Aspirin is the common name for acetylsalicylic acid. It has been used for over a century to reduce pains, aches, and fevers. One beneficial side effect is that aspirin can reduce the risk of heart attack. But there can be negative side effects as well. Stomach and intestinal bleeding can occur due to irritation of the wall lining. Special additives and coatings have been utilized to try to mitigate this effect in susceptible people. Chronic abuse of aspirin can also lead to anemia (iron deficiency in the blood). Hives and swelling can also occur. Reye's syndrome has also been detected in children (though this is very rare). So you can see that even something as simple and common as aspirin can still produce problems sometimes.