Aspirin is safe for dogs to consume on a short-term basis and under the direction of a veterinarian, according to WebMD. Prolonged use of aspirin may result in side effects such as internal bleeding. Coated aspirin is gentler on a dog's stomach, as is taking the medicine with food.
Aspirin relieves pain in dogs that suffer from arthritis or those recovering from surgery. The nonsteroidal drug reduces inflammation, swelling and stiffness of the joints, explains WebMD. A dog may be exhibiting signs of a bad reaction to aspirin if it suffers from changes in behavior, a decrease in appetite, redness of the skin or tarry bowel movements.