With vitamin antioxidants, “more” is not automatically better—especially for fat-soluble vitamins. Vitamin E and vitamin A (including some carotenoid-derived vitamin A activity) can contribute to fat-soluble accumulation over time. That matters most when owners unintentionally stack sources: a complete-and-balanced diet already fortified with vitamins, plus a multivitamin, plus an “antioxidant” chew, plus add-ons like oils or treats that also contain added vitamins. This stacking is a common way dogs drift above intended intake without any single product looking excessive.
Interaction caution is also important. Higher vitamin E intake may be a concern for dogs on anticoagulant medications or with clotting disorders, and any dog with liver disease, pancreatitis, or other chronic conditions should have vet-led dosing and monitoring.
Vitamin C is less likely to accumulate, but it can still cause problems at higher amounts—most commonly GI upset (loose stool, gas, vomiting). If a new antioxidant vitamin product is started, monitor stool quality and appetite, and stop and consult your veterinarian if digestive signs appear.
Practical takeaway: choose one primary fortified source at a time, avoid overlapping formulas, and use your veterinarian to set a conservative, individualized plan based on diet, size, and medical history (NRC, 2006).