Ibuprofen is toxic to dogs. Although it is a common over-the-counter pain reliever and considered generally safe for use in people, dogs have different body chemistry and cannot tolerate the drug.
Ibuprofen belongs to a class of medications called NSAIDs, which are chemicals that inhibit a group of enzymes that cause pain and inflammation. However, some of these enzymes also help the stomach and kidneys function properly. Some NSAIDs are selective, meaning they only affect the enzymes that cause pain. Ibuprofen is non-selective, so it interferes with stomach and kidney function as well as providing pain relief. Humans are able to tolerate this effect well enough that ibuprofen is considered generally safe for use in people, but cats and dogs are especially sensitive. Dogs can experience kidney failure and stomach ulcers if given ibuprofen.