If you hit your finger with a hammer and it's bleeding, an aspirin may help the pain and swelling, but the wound may take longer to clot and stop bleeding. Also, it can be very upsetting to the stomach, especially at the high doses often used in arthritis. Aspirin also isn't used as much for fevers in children since research has suggested that aspirin given to kids with flu, chickenpox, or other viral sicknesses may cause a potentially deadly problem called Reye syndrome. Aspirin also changes the way your kidneys make urine, can cause some people to have trouble breathing (rarely), and can be dangerous at very high doses.