The exact cause of Reye syndrome is unknown. Viral infections (e.g., colds, the flu, chickenpox) combined with the use of aspirin or aspirin-containing medication appear to be related to the onset of the syndrome.

Aspirin, a type of nonsteroidal anti-inflammatory drug (NSAID), may also be referred to as salicylic acid, salicylate, acetylsalicylic acid or acetylsalicylate. Aspirin has anti-inflammatory, pain-relieving (analgesic), and fever-reducing (antipyretic) properties – but appears to have a toxic effect in children and teens with viral illnesses. Various medications may contain aspirin, including antidiarrheals, antacids, antinausea drugs and a variety of cold and flu medications. Bismuth subsalicylate is an example of an over-the-counter medication for upset stomach that contains aspirin.

Certain factors are associated with an increased risk of Reye syndrome. These risk factors include:

• Use of aspirin to treat viral infections. Aspirin (or any medication derived from salicylic acid) used to treat viral illnesses has been associated with an increased risk of Reye syndrome.
  
  
• Age. Most cases of Reye syndrome occur in children and teenagers – especially those under 15 years old. Almost half of all cases in children occur in those under 5 years of age, according to the Centers for Disease Control and Prevention (CDC).
  
  
• Season. Reye syndrome occurs more often in the winter months when viral illnesses are more easily spread due to close contact indoors with others.