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Mononucleosis (mono) may cause various complications that can have serious health consequences. Inflammation of the liver is common and may result in jaundice, which causes yellowing of the skin and the whites of the eyes. Many people with mono also experience low platelet counts, which may interfere with the ability of blood to clot

In rare cases, enlargement of the spleen may lead to rupture. The spleen is located in the upper part of the abdomen on the left side. It helps to filter the blood of impurities and pathogens. When the spleen ruptures, it may feel like a sharp, sudden pain in the left side of the upper abdomen. Other symptoms associated with rupture of the spleen include lightheadedness, feeling like the heart is beating fast and hard, a tendency to bleed more easily than usual and breathing problems. This is a potentially serious condition that requires immediate emergency medical attention. Because of the risk of trauma to the enlarged spleen (splenomegaly), it is recommended that contact sports be avoided until the mononucleosis has resolved and enlargement of the spleen is no longer present or suspected.

Other uncommon complications associated with mono are:

• Anemia (reduction of red blood cells)
  
  
• Inflammation of the heart
  
  
• Pink rash (may develop in people with mono who are also taking various antibiotics)
  
  
• Nervous system problems (e.g., Bell's palsy, encephalitis, Guillain-Barre syndrome, meningitis, seizures)
  
  
• Swollen tonsils that cause obstructed breathing and swallowing difficulties that can lead to dehydration

Severe complications are most likely to appear in people who have impaired immune systems or who are taking medications to suppress their immune systems after an organ transplant. People with the human immunodeficiency virus (HIV) may have white tongue lesions (oral hairy leukoplakia) that contain the Epstein-Barr virus.

In very rare cases, chronic mono appears to be related to the development of Burkitt's lymphoma and nasopharyngeal carcinoma, two rare cancers that typically do not occur in the United States. Burkitt’s lymphoma occurs in certain regions of Africa and the Caribbean, and it has the same geographical distribution as malaria in that locale.

The Epstein-Barr virus has been linked to several other conditions, including several cancers and multiple sclerosis. These links are not well understood. Many studies have been and are being conducted to investigate these relationships.