What is the most common clinical sign of cancer in dogs?

Weight loss is considered the most common clinical sign of cancer in dogs due to the metabolic changes and physiological stress that cancer causes. When a dog has cancer, the tumor and the body's efforts to fight it can lead to increased metabolic demands, resulting in the body using its energy reserves more rapidly. This can manifest as a noticeable loss of weight, as the dog may struggle to maintain its typical body condition.

Additionally, cancer can impact a dog's ability to absorb nutrients effectively, especially if the gastrointestinal system is involved or if the dog is experiencing other symptoms such as vomiting or discomfort, which can further exacerbate weight loss. As tumors grow, they can also interfere with normal bodily functions and overall physical health.

While vomiting, increased appetite, and coughing may also occur in dogs with cancer, they are not as universally prominent or defining as weight loss. Weight loss is a more consistent and significant indicator that warrants further investigation for underlying disease processes, including cancer.