Dogs and humans have similar physiology, so many medications are acceptable for both species. Aspirin is OK to give to your dog unless it is taking another non-steroidal anti-inflammatory drug (NSAID) such as Rimadyl, a common medication prescribed for arthritis pain in canines. Since aspirin is also a NSAID, combining it with another NSAID increases the risk of stomach bleeding and liver damage. Dogs should never take ibuprofen because they don't metabolize it rapidly enough to avoid kidney and liver damage.
Aspirin can be beneficial to soothe inflammation from injuries or to ease the pain of arthritis in dogs, and is usually the first drug of choice for dogs beginning to show signs of the disease.
All dogs should start with 81 mg, available in a buffered children's aspirin, or as a specially formulated dog aspirin available through most vets and some specialty pet stores.
Dogs must have buffered aspirin because they cannot as thoroughly absorb enteric-coated aspirin, thus increasing the risk of accumulation in the gastrointestinal system that could result in an overdose.
Ascriptin is a name-brand aspirin product that also contains Maalox to protect the stomach. This eases gastrointestinal upset, but shouldn't be given more than three times daily for 10 days because it contains full-strength aspirin (325 mg per dose).
ASPCA's National Animal Poison Control Center cautions against giving ibuprofen to dogs, since less than 200 mg can cause an ulcer in a small dog, and 1,200 mg can be fatal.