Pain Answer: Like any other medicine, aspirin has negative side effects in certain situations - usually with excessive doses.

(1) It may cause stomach problems and kidney problems. This is because both of these organs require COX-1 for their normal functioning (see below).

(2) It may cause metabolic acidosis - a drop in the blood pH. This occurs when an excess of aspirin, an acid, overwhelms the blood's bicarbonate buffer system.

(3) It may cause hyperpyrexia - elevated body temperatures. To explain how we need to examine how the mitochondria in our cells synthesize ATP using the process of cellular respiration. Mitochondria have a proton gradient across their inner membrane. This gradient is made by electron transport chains (ETCs) using electrons removed from organic molecules. This proton gradient powers ATP Synthases which make ATP. However, aspirin molecules can carry protons across the inner mitochondrial membrane allowing them to bypass the ATP Synthases. In this situation the ETCs and the ATP Synthases are said to be uncoupled. As the efficiency of cellular respiration decreases the amount of heat produced increases. The result is hyperprexia. This situation usually occurs when small children eat aspirin tablets thinking that they are candy.

(4) In rare cases aspirin causes Reye's syndrome if it is given to children to treat chickenpox (varicella virus infection) or the flu (influenza virus infection). It is a severe and often fatal disorder characterized by deterioration of the brain and liver. Its cause is unknown. This is why aspirin, even children's aspirin, should never be given to a child under the age of 17 unless directed to by a physician.

(5) Aspirin may also trigger an asthma attack in people with asthma. How this happens is not clear.