Identification

• One of the most harmful potential side effect of Xolair is that of an anaphylactic allergic reaction.
  
  

Types of Symptoms

• Symptoms of anaphylaxis due to Xolair include shortness of breath, swelling of your face, tongue, throat or extremities, low blood pressure, fainting and skin hives. These symptoms require immediate medical attention as untreated anaphylaxis can be fatal.
  
  

Significance

• Approximately .2 percent of patients who use Xolair experience anaphylactic reactions, according to RxList.
  
  

Risks

• There is also a slight risk of cancer associated with the use of Xolair, particularly tumors that form in your breasts and prostate as well as on your skin, cautions RxList.
  
  

Considerations

• Because Xolair is known to cross both into the placenta and breast milk, doctors rarely prescribe the drug for use by pregnant or nursing women. The drug is also not approved for use in children under the age of twelve, cautions the Mayo Clinic.