthritis in dogs, improving their quality of life.